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XFS docs are currently in upper-level Documentation/filesystems. Although these are currently 4 docs, they are already outstanding as a group and can be moved to its own subdirectory. Consolidate them into Documentation/filesystems/xfs/. Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com> Reviewed-by: Bill O'Donnell <bodonnel@redhat.com> Reviewed-by: "Darrick J. Wong" <djwong@kernel.org> Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>
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ReStructuredText
1088 lines
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ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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==================
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XFS Logging Design
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==================
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Preamble
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========
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This document describes the design and algorithms that the XFS journalling
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subsystem is based on. This document describes the design and algorithms that
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the XFS journalling subsystem is based on so that readers may familiarize
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themselves with the general concepts of how transaction processing in XFS works.
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We begin with an overview of transactions in XFS, followed by describing how
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transaction reservations are structured and accounted, and then move into how we
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guarantee forwards progress for long running transactions with finite initial
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reservations bounds. At this point we need to explain how relogging works. With
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the basic concepts covered, the design of the delayed logging mechanism is
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documented.
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Introduction
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============
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XFS uses Write Ahead Logging for ensuring changes to the filesystem metadata
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are atomic and recoverable. For reasons of space and time efficiency, the
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logging mechanisms are varied and complex, combining intents, logical and
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physical logging mechanisms to provide the necessary recovery guarantees the
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filesystem requires.
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Some objects, such as inodes and dquots, are logged in logical format where the
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details logged are made up of the changes to in-core structures rather than
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on-disk structures. Other objects - typically buffers - have their physical
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changes logged. Long running atomic modifications have individual changes
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chained together by intents, ensuring that journal recovery can restart and
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finish an operation that was only partially done when the system stopped
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functioning.
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The reason for these differences is to keep the amount of log space and CPU time
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required to process objects being modified as small as possible and hence the
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logging overhead as low as possible. Some items are very frequently modified,
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and some parts of objects are more frequently modified than others, so keeping
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the overhead of metadata logging low is of prime importance.
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The method used to log an item or chain modifications together isn't
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particularly important in the scope of this document. It suffices to know that
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the method used for logging a particular object or chaining modifications
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together are different and are dependent on the object and/or modification being
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performed. The logging subsystem only cares that certain specific rules are
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followed to guarantee forwards progress and prevent deadlocks.
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Transactions in XFS
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===================
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XFS has two types of high level transactions, defined by the type of log space
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reservation they take. These are known as "one shot" and "permanent"
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transactions. Permanent transaction reservations can take reservations that span
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commit boundaries, whilst "one shot" transactions are for a single atomic
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modification.
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The type and size of reservation must be matched to the modification taking
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place. This means that permanent transactions can be used for one-shot
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modifications, but one-shot reservations cannot be used for permanent
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transactions.
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In the code, a one-shot transaction pattern looks somewhat like this::
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tp = xfs_trans_alloc(<reservation>)
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<lock items>
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<join item to transaction>
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<do modification>
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xfs_trans_commit(tp);
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As items are modified in the transaction, the dirty regions in those items are
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tracked via the transaction handle. Once the transaction is committed, all
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resources joined to it are released, along with the remaining unused reservation
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space that was taken at the transaction allocation time.
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In contrast, a permanent transaction is made up of multiple linked individual
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transactions, and the pattern looks like this::
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tp = xfs_trans_alloc(<reservation>)
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xfs_ilock(ip, XFS_ILOCK_EXCL)
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loop {
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xfs_trans_ijoin(tp, 0);
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<do modification>
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xfs_trans_log_inode(tp, ip);
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xfs_trans_roll(&tp);
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}
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xfs_trans_commit(tp);
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xfs_iunlock(ip, XFS_ILOCK_EXCL);
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While this might look similar to a one-shot transaction, there is an important
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difference: xfs_trans_roll() performs a specific operation that links two
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transactions together::
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ntp = xfs_trans_dup(tp);
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xfs_trans_commit(tp);
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xfs_trans_reserve(ntp);
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This results in a series of "rolling transactions" where the inode is locked
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across the entire chain of transactions. Hence while this series of rolling
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transactions is running, nothing else can read from or write to the inode and
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this provides a mechanism for complex changes to appear atomic from an external
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observer's point of view.
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It is important to note that a series of rolling transactions in a permanent
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transaction does not form an atomic change in the journal. While each
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individual modification is atomic, the chain is *not atomic*. If we crash half
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way through, then recovery will only replay up to the last transactional
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modification the loop made that was committed to the journal.
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This affects long running permanent transactions in that it is not possible to
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predict how much of a long running operation will actually be recovered because
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there is no guarantee of how much of the operation reached stale storage. Hence
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if a long running operation requires multiple transactions to fully complete,
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the high level operation must use intents and deferred operations to guarantee
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recovery can complete the operation once the first transactions is persisted in
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the on-disk journal.
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Transactions are Asynchronous
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=============================
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In XFS, all high level transactions are asynchronous by default. This means that
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xfs_trans_commit() does not guarantee that the modification has been committed
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to stable storage when it returns. Hence when a system crashes, not all the
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completed transactions will be replayed during recovery.
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However, the logging subsystem does provide global ordering guarantees, such
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that if a specific change is seen after recovery, all metadata modifications
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that were committed prior to that change will also be seen.
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For single shot operations that need to reach stable storage immediately, or
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ensuring that a long running permanent transaction is fully committed once it is
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complete, we can explicitly tag a transaction as synchronous. This will trigger
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a "log force" to flush the outstanding committed transactions to stable storage
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in the journal and wait for that to complete.
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Synchronous transactions are rarely used, however, because they limit logging
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throughput to the IO latency limitations of the underlying storage. Instead, we
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tend to use log forces to ensure modifications are on stable storage only when
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a user operation requires a synchronisation point to occur (e.g. fsync).
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Transaction Reservations
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========================
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It has been mentioned a number of times now that the logging subsystem needs to
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provide a forwards progress guarantee so that no modification ever stalls
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because it can't be written to the journal due to a lack of space in the
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journal. This is achieved by the transaction reservations that are made when
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a transaction is first allocated. For permanent transactions, these reservations
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are maintained as part of the transaction rolling mechanism.
