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Requests that triggers flushing volatile writeback cache to disk (barriers) have significant effect to overall performance. Block layer has sophisticated engine for combining several flush requests into one. But there is no statistics for actual flushes executed by disk. Requests which trigger flushes usually are barriers - zero-size writes. This patch adds two iostat counters into /sys/class/block/$dev/stat and /proc/diskstats - count of completed flush requests and their total time. Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru> Signed-off-by: Jens Axboe <axboe@kernel.dk>
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=====================
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I/O statistics fields
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=====================
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Since 2.4.20 (and some versions before, with patches), and 2.5.45,
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more extensive disk statistics have been introduced to help measure disk
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activity. Tools such as ``sar`` and ``iostat`` typically interpret these and do
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the work for you, but in case you are interested in creating your own
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tools, the fields are explained here.
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In 2.4 now, the information is found as additional fields in
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``/proc/partitions``. In 2.6 and upper, the same information is found in two
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places: one is in the file ``/proc/diskstats``, and the other is within
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the sysfs file system, which must be mounted in order to obtain
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the information. Throughout this document we'll assume that sysfs
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is mounted on ``/sys``, although of course it may be mounted anywhere.
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Both ``/proc/diskstats`` and sysfs use the same source for the information
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and so should not differ.
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Here are examples of these different formats::
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2.4:
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3 0 39082680 hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
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3 1 9221278 hda1 35486 0 35496 38030 0 0 0 0 0 38030 38030
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2.6+ sysfs:
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446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
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35486 38030 38030 38030
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2.6+ diskstats:
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3 0 hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
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3 1 hda1 35486 38030 38030 38030
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4.18+ diskstats:
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3 0 hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160 0 0 0 0
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On 2.4 you might execute ``grep 'hda ' /proc/partitions``. On 2.6+, you have
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a choice of ``cat /sys/block/hda/stat`` or ``grep 'hda ' /proc/diskstats``.
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The advantage of one over the other is that the sysfs choice works well
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if you are watching a known, small set of disks. ``/proc/diskstats`` may
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be a better choice if you are watching a large number of disks because
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you'll avoid the overhead of 50, 100, or 500 or more opens/closes with
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each snapshot of your disk statistics.
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In 2.4, the statistics fields are those after the device name. In
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the above example, the first field of statistics would be 446216.
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By contrast, in 2.6+ if you look at ``/sys/block/hda/stat``, you'll
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find just the eleven fields, beginning with 446216. If you look at
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``/proc/diskstats``, the eleven fields will be preceded by the major and
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minor device numbers, and device name. Each of these formats provides
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eleven fields of statistics, each meaning exactly the same things.
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All fields except field 9 are cumulative since boot. Field 9 should
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go to zero as I/Os complete; all others only increase (unless they
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overflow and wrap). Yes, these are (32-bit or 64-bit) unsigned long
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(native word size) numbers, and on a very busy or long-lived system they
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may wrap. Applications should be prepared to deal with that; unless
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your observations are measured in large numbers of minutes or hours,
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they should not wrap twice before you notice them.
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Each set of stats only applies to the indicated device; if you want
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system-wide stats you'll have to find all the devices and sum them all up.
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Field 1 -- # of reads completed
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This is the total number of reads completed successfully.
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Field 2 -- # of reads merged, field 6 -- # of writes merged
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Reads and writes which are adjacent to each other may be merged for
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efficiency. Thus two 4K reads may become one 8K read before it is
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ultimately handed to the disk, and so it will be counted (and queued)
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as only one I/O. This field lets you know how often this was done.
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Field 3 -- # of sectors read
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This is the total number of sectors read successfully.
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Field 4 -- # of milliseconds spent reading
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This is the total number of milliseconds spent by all reads (as
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measured from __make_request() to end_that_request_last()).
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Field 5 -- # of writes completed
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This is the total number of writes completed successfully.
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Field 6 -- # of writes merged
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See the description of field 2.
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Field 7 -- # of sectors written
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This is the total number of sectors written successfully.
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Field 8 -- # of milliseconds spent writing
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This is the total number of milliseconds spent by all writes (as
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measured from __make_request() to end_that_request_last()).
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Field 9 -- # of I/Os currently in progress
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The only field that should go to zero. Incremented as requests are
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given to appropriate struct request_queue and decremented as they finish.
