194 lines
8.7 KiB
Plaintext
194 lines
8.7 KiB
Plaintext
=====================
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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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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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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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