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A transaction reservation provides a guarantee that there is physical log space
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available to write the modification into the journal before we start making
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modifications to objects and items. As such, the reservation needs to be large
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enough to take into account the amount of metadata that the change might need to
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log in the worst case. This means that if we are modifying a btree in the
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transaction, we have to reserve enough space to record a full leaf-to-root split
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of the btree. As such, the reservations are quite complex because we have to
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take into account all the hidden changes that might occur.
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For example, a user data extent allocation involves allocating an extent from
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free space, which modifies the free space trees. That's two btrees. Inserting
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the extent into the inode's extent map might require a split of the extent map
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btree, which requires another allocation that can modify the free space trees
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again. Then we might have to update reverse mappings, which modifies yet
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another btree which might require more space. And so on. Hence the amount of
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metadata that a "simple" operation can modify can be quite large.
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This "worst case" calculation provides us with the static "unit reservation"
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for the transaction that is calculated at mount time. We must guarantee that the
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log has this much space available before the transaction is allowed to proceed
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so that when we come to write the dirty metadata into the log we don't run out
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of log space half way through the write.
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For one-shot transactions, a single unit space reservation is all that is
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required for the transaction to proceed. For permanent transactions, however, we
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also have a "log count" that affects the size of the reservation that is to be
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made.
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While a permanent transaction can get by with a single unit of space
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reservation, it is somewhat inefficient to do this as it requires the
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transaction rolling mechanism to re-reserve space on every transaction roll. We
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know from the implementation of the permanent transactions how many transaction
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rolls are likely for the common modifications that need to be made.
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For example, an inode allocation is typically two transactions - one to
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physically allocate a free inode chunk on disk, and another to allocate an inode
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from an inode chunk that has free inodes in it. Hence for an inode allocation
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transaction, we might set the reservation log count to a value of 2 to indicate
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that the common/fast path transaction will commit two linked transactions in a
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chain. Each time a permanent transaction rolls, it consumes an entire unit
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reservation.
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Hence when the permanent transaction is first allocated, the log space
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reservation is increased from a single unit reservation to multiple unit
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reservations. That multiple is defined by the reservation log count, and this
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means we can roll the transaction multiple times before we have to re-reserve
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log space when we roll the transaction. This ensures that the common
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modifications we make only need to reserve log space once.
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If the log count for a permanent transaction reaches zero, then it needs to
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re-reserve physical space in the log. This is somewhat complex, and requires
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an understanding of how the log accounts for space that has been reserved.
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Log Space Accounting
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====================
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The position in the log is typically referred to as a Log Sequence Number (LSN).
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The log is circular, so the positions in the log are defined by the combination
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of a cycle number - the number of times the log has been overwritten - and the
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offset into the log. A LSN carries the cycle in the upper 32 bits and the
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offset in the lower 32 bits. The offset is in units of "basic blocks" (512
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bytes). Hence we can do realtively simple LSN based math to keep track of
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available space in the log.
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Log space accounting is done via a pair of constructs called "grant heads". The
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position of the grant heads is an absolute value, so the amount of space
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available in the log is defined by the distance between the position of the
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grant head and the current log tail. That is, how much space can be
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reserved/consumed before the grant heads would fully wrap the log and overtake
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the tail position.
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The first grant head is the "reserve" head. This tracks the byte count of the
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reservations currently held by active transactions. It is a purely in-memory
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accounting of the space reservation and, as such, actually tracks byte offsets
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into the log rather than basic blocks. Hence it technically isn't using LSNs to
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represent the log position, but it is still treated like a split {cycle,offset}
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tuple for the purposes of tracking reservation space.
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The reserve grant head is used to accurately account for exact transaction
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reservations amounts and the exact byte count that modifications actually make
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and need to write into the log. The reserve head is used to prevent new
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transactions from taking new reservations when the head reaches the current
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tail. It will block new reservations in a FIFO queue and as the log tail moves
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forward it will wake them in order once sufficient space is available. This FIFO
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mechanism ensures no transaction is starved of resources when log space
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shortages occur.
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The other grant head is the "write" head. Unlike the reserve head, this grant
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head contains an LSN and it tracks the physical space usage in the log. While
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this might sound like it is accounting the same state as the reserve grant head
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- and it mostly does track exactly the same location as the reserve grant head -
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there are critical differences in behaviour between them that provides the
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forwards progress guarantees that rolling permanent transactions require.
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These differences when a permanent transaction is rolled and the internal "log
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count" reaches zero and the initial set of unit reservations have been
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exhausted. At this point, we still require a log space reservation to continue
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the next transaction in the sequeunce, but we have none remaining. We cannot
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sleep during the transaction commit process waiting for new log space to become
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available, as we may end up on the end of the FIFO queue and the items we have
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locked while we sleep could end up pinning the tail of the log before there is
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enough free space in the log to fulfill all of the pending reservations and
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then wake up transaction commit in progress.
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To take a new reservation without sleeping requires us to be able to take a
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reservation even if there is no reservation space currently available. That is,
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we need to be able to *overcommit* the log reservation space. As has already
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been detailed, we cannot overcommit physical log space. However, the reserve
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grant head does not track physical space - it only accounts for the amount of
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reservations we currently have outstanding. Hence if the reserve head passes
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over the tail of the log all it means is that new reservations will be throttled
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immediately and remain throttled until the log tail is moved forward far enough
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to remove the overcommit and start taking new reservations. In other words, we
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can overcommit the reserve head without violating the physical log head and tail
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rules.
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As a result, permanent transactions only "regrant" reservation space during
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xfs_trans_commit() calls, while the physical log space reservation - tracked by
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the write head - is then reserved separately by a call to xfs_log_reserve()
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after the commit completes. Once the commit completes, we can sleep waiting for
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physical log space to be reserved from the write grant head, but only if one
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critical rule has been observed::
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Code using permanent reservations must always log the items they hold
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locked across each transaction they roll in the chain.
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"Re-logging" the locked items on every transaction roll ensures that the items
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attached to the transaction chain being rolled are always relocated to the
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physical head of the log and so do not pin the tail of the log. If a locked item
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pins the tail of the log when we sleep on the write reservation, then we will
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deadlock the log as we cannot take the locks needed to write back that item and
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move the tail of the log forwards to free up write grant space. Re-logging the
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locked items avoids this deadlock and guarantees that the log reservation we are
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making cannot self-deadlock.
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If all rolling transactions obey this rule, then they can all make forwards
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progress independently because nothing will block the progress of the log
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tail moving forwards and hence ensuring that write grant space is always
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(eventually) made available to permanent transactions no matter how many times
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they roll.
