mirror of https://github.com/torvalds/linux.git
188 lines
8.4 KiB
ReStructuredText
188 lines
8.4 KiB
ReStructuredText
=====================
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I/O statistics fields
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=====================
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The kernel exposes disk statistics via ``/proc/diskstats`` and
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``/sys/block/<device>/stat``. These stats are usually accessed via tools
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such as ``sar`` and ``iostat``.
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Here are examples using a disk with two partitions::
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/proc/diskstats:
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259 0 nvme0n1 255999 814 12369153 47919 996852 81 36123024 425995 0 301795 580470 0 0 0 0 60602 106555
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259 1 nvme0n1p1 492 813 17572 96 848 81 108288 210 0 76 307 0 0 0 0 0 0
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259 2 nvme0n1p2 255401 1 12343477 47799 996004 0 36014736 425784 0 344336 473584 0 0 0 0 0 0
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/sys/block/nvme0n1/stat:
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255999 814 12369153 47919 996858 81 36123056 426009 0 301809 580491 0 0 0 0 60605 106562
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/sys/block/nvme0n1/nvme0n1p1/stat:
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492 813 17572 96 848 81 108288 210 0 76 307 0 0 0 0 0 0
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Both files contain the same 17 statistics. ``/sys/block/<device>/stat``
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contains the fields for ``<device>``. In ``/proc/diskstats`` the fields
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are prefixed with the major and minor device numbers and the device
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name. In the example above, the first stat value for ``nvme0n1`` is
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255999 in both files.
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The sysfs ``stat`` file is efficient for monitoring a small, known set
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of disks. If you're tracking a large number of devices,
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``/proc/diskstats`` is often the better choice since it avoids the
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overhead of opening and closing multiple files for each snapshot.
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All fields are cumulative, monotonic counters, except for field 9, which
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resets to zero as I/Os complete. The remaining fields reset at boot, on
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device reattachment or reinitialization, or when the underlying counter
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overflows. Applications reading these counters should detect and handle
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resets when comparing stat snapshots.
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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 (unsigned long)
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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 (unsigned long)
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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 (unsigned long)
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This is the total number of sectors read successfully.
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Field 4 -- # of milliseconds spent reading (unsigned int)
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This is the total number of milliseconds spent by all reads (as
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measured from blk_mq_alloc_request() to __blk_mq_end_request()).
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Field 5 -- # of writes completed (unsigned long)
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This is the total number of writes completed successfully.
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Field 6 -- # of writes merged (unsigned long)
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See the description of field 2.
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Field 7 -- # of sectors written (unsigned long)
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This is the total number of sectors written successfully.
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Field 8 -- # of milliseconds spent writing (unsigned int)
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This is the total number of milliseconds spent by all writes (as
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measured from blk_mq_alloc_request() to __blk_mq_end_request()).
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Field 9 -- # of I/Os currently in progress (unsigned int)
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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 (unsigned int)
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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 might be not accounted in case of concurrent requests.
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Field 11 -- weighted # of milliseconds spent doing I/Os (unsigned int)
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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 (unsigned long)
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This is the total number of discards completed successfully.
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Field 13 -- # of discards merged (unsigned long)
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See the description of field 2
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Field 14 -- # of sectors discarded (unsigned long)
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This is the total number of sectors discarded successfully.
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Field 15 -- # of milliseconds spent discarding (unsigned int)
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This is the total number of milliseconds spent by all discards (as
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measured from blk_mq_alloc_request() to __blk_mq_end_request()).
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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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Since 4.19 request times are measured with nanoseconds precision and
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truncated to milliseconds before showing in this interface.
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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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