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ext4: update journal documentation

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Add a section about journal checkpointing, including information about
the ioctl EXT4_IOC_CHECKPOINT which can be used to trigger a journal
checkpoint from userspace.

Also, update the journal allocation information to reflect that up to
10240000 blocks are used for the journal and that the journal is not
necessarily contiguous.

Signed-off-by: Leah Rumancik <leah.rumancik@gmail.com>

Changes in v5:
- clarify behavior of DRY_RUN flag
Link: https://lore.kernel.org/r/20210518151327.130198-3-leah.rumancik@gmail.com

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
This commit is contained in:
Leah Rumancik 2021-05-18 15:13:27 +00:00 committed by Theodore Ts'o
parent 351a0a3fbc
commit fd7b23be92
1 changed files with 31 additions and 8 deletions
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39
Documentation/filesystems/ext4/journal.rst
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@ -4,14 +4,14 @@ Journal (jbd2)
--------------
Introduced in ext3, the ext4 filesystem employs a journal to protect the
filesystem against corruption in the case of a system crash. A small
continuous region of disk (default 128MiB) is reserved inside the
filesystem as a place to land “important” data writes on-disk as quickly
as possible. Once the important data transaction is fully written to the
disk and flushed from the disk write cache, a record of the data being
committed is also written to the journal. At some later point in time,
the journal code writes the transactions to their final locations on
disk (this could involve a lot of seeking or a lot of small
filesystem against metadata inconsistencies in the case of a system crash. Up
to 10,240,000 file system blocks (see man mke2fs(8) for more details on journal
size limits) can be reserved inside the filesystem as a place to land
“important” data writes on-disk as quickly as possible. Once the important
data transaction is fully written to the disk and flushed from the disk write
cache, a record of the data being committed is also written to the journal. At
some later point in time, the journal code writes the transactions to their
final locations on disk (this could involve a lot of seeking or a lot of small
read-write-erases) before erasing the commit record. Should the system
crash during the second slow write, the journal can be replayed all the
way to the latest commit record, guaranteeing the atomicity of whatever
@ -731,3 +731,26 @@ point, the refcount for inode 11 is not reliable, but that gets fixed by the
replay of last inode 11 tag. Thus, by converting a non-idempotent procedure
into a series of idempotent outcomes, fast commits ensured idempotence during
the replay.
Journal Checkpoint
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Checkpointing the journal ensures all transactions and their associated buffers
are submitted to the disk. In-progress transactions are waited upon and included
in the checkpoint. Checkpointing is used internally during critical updates to
the filesystem including journal recovery, filesystem resizing, and freeing of
the journal_t structure.
A journal checkpoint can be triggered from userspace via the ioctl
EXT4_IOC_CHECKPOINT. This ioctl takes a single, u64 argument for flags.
Currently, three flags are supported. First, EXT4_IOC_CHECKPOINT_FLAG_DRY_RUN
can be used to verify input to the ioctl. It returns error if there is any
invalid input, otherwise it returns success without performing
any checkpointing. This can be used to check whether the ioctl exists on a
system and to verify there are no issues with arguments or flags. The
other two flags are EXT4_IOC_CHECKPOINT_FLAG_DISCARD and
EXT4_IOC_CHECKPOINT_FLAG_ZEROOUT. These flags cause the journal blocks to be
discarded or zero-filled, respectively, after the journal checkpoint is
complete. EXT4_IOC_CHECKPOINT_FLAG_DISCARD and EXT4_IOC_CHECKPOINT_FLAG_ZEROOUT
cannot both be set. The ioctl may be useful when snapshotting a system or for
complying with content deletion SLOs.