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Re-logging Explained
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====================
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XFS allows multiple separate modifications to a single object to be carried in
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the log at any given time. This allows the log to avoid needing to flush each
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change to disk before recording a new change to the object. XFS does this via a
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method called "re-logging". Conceptually, this is quite simple - all it requires
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is that any new change to the object is recorded with a *new copy* of all the
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existing changes in the new transaction that is written to the log.
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That is, if we have a sequence of changes A through to F, and the object was
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written to disk after change D, we would see in the log the following series
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of transactions, their contents and the log sequence number (LSN) of the
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transaction::
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Transaction Contents LSN
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A A X
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B A+B X+n
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C A+B+C X+n+m
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D A+B+C+D X+n+m+o
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<object written to disk>
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E E Y (> X+n+m+o)
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F E+F Y+p
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In other words, each time an object is relogged, the new transaction contains
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the aggregation of all the previous changes currently held only in the log.
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This relogging technique allows objects to be moved forward in the log so that
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an object being relogged does not prevent the tail of the log from ever moving
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forward. This can be seen in the table above by the changing (increasing) LSN
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of each subsequent transaction, and it's the technique that allows us to
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implement long-running, multiple-commit permanent transactions.
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A typical example of a rolling transaction is the removal of extents from an
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inode which can only be done at a rate of two extents per transaction because
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of reservation size limitations. Hence a rolling extent removal transaction
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keeps relogging the inode and btree buffers as they get modified in each
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removal operation. This keeps them moving forward in the log as the operation
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progresses, ensuring that current operation never gets blocked by itself if the
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log wraps around.
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Hence it can be seen that the relogging operation is fundamental to the correct
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working of the XFS journalling subsystem. From the above description, most
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people should be able to see why the XFS metadata operations writes so much to
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the log - repeated operations to the same objects write the same changes to
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the log over and over again. Worse is the fact that objects tend to get
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dirtier as they get relogged, so each subsequent transaction is writing more
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metadata into the log.
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It should now also be obvious how relogging and asynchronous transactions go
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hand in hand. That is, transactions don't get written to the physical journal
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until either a log buffer is filled (a log buffer can hold multiple
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transactions) or a synchronous operation forces the log buffers holding the
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transactions to disk. This means that XFS is doing aggregation of transactions
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in memory - batching them, if you like - to minimise the impact of the log IO on
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transaction throughput.
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The limitation on asynchronous transaction throughput is the number and size of
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log buffers made available by the log manager. By default there are 8 log
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buffers available and the size of each is 32kB - the size can be increased up
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to 256kB by use of a mount option.
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Effectively, this gives us the maximum bound of outstanding metadata changes
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that can be made to the filesystem at any point in time - if all the log
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buffers are full and under IO, then no more transactions can be committed until
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the current batch completes. It is now common for a single current CPU core to
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be to able to issue enough transactions to keep the log buffers full and under
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IO permanently. Hence the XFS journalling subsystem can be considered to be IO
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bound.
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Delayed Logging: Concepts
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=========================
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The key thing to note about the asynchronous logging combined with the
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relogging technique XFS uses is that we can be relogging changed objects
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multiple times before they are committed to disk in the log buffers. If we
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return to the previous relogging example, it is entirely possible that
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transactions A through D are committed to disk in the same log buffer.
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That is, a single log buffer may contain multiple copies of the same object,
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but only one of those copies needs to be there - the last one "D", as it
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contains all the changes from the previous changes. In other words, we have one
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necessary copy in the log buffer, and three stale copies that are simply
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wasting space. When we are doing repeated operations on the same set of
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objects, these "stale objects" can be over 90% of the space used in the log
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buffers. It is clear that reducing the number of stale objects written to the
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log would greatly reduce the amount of metadata we write to the log, and this
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is the fundamental goal of delayed logging.
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From a conceptual point of view, XFS is already doing relogging in memory (where
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memory == log buffer), only it is doing it extremely inefficiently. It is using
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logical to physical formatting to do the relogging because there is no
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infrastructure to keep track of logical changes in memory prior to physically
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formatting the changes in a transaction to the log buffer. Hence we cannot avoid
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accumulating stale objects in the log buffers.
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Delayed logging is the name we've given to keeping and tracking transactional
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changes to objects in memory outside the log buffer infrastructure. Because of
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the relogging concept fundamental to the XFS journalling subsystem, this is
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actually relatively easy to do - all the changes to logged items are already
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tracked in the current infrastructure. The big problem is how to accumulate
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them and get them to the log in a consistent, recoverable manner.
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Describing the problems and how they have been solved is the focus of this
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document.
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One of the key changes that delayed logging makes to the operation of the
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journalling subsystem is that it disassociates the amount of outstanding
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metadata changes from the size and number of log buffers available. In other
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words, instead of there only being a maximum of 2MB of transaction changes not
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written to the log at any point in time, there may be a much greater amount
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being accumulated in memory. Hence the potential for loss of metadata on a
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crash is much greater than for the existing logging mechanism.
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It should be noted that this does not change the guarantee that log recovery
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will result in a consistent filesystem. What it does mean is that as far as the
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recovered filesystem is concerned, there may be many thousands of transactions
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that simply did not occur as a result of the crash. This makes it even more
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important that applications that care about their data use fsync() where they
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need to ensure application level data integrity is maintained.
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It should be noted that delayed logging is not an innovative new concept that
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warrants rigorous proofs to determine whether it is correct or not. The method
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of accumulating changes in memory for some period before writing them to the
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log is used effectively in many filesystems including ext3 and ext4. Hence
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no time is spent in this document trying to convince the reader that the
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concept is sound. Instead it is simply considered a "solved problem" and as
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such implementing it in XFS is purely an exercise in software engineering.
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The fundamental requirements for delayed logging in XFS are simple:
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1. Reduce the amount of metadata written to the log by at least
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an order of magnitude.
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2. Supply sufficient statistics to validate Requirement #1.