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Field 10 -- # of milliseconds spent doing I/Os
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This field increases so long as field 9 is nonzero.
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Since 5.0 this field counts jiffies when at least one request was
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started or completed. If request runs more than 2 jiffies then some
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I/O time will not be accounted unless there are other requests.
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Field 11 -- weighted # of milliseconds spent doing I/Os
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This field is incremented at each I/O start, I/O completion, I/O
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merge, or read of these stats by the number of I/Os in progress
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(field 9) times the number of milliseconds spent doing I/O since the
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last update of this field. This can provide an easy measure of both
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I/O completion time and the backlog that may be accumulating.
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Field 12 -- # of discards completed
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This is the total number of discards completed successfully.
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Field 13 -- # of discards merged
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See the description of field 2
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Field 14 -- # of sectors discarded
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This is the total number of sectors discarded successfully.
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Field 15 -- # of milliseconds spent discarding
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This is the total number of milliseconds spent by all discards (as
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measured from __make_request() to end_that_request_last()).
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Field 16 -- # of flush requests completed
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This is the total number of flush requests completed successfully.
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Block layer combines flush requests and executes at most one at a time.
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This counts flush requests executed by disk. Not tracked for partitions.
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Field 17 -- # of milliseconds spent flushing
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This is the total number of milliseconds spent by all flush requests.
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To avoid introducing performance bottlenecks, no locks are held while
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modifying these counters. This implies that minor inaccuracies may be
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introduced when changes collide, so (for instance) adding up all the
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read I/Os issued per partition should equal those made to the disks ...
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but due to the lack of locking it may only be very close.
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In 2.6+, there are counters for each CPU, which make the lack of locking
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almost a non-issue. When the statistics are read, the per-CPU counters
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are summed (possibly overflowing the unsigned long variable they are
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summed to) and the result given to the user. There is no convenient
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user interface for accessing the per-CPU counters themselves.
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Disks vs Partitions
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-------------------
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There were significant changes between 2.4 and 2.6+ in the I/O subsystem.
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As a result, some statistic information disappeared. The translation from
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a disk address relative to a partition to the disk address relative to
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the host disk happens much earlier. All merges and timings now happen
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at the disk level rather than at both the disk and partition level as
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in 2.4. Consequently, you'll see a different statistics output on 2.6+ for
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partitions from that for disks. There are only *four* fields available
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for partitions on 2.6+ machines. This is reflected in the examples above.
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Field 1 -- # of reads issued
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This is the total number of reads issued to this partition.
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Field 2 -- # of sectors read
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This is the total number of sectors requested to be read from this
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partition.
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Field 3 -- # of writes issued
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This is the total number of writes issued to this partition.
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Field 4 -- # of sectors written
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This is the total number of sectors requested to be written to
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this partition.
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Note that since the address is translated to a disk-relative one, and no
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record of the partition-relative address is kept, the subsequent success
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or failure of the read cannot be attributed to the partition. In other
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words, the number of reads for partitions is counted slightly before time
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of queuing for partitions, and at completion for whole disks. This is
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a subtle distinction that is probably uninteresting for most cases.
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More significant is the error induced by counting the numbers of
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reads/writes before merges for partitions and after for disks. Since a
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typical workload usually contains a lot of successive and adjacent requests,
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the number of reads/writes issued can be several times higher than the
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number of reads/writes completed.
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In 2.6.25, the full statistic set is again available for partitions and
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disk and partition statistics are consistent again. Since we still don't
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keep record of the partition-relative address, an operation is attributed to
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the partition which contains the first sector of the request after the
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eventual merges. As requests can be merged across partition, this could lead
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to some (probably insignificant) inaccuracy.
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Additional notes
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----------------
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In 2.6+, sysfs is not mounted by default. If your distribution of
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Linux hasn't added it already, here's the line you'll want to add to
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your ``/etc/fstab``::
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none /sys sysfs defaults 0 0
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In 2.6+, all disk statistics were removed from ``/proc/stat``. In 2.4, they
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appear in both ``/proc/partitions`` and ``/proc/stat``, although the ones in
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``/proc/stat`` take a very different format from those in ``/proc/partitions``
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(see proc(5), if your system has it.)
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-- ricklind@us.ibm.com
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