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3. Supply sufficient new tracing infrastructure to be able to debug
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problems with the new code.
|
|
4. No on-disk format change (metadata or log format).
|
|
5. Enable and disable with a mount option.
|
|
6. No performance regressions for synchronous transaction workloads.
|
|
|
|
Delayed Logging: Design
|
|
=======================
|
|
|
|
Storing Changes
|
|
---------------
|
|
|
|
The problem with accumulating changes at a logical level (i.e. just using the
|
|
existing log item dirty region tracking) is that when it comes to writing the
|
|
changes to the log buffers, we need to ensure that the object we are formatting
|
|
is not changing while we do this. This requires locking the object to prevent
|
|
concurrent modification. Hence flushing the logical changes to the log would
|
|
require us to lock every object, format them, and then unlock them again.
|
|
|
|
This introduces lots of scope for deadlocks with transactions that are already
|
|
running. For example, a transaction has object A locked and modified, but needs
|
|
the delayed logging tracking lock to commit the transaction. However, the
|
|
flushing thread has the delayed logging tracking lock already held, and is
|
|
trying to get the lock on object A to flush it to the log buffer. This appears
|
|
to be an unsolvable deadlock condition, and it was solving this problem that
|
|
was the barrier to implementing delayed logging for so long.
|
|
|
|
The solution is relatively simple - it just took a long time to recognise it.
|
|
Put simply, the current logging code formats the changes to each item into an
|
|
vector array that points to the changed regions in the item. The log write code
|
|
simply copies the memory these vectors point to into the log buffer during
|
|
transaction commit while the item is locked in the transaction. Instead of
|
|
using the log buffer as the destination of the formatting code, we can use an
|
|
allocated memory buffer big enough to fit the formatted vector.
|
|
|
|
If we then copy the vector into the memory buffer and rewrite the vector to
|
|
point to the memory buffer rather than the object itself, we now have a copy of
|
|
the changes in a format that is compatible with the log buffer writing code.
|
|
that does not require us to lock the item to access. This formatting and
|
|
rewriting can all be done while the object is locked during transaction commit,
|
|
resulting in a vector that is transactionally consistent and can be accessed
|
|
without needing to lock the owning item.
|
|
|
|
Hence we avoid the need to lock items when we need to flush outstanding
|
|
asynchronous transactions to the log. The differences between the existing
|
|
formatting method and the delayed logging formatting can be seen in the
|
|
diagram below.
|
|
|
|
Current format log vector::
|
|
|
|
Object +---------------------------------------------+
|
|
Vector 1 +----+
|
|
Vector 2 +----+
|
|
Vector 3 +----------+
|
|
|
|
After formatting::
|
|
|
|
Log Buffer +-V1-+-V2-+----V3----+
|
|
|
|
Delayed logging vector::
|
|
|
|
Object +---------------------------------------------+
|
|
Vector 1 +----+
|
|
Vector 2 +----+
|
|
Vector 3 +----------+
|
|
|
|
After formatting::
|
|
|
|
Memory Buffer +-V1-+-V2-+----V3----+
|
|
Vector 1 +----+
|
|
Vector 2 +----+
|
|
Vector 3 +----------+
|
|
|
|
The memory buffer and associated vector need to be passed as a single object,
|
|
but still need to be associated with the parent object so if the object is
|
|
relogged we can replace the current memory buffer with a new memory buffer that
|
|
contains the latest changes.
|
|
|
|
The reason for keeping the vector around after we've formatted the memory
|
|
buffer is to support splitting vectors across log buffer boundaries correctly.
|
|
If we don't keep the vector around, we do not know where the region boundaries
|
|
are in the item, so we'd need a new encapsulation method for regions in the log
|
|
buffer writing (i.e. double encapsulation). This would be an on-disk format
|
|
change and as such is not desirable. It also means we'd have to write the log
|
|
region headers in the formatting stage, which is problematic as there is per
|
|
region state that needs to be placed into the headers during the log write.
|
|
|
|
Hence we need to keep the vector, but by attaching the memory buffer to it and
|
|
rewriting the vector addresses to point at the memory buffer we end up with a
|
|
self-describing object that can be passed to the log buffer write code to be
|
|
handled in exactly the same manner as the existing log vectors are handled.
|
|
Hence we avoid needing a new on-disk format to handle items that have been
|
|
relogged in memory.
|
|
|
|
|
|
Tracking Changes
|
|
----------------
|
|
|
|
Now that we can record transactional changes in memory in a form that allows
|
|
them to be used without limitations, we need to be able to track and accumulate
|
|
them so that they can be written to the log at some later point in time. The
|
|
log item is the natural place to store this vector and buffer, and also makes sense
|
|
to be the object that is used to track committed objects as it will always
|
|
exist once the object has been included in a transaction.
|
|
|
|
The log item is already used to track the log items that have been written to
|
|
the log but not yet written to disk. Such log items are considered "active"
|
|
and as such are stored in the Active Item List (AIL) which is a LSN-ordered
|
|
double linked list. Items are inserted into this list during log buffer IO
|
|
completion, after which they are unpinned and can be written to disk. An object
|
|
that is in the AIL can be relogged, which causes the object to be pinned again
|
|
and then moved forward in the AIL when the log buffer IO completes for that
|
|
transaction.
|
|
|
|
Essentially, this shows that an item that is in the AIL can still be modified
|
|
and relogged, so any tracking must be separate to the AIL infrastructure. As
|
|
such, we cannot reuse the AIL list pointers for tracking committed items, nor
|
|
can we store state in any field that is protected by the AIL lock. Hence the
|
|
committed item tracking needs its own locks, lists and state fields in the log
|
|
item.
|
|
|
|
Similar to the AIL, tracking of committed items is done through a new list
|
|
called the Committed Item List (CIL). The list tracks log items that have been
|
|
committed and have formatted memory buffers attached to them. It tracks objects
|
|
in transaction commit order, so when an object is relogged it is removed from
|
|
its place in the list and re-inserted at the tail. This is entirely arbitrary
|
|
and done to make it easy for debugging - the last items in the list are the
|
|
ones that are most recently modified. Ordering of the CIL is not necessary for
|
|
transactional integrity (as discussed in the next section) so the ordering is
|
|
done for convenience/sanity of the developers.
|
|
|
|
|
|
Delayed Logging: Checkpoints
|
|
----------------------------
|
|
|
|
When we have a log synchronisation event, commonly known as a "log force",
|
|
all the items in the CIL must be written into the log via the log buffers.
|
|
We need to write these items in the order that they exist in the CIL, and they
|
|
need to be written as an atomic transaction. The need for all the objects to be
|
|
written as an atomic transaction comes from the requirements of relogging and
|
|
log replay - all the changes in all the objects in a given transaction must
|
|
either be completely replayed during log recovery, or not replayed at all. If
|
|
a transaction is not replayed because it is not complete in the log, then
|
|
no later transactions should be replayed, either.
|
|
|
|
To fulfill this requirement, we need to write the entire CIL in a single log
|
|
transaction. Fortunately, the XFS log code has no fixed limit on the size of a
|
|
transaction, nor does the log replay code. The only fundamental limit is that
|
|
the transaction cannot be larger than just under half the size of the log. The
|
|
reason for this limit is that to find the head and tail of the log, there must
|
|
be at least one complete transaction in the log at any given time. If a
|
|
transaction is larger than half the log, then there is the possibility that a
|
|
crash during the write of a such a transaction could partially overwrite the
|
|
only complete previous transaction in the log. This will result in a recovery
|
|
failure and an inconsistent filesystem and hence we must enforce the maximum
|
|
size of a checkpoint to be slightly less than a half the log.
|
|
|
|
Apart from this size requirement, a checkpoint transaction looks no different
|
|
to any other transaction - it contains a transaction header, a series of
|
|
formatted log items and a commit record at the tail. From a recovery
|
|
perspective, the checkpoint transaction is also no different - just a lot
|
|
bigger with a lot more items in it. The worst case effect of this is that we
|
|
might need to tune the recovery transaction object hash size.
|
|
|
|
Because the checkpoint is just another transaction and all the changes to log
|
|
items are stored as log vectors, we can use the existing log buffer writing
|
|
code to write the changes into the log. To do this efficiently, we need to
|
|
minimise the time we hold the CIL locked while writing the checkpoint
|
|
transaction. The current log write code enables us to do this easily with the
|
|
way it separates the writing of the transaction contents (the log vectors) from
|
|
the transaction commit record, but tracking this requires us to have a
|
|
per-checkpoint context that travels through the log write process through to
|
|
checkpoint completion.
|
|
|
|
Hence a checkpoint has a context that tracks the state of the current
|
|
checkpoint from initiation to checkpoint completion. A new context is initiated
|
|
at the same time a checkpoint transaction is started. That is, when we remove
|
|
all the current items from the CIL during a checkpoint operation, we move all
|
|
those changes into the current checkpoint context. We then initialise a new
|
|
context and attach that to the CIL for aggregation of new transactions.
|
|
|
|
This allows us to unlock the CIL immediately after transfer of all the
|
|
committed items and effectively allows new transactions to be issued while we
|
|
are formatting the checkpoint into the log. It also allows concurrent
|
|
checkpoints to be written into the log buffers in the case of log force heavy
|
|
workloads, just like the existing transaction commit code does. This, however,
|
|
requires that we strictly order the commit records in the log so that
|
|
checkpoint sequence order is maintained during log replay.
|
|
|
|
To ensure that we can be writing an item into a checkpoint transaction at
|
|
the same time another transaction modifies the item and inserts the log item
|
|
into the new CIL, then checkpoint transaction commit code cannot use log items
|
|
to store the list of log vectors that need to be written into the transaction.
|
|
Hence log vectors need to be able to be chained together to allow them to be
|
|
detached from the log items. That is, when the CIL is flushed the memory
|
|
buffer and log vector attached to each log item needs to be attached to the
|
|
checkpoint context so that the log item can be released. In diagrammatic form,
|
|
the CIL would look like this before the flush::
|
|
|
|
CIL Head
|
|
|
|
|
V
|
|
Log Item <-> log vector 1 -> memory buffer
|
|
| -> vector array
|
|
V
|
|
Log Item <-> log vector 2 -> memory buffer
|
|
| -> vector array
|
|
V
|
|
......
|
|
|
|
|
V
|
|
Log Item <-> log vector N-1 -> memory buffer
|
|
| -> vector array
|
|
V
|
|
Log Item <-> log vector N -> memory buffer
|
|
-> vector array
|
|
|
|
And after the flush the CIL head is empty, and the checkpoint context log
|
|
vector list would look like::
|
|
|
|
Checkpoint Context
|
|
|
|
|
V
|
|
log vector 1 -> memory buffer
|
|
| -> vector array
|
|
| -> Log Item
|
|
V
|
|
log vector 2 -> memory buffer
|
|
| -> vector array
|
|
| -> Log Item
|
|
V
|
|
......
|
|
|
|
|
V
|
|
log vector N-1 -> memory buffer
|
|
| -> vector array
|
|
| -> Log Item
|
|
V
|
|
log vector N -> memory buffer
|
|
-> vector array
|
|
-> Log Item
|
|
|
|
Once this transfer is done, the CIL can be unlocked and new transactions can
|
|
start, while the checkpoint flush code works over the log vector chain to
|
|
commit the checkpoint.
|
|
|
|
Once the checkpoint is written into the log buffers, the checkpoint context is
|
|
attached to the log buffer that the commit record was written to along with a
|
|
completion callback. Log IO completion will call that callback, which can then
|
|
run transaction committed processing for the log items (i.e. insert into AIL
|
|
and unpin) in the log vector chain and then free the log vector chain and
|
|
checkpoint context.
|
|
|
|
Discussion Point: I am uncertain as to whether the log item is the most
|
|
efficient way to track vectors, even though it seems like the natural way to do
|
|
it. The fact that we walk the log items (in the CIL) just to chain the log
|
|
vectors and break the link between the log item and the log vector means that
|
|
we take a cache line hit for the log item list modification, then another for
|
|
the log vector chaining. If we track by the log vectors, then we only need to
|
|
break the link between the log item and the log vector, which means we should
|
|
dirty only the log item cachelines. Normally I wouldn't be concerned about one
|
|
vs two dirty cachelines except for the fact I've seen upwards of 80,000 log
|
|
vectors in one checkpoint transaction. I'd guess this is a "measure and
|
|
compare" situation that can be done after a working and reviewed implementation
|
|
is in the dev tree....
|
|
|
|
Delayed Logging: Checkpoint Sequencing
|
|
--------------------------------------
|
|
|
|
One of the key aspects of the XFS transaction subsystem is that it tags
|
|
committed transactions with the log sequence number of the transaction commit.
|
|
This allows transactions to be issued asynchronously even though there may be
|
|
future operations that cannot be completed until that transaction is fully
|
|
committed to the log. In the rare case that a dependent operation occurs (e.g.
|
|
re-using a freed metadata extent for a data extent), a special, optimised log
|
|
force can be issued to force the dependent transaction to disk immediately.
|
|
|
|
To do this, transactions need to record the LSN of the commit record of the
|
|
transaction. This LSN comes directly from the log buffer the transaction is
|
|
written into. While this works just fine for the existing transaction
|
|
mechanism, it does not work for delayed logging because transactions are not
|
|
written directly into the log buffers. Hence some other method of sequencing
|
|
transactions is required.
|
|
|
|
As discussed in the checkpoint section, delayed logging uses per-checkpoint
|
|
contexts, and as such it is simple to assign a sequence number to each
|
|
checkpoint. Because the switching of checkpoint contexts must be done
|
|
atomically, it is simple to ensure that each new context has a monotonically
|
|
increasing sequence number assigned to it without the need for an external
|
|
atomic counter - we can just take the current context sequence number and add
|
|
one to it for the new context.
|
|
|
|
Then, instead of assigning a log buffer LSN to the transaction commit LSN
|
|
during the commit, we can assign the current checkpoint sequence. This allows
|
|
operations that track transactions that have not yet completed know what
|
|
checkpoint sequence needs to be committed before they can continue. As a
|
|
result, the code that forces the log to a specific LSN now needs to ensure that
|
|
the log forces to a specific checkpoint.
|
|
|
|
To ensure that we can do this, we need to track all the checkpoint contexts
|
|
that are currently committing to the log. When we flush a checkpoint, the
|
|
context gets added to a "committing" list which can be searched. When a
|
|
checkpoint commit completes, it is removed from the committing list. Because
|
|
the checkpoint context records the LSN of the commit record for the checkpoint,
|
|
we can also wait on the log buffer that contains the commit record, thereby
|
|
using the existing log force mechanisms to execute synchronous forces.
|
|
|
|
It should be noted that the synchronous forces may need to be extended with
|
|
mitigation algorithms similar to the current log buffer code to allow
|
|
aggregation of multiple synchronous transactions if there are already
|
|
synchronous transactions being flushed. Investigation of the performance of the
|
|
current design is needed before making any decisions here.
|
|
|
|
The main concern with log forces is to ensure that all the previous checkpoints
|
|
are also committed to disk before the one we need to wait for. Therefore we
|
|
need to check that all the prior contexts in the committing list are also
|
|
complete before waiting on the one we need to complete. We do this
|
|
synchronisation in the log force code so that we don't need to wait anywhere
|
|
else for such serialisation - it only matters when we do a log force.
|
|
|
|
The only remaining complexity is that a log force now also has to handle the
|
|
case where the forcing sequence number is the same as the current context. That
|
|
is, we need to flush the CIL and potentially wait for it to complete. This is a
|
|
simple addition to the existing log forcing code to check the sequence numbers
|
|
and push if required. Indeed, placing the current sequence checkpoint flush in
|
|
the log force code enables the current mechanism for issuing synchronous
|
|
transactions to remain untouched (i.e. commit an asynchronous transaction, then
|
|
force the log at the LSN of that transaction) and so the higher level code
|
|
behaves the same regardless of whether delayed logging is being used or not.
|
|
|
|
Delayed Logging: Checkpoint Log Space Accounting
|
|
------------------------------------------------
|
|
|
|
The big issue for a checkpoint transaction is the log space reservation for the
|
|
transaction. We don't know how big a checkpoint transaction is going to be
|
|
ahead of time, nor how many log buffers it will take to write out, nor the
|
|
number of split log vector regions are going to be used. We can track the
|
|
amount of log space required as we add items to the commit item list, but we
|
|
still need to reserve the space in the log for the checkpoint.
|
|
|
|
A typical transaction reserves enough space in the log for the worst case space
|
|
usage of the transaction. The reservation accounts for log record headers,
|
|
transaction and region headers, headers for split regions, buffer tail padding,
|
|
etc. as well as the actual space for all the changed metadata in the
|
|
transaction. While some of this is fixed overhead, much of it is dependent on
|
|
the size of the transaction and the number of regions being logged (the number
|
|
of log vectors in the transaction).
|
|
|
|
An example of the differences would be logging directory changes versus logging
|
|
inode changes. If you modify lots of inode cores (e.g. ``chmod -R g+w *``), then
|
|
there are lots of transactions that only contain an inode core and an inode log
|
|
format structure. That is, two vectors totaling roughly 150 bytes. If we modify
|
|
10,000 inodes, we have about 1.5MB of metadata to write in 20,000 vectors. Each
|
|
vector is 12 bytes, so the total to be logged is approximately 1.75MB. In
|
|
comparison, if we are logging full directory buffers, they are typically 4KB
|
|
each, so we in 1.5MB of directory buffers we'd have roughly 400 buffers and a
|
|
buffer format structure for each buffer - roughly 800 vectors or 1.51MB total
|
|
space. From this, it should be obvious that a static log space reservation is
|
|
not particularly flexible and is difficult to select the "optimal value" for
|
|
all workloads.
|
|
|
|
Further, if we are going to use a static reservation, which bit of the entire
|
|
reservation does it cover? We account for space used by the transaction
|
|
reservation by tracking the space currently used by the object in the CIL and
|
|
then calculating the increase or decrease in space used as the object is
|
|
relogged. This allows for a checkpoint reservation to only have to account for
|
|
log buffer metadata used such as log header records.
|
|
|
|
However, even using a static reservation for just the log metadata is
|
|
problematic. Typically log record headers use at least 16KB of log space per
|
|
1MB of log space consumed (512 bytes per 32k) and the reservation needs to be
|
|
large enough to handle arbitrary sized checkpoint transactions. This
|
|
reservation needs to be made before the checkpoint is started, and we need to
|
|
be able to reserve the space without sleeping. For a 8MB checkpoint, we need a
|
|
reservation of around 150KB, which is a non-trivial amount of space.
|
|
|
|
A static reservation needs to manipulate the log grant counters - we can take a
|
|
permanent reservation on the space, but we still need to make sure we refresh
|
|
the write reservation (the actual space available to the transaction) after
|
|
every checkpoint transaction completion. Unfortunately, if this space is not
|
|
available when required, then the regrant code will sleep waiting for it.
|
|
|
|
The problem with this is that it can lead to deadlocks as we may need to commit
|
|
checkpoints to be able to free up log space (refer back to the description of
|
|
rolling transactions for an example of this). Hence we *must* always have
|
|
space available in the log if we are to use static reservations, and that is
|
|
very difficult and complex to arrange. It is possible to do, but there is a
|
|
simpler way.
|
|
|
|
The simpler way of doing this is tracking the entire log space used by the
|
|
items in the CIL and using this to dynamically calculate the amount of log
|
|
space required by the log metadata. If this log metadata space changes as a
|
|
result of a transaction commit inserting a new memory buffer into the CIL, then
|
|
the difference in space required is removed from the transaction that causes
|
|
the change. Transactions at this level will *always* have enough space
|
|
available in their reservation for this as they have already reserved the
|
|
maximal amount of log metadata space they require, and such a delta reservation
|
|
will always be less than or equal to the maximal amount in the reservation.
|
|
|
|
Hence we can grow the checkpoint transaction reservation dynamically as items
|
|
are added to the CIL and avoid the need for reserving and regranting log space
|
|
up front. This avoids deadlocks and removes a blocking point from the
|
|
checkpoint flush code.
|
|
|
|
As mentioned early, transactions can't grow to more than half the size of the
|
|
log. Hence as part of the reservation growing, we need to also check the size
|
|
of the reservation against the maximum allowed transaction size. If we reach
|
|
the maximum threshold, we need to push the CIL to the log. This is effectively
|
|
a "background flush" and is done on demand. This is identical to
|
|
a CIL push triggered by a log force, only that there is no waiting for the
|
|
checkpoint commit to complete. This background push is checked and executed by
|
|
transaction commit code.
|
|
|
|
If the transaction subsystem goes idle while we still have items in the CIL,
|
|
they will be flushed by the periodic log force issued by the xfssyncd. This log
|
|
force will push the CIL to disk, and if the transaction subsystem stays idle,
|
|
allow the idle log to be covered (effectively marked clean) in exactly the same
|
|
manner that is done for the existing logging method. A discussion point is
|
|
whether this log force needs to be done more frequently than the current rate
|
|
which is once every 30s.
|
|
|
|
|
|
Delayed Logging: Log Item Pinning
|
|
---------------------------------
|
|
|
|
Currently log items are pinned during transaction commit while the items are
|
|
still locked. This happens just after the items are formatted, though it could
|
|
be done any time before the items are unlocked. The result of this mechanism is
|
|
that items get pinned once for every transaction that is committed to the log
|
|
buffers. Hence items that are relogged in the log buffers will have a pin count
|
|
for every outstanding transaction they were dirtied in. When each of these
|
|
transactions is completed, they will unpin the item once. As a result, the item
|
|
only becomes unpinned when all the transactions complete and there are no
|
|
pending transactions. Thus the pinning and unpinning of a log item is symmetric
|
|
as there is a 1:1 relationship with transaction commit and log item completion.
|
|
|
|
For delayed logging, however, we have an asymmetric transaction commit to
|
|
completion relationship. Every time an object is relogged in the CIL it goes
|
|
through the commit process without a corresponding completion being registered.
|
|
That is, we now have a many-to-one relationship between transaction commit and
|
|
log item completion. The result of this is that pinning and unpinning of the
|
|
log items becomes unbalanced if we retain the "pin on transaction commit, unpin
|
|
on transaction completion" model.
|
|
|
|
To keep pin/unpin symmetry, the algorithm needs to change to a "pin on
|
|
insertion into the CIL, unpin on checkpoint completion". In other words, the
|
|
pinning and unpinning becomes symmetric around a checkpoint context. We have to
|
|
pin the object the first time it is inserted into the CIL - if it is already in
|
|
the CIL during a transaction commit, then we do not pin it again. Because there
|
|
can be multiple outstanding checkpoint contexts, we can still see elevated pin
|
|
counts, but as each checkpoint completes the pin count will retain the correct
|
|
value according to its context.
|
|
|
|
Just to make matters slightly more complex, this checkpoint level context
|
|
for the pin count means that the pinning of an item must take place under the
|
|
CIL commit/flush lock. If we pin the object outside this lock, we cannot
|
|
guarantee which context the pin count is associated with. This is because of
|
|
the fact pinning the item is dependent on whether the item is present in the
|
|
current CIL or not. If we don't pin the CIL first before we check and pin the
|
|
object, we have a race with CIL being flushed between the check and the pin
|
|
(or not pinning, as the case may be). Hence we must hold the CIL flush/commit
|
|
lock to guarantee that we pin the items correctly.
|
|
|
|
Delayed Logging: Concurrent Scalability
|
|
---------------------------------------
|
|
|
|
A fundamental requirement for the CIL is that accesses through transaction
|
|
commits must scale to many concurrent commits. The current transaction commit
|
|
code does not break down even when there are transactions coming from 2048
|
|
processors at once. The current transaction code does not go any faster than if
|
|
there was only one CPU using it, but it does not slow down either.
|
|
|
|
As a result, the delayed logging transaction commit code needs to be designed
|
|
for concurrency from the ground up. It is obvious that there are serialisation
|
|
points in the design - the three important ones are:
|
|
|
|
1. Locking out new transaction commits while flushing the CIL
|
|
2. Adding items to the CIL and updating item space accounting
|
|
3. Checkpoint commit ordering
|
|
|
|
Looking at the transaction commit and CIL flushing interactions, it is clear
|
|
that we have a many-to-one interaction here. That is, the only restriction on
|
|
the number of concurrent transactions that can be trying to commit at once is
|
|
the amount of space available in the log for their reservations. The practical
|
|
limit here is in the order of several hundred concurrent transactions for a
|
|
128MB log, which means that it is generally one per CPU in a machine.
|
|
|
|
The amount of time a transaction commit needs to hold out a flush is a
|
|
relatively long period of time - the pinning of log items needs to be done
|
|
while we are holding out a CIL flush, so at the moment that means it is held
|
|
across the formatting of the objects into memory buffers (i.e. while memcpy()s
|
|
are in progress). Ultimately a two pass algorithm where the formatting is done
|
|
separately to the pinning of objects could be used to reduce the hold time of
|
|
the transaction commit side.
|
|
|
|
Because of the number of potential transaction commit side holders, the lock
|
|
really needs to be a sleeping lock - if the CIL flush takes the lock, we do not
|
|
want every other CPU in the machine spinning on the CIL lock. Given that
|
|
flushing the CIL could involve walking a list of tens of thousands of log
|
|
items, it will get held for a significant time and so spin contention is a
|
|
significant concern. Preventing lots of CPUs spinning doing nothing is the
|
|
main reason for choosing a sleeping lock even though nothing in either the
|
|
transaction commit or CIL flush side sleeps with the lock held.
|
|
|
|
It should also be noted that CIL flushing is also a relatively rare operation
|
|
compared to transaction commit for asynchronous transaction workloads - only
|
|
time will tell if using a read-write semaphore for exclusion will limit
|
|
transaction commit concurrency due to cache line bouncing of the lock on the
|
|
read side.
|
|
|
|
The second serialisation point is on the transaction commit side where items
|
|
are inserted into the CIL. Because transactions can enter this code
|
|
concurrently, the CIL needs to be protected separately from the above
|
|
commit/flush exclusion. It also needs to be an exclusive lock but it is only
|
|
held for a very short time and so a spin lock is appropriate here. It is
|
|
possible that this lock will become a contention point, but given the short
|
|
hold time once per transaction I think that contention is unlikely.
|
|
|
|
The final serialisation point is the checkpoint commit record ordering code
|
|
that is run as part of the checkpoint commit and log force sequencing. The code
|
|
path that triggers a CIL flush (i.e. whatever triggers the log force) will enter
|
|
an ordering loop after writing all the log vectors into the log buffers but
|
|
before writing the commit record. This loop walks the list of committing
|
|
checkpoints and needs to block waiting for checkpoints to complete their commit
|
|
record write. As a result it needs a lock and a wait variable. Log force
|
|
sequencing also requires the same lock, list walk, and blocking mechanism to
|
|
ensure completion of checkpoints.
|
|
|
|
These two sequencing operations can use the mechanism even though the
|
|
events they are waiting for are different. The checkpoint commit record
|
|
sequencing needs to wait until checkpoint contexts contain a commit LSN
|
|
(obtained through completion of a commit record write) while log force
|
|
sequencing needs to wait until previous checkpoint contexts are removed from
|
|
the committing list (i.e. they've completed). A simple wait variable and
|
|
broadcast wakeups (thundering herds) has been used to implement these two
|
|
serialisation queues. They use the same lock as the CIL, too. If we see too
|
|
much contention on the CIL lock, or too many context switches as a result of
|
|
the broadcast wakeups these operations can be put under a new spinlock and
|
|
given separate wait lists to reduce lock contention and the number of processes
|
|
woken by the wrong event.
|
|
|
|
|
|
Lifecycle Changes
|
|
-----------------
|
|
|
|
The existing log item life cycle is as follows::
|
|
|
|
1. Transaction allocate
|
|
2. Transaction reserve
|
|
3. Lock item
|
|
4. Join item to transaction
|
|
If not already attached,
|
|
Allocate log item
|
|
Attach log item to owner item
|
|
Attach log item to transaction
|
|
5. Modify item
|
|
Record modifications in log item
|
|
6. Transaction commit
|
|
Pin item in memory
|
|
Format item into log buffer
|
|
Write commit LSN into transaction
|
|
Unlock item
|
|
Attach transaction to log buffer
|
|
|
|
<log buffer IO dispatched>
|
|
<log buffer IO completes>
|
|
|
|
7. Transaction completion
|
|
Mark log item committed
|
|
Insert log item into AIL
|
|
Write commit LSN into log item
|
|
Unpin log item
|
|
8. AIL traversal
|
|
Lock item
|
|
Mark log item clean
|
|
Flush item to disk
|
|
|
|
<item IO completion>
|
|
|
|
9. Log item removed from AIL
|
|
Moves log tail
|
|
Item unlocked
|
|
|
|
Essentially, steps 1-6 operate independently from step 7, which is also
|
|
independent of steps 8-9. An item can be locked in steps 1-6 or steps 8-9
|
|
at the same time step 7 is occurring, but only steps 1-6 or 8-9 can occur
|
|
at the same time. If the log item is in the AIL or between steps 6 and 7
|
|
and steps 1-6 are re-entered, then the item is relogged. Only when steps 8-9
|
|
are entered and completed is the object considered clean.
|
|
|
|
With delayed logging, there are new steps inserted into the life cycle::
|
|
|
|
1. Transaction allocate
|
|
2. Transaction reserve
|
|
3. Lock item
|
|
4. Join item to transaction
|
|
If not already attached,
|
|
Allocate log item
|
|
Attach log item to owner item
|
|
Attach log item to transaction
|
|
5. Modify item
|
|
Record modifications in log item
|
|
6. Transaction commit
|
|
Pin item in memory if not pinned in CIL
|
|
Format item into log vector + buffer
|
|
Attach log vector and buffer to log item
|
|
Insert log item into CIL
|
|
Write CIL context sequence into transaction
|
|
Unlock item
|
|
|
|
<next log force>
|
|
|
|
7. CIL push
|
|
lock CIL flush
|
|
Chain log vectors and buffers together
|
|
Remove items from CIL
|
|
unlock CIL flush
|
|
write log vectors into log
|
|
sequence commit records
|
|
attach checkpoint context to log buffer
|
|
|
|
<log buffer IO dispatched>
|
|
<log buffer IO completes>
|
|
|
|
8. Checkpoint completion
|
|
Mark log item committed
|
|
Insert item into AIL
|
|
Write commit LSN into log item
|
|
Unpin log item
|
|
9. AIL traversal
|
|
Lock item
|
|
Mark log item clean
|
|
Flush item to disk
|
|
<item IO completion>
|
|
10. Log item removed from AIL
|
|
Moves log tail
|
|
Item unlocked
|
|
|
|
From this, it can be seen that the only life cycle differences between the two
|
|
logging methods are in the middle of the life cycle - they still have the same
|
|
beginning and end and execution constraints. The only differences are in the
|
|
committing of the log items to the log itself and the completion processing.
|
|
Hence delayed logging should not introduce any constraints on log item
|
|
behaviour, allocation or freeing that don't already exist.
|
|
|
|
As a result of this zero-impact "insertion" of delayed logging infrastructure
|
|
and the design of the internal structures to avoid on disk format changes, we
|
|
can basically switch between delayed logging and the existing mechanism with a
|
|
mount option. Fundamentally, there is no reason why the log manager would not
|
|
be able to swap methods automatically and transparently depending on load
|
|
characteristics, but this should not be necessary if delayed logging works as
|
|
designed.
|