Documentation/admin-guide/iostats.rst - pub/scm/linux/kernel/git/gregkh/tty - Git at Google

 =====================
 I/O statistics fields
 =====================

 The kernel exposes disk statistics via ``/proc/diskstats`` and
 ``/sys/block/<device>/stat``. These stats are usually accessed via tools
 such as ``sar`` and ``iostat``.

 Here are examples using a disk with two partitions::

    /proc/diskstats:
      259       0 nvme0n1 255999 814 12369153 47919 996852 81 36123024 425995 0 301795 580470 0 0 0 0 60602 106555
      259       1 nvme0n1p1 492 813 17572 96 848 81 108288 210 0 76 307 0 0 0 0 0 0
      259       2 nvme0n1p2 255401 1 12343477 47799 996004 0 36014736 425784 0 344336 473584 0 0 0 0 0 0

    /sys/block/nvme0n1/stat:
      255999 814 12369153 47919 996858 81 36123056 426009 0 301809 580491 0 0 0 0 60605 106562

    /sys/block/nvme0n1/nvme0n1p1/stat:
      492 813 17572 96 848 81 108288 210 0 76 307 0 0 0 0 0 0

 Both files contain the same 17 statistics. ``/sys/block/<device>/stat``
 contains the fields for ``<device>``. In ``/proc/diskstats`` the fields
 are prefixed with the major and minor device numbers and the device
 name. In the example above, the first stat value for ``nvme0n1`` is
 255999 in both files.

 The sysfs ``stat`` file is efficient for monitoring a small, known set
 of disks. If you're tracking a large number of devices,
 ``/proc/diskstats`` is often the better choice since it avoids the
 overhead of opening and closing multiple files for each snapshot.

 All fields are cumulative, monotonic counters, except for field 9, which
 resets to zero as I/Os complete. The remaining fields reset at boot, on
 device reattachment or reinitialization, or when the underlying counter
 overflows. Applications reading these counters should detect and handle
 resets when comparing stat snapshots.

 Each set of stats only applies to the indicated device; if you want
 system-wide stats you'll have to find all the devices and sum them all up.

 Field  1 -- # of reads completed (unsigned long)
     This is the total number of reads completed successfully.

 Field  2 -- # of reads merged, field 6 -- # of writes merged (unsigned long)
     Reads and writes which are adjacent to each other may be merged for
     efficiency.  Thus two 4K reads may become one 8K read before it is
     ultimately handed to the disk, and so it will be counted (and queued)
     as only one I/O.  This field lets you know how often this was done.

 Field  3 -- # of sectors read (unsigned long)
     This is the total number of sectors read successfully.

 Field  4 -- # of milliseconds spent reading (unsigned int)
     This is the total number of milliseconds spent by all reads (as
     measured from blk_mq_alloc_request() to __blk_mq_end_request()).

 Field  5 -- # of writes completed (unsigned long)
     This is the total number of writes completed successfully.

 Field  6 -- # of writes merged  (unsigned long)
     See the description of field 2.

 Field  7 -- # of sectors written (unsigned long)
     This is the total number of sectors written successfully.

 Field  8 -- # of milliseconds spent writing (unsigned int)
     This is the total number of milliseconds spent by all writes (as
     measured from blk_mq_alloc_request() to __blk_mq_end_request()).

 Field  9 -- # of I/Os currently in progress (unsigned int)
     The only field that should go to zero. Incremented as requests are
     given to appropriate struct request_queue and decremented as they finish.

 Field 10 -- # of milliseconds spent doing I/Os (unsigned int)
     This field increases so long as field 9 is nonzero.

     Since 5.0 this field counts jiffies when at least one request was
     started or completed. If request runs more than 2 jiffies then some
     I/O time might be not accounted in case of concurrent requests.

 Field 11 -- weighted # of milliseconds spent doing I/Os (unsigned int)
     This field is incremented at each I/O start, I/O completion, I/O
     merge, or read of these stats by the number of I/Os in progress
     (field 9) times the number of milliseconds spent doing I/O since the
     last update of this field.  This can provide an easy measure of both
     I/O completion time and the backlog that may be accumulating.

 Field 12 -- # of discards completed (unsigned long)
     This is the total number of discards completed successfully.

 Field 13 -- # of discards merged (unsigned long)
     See the description of field 2

 Field 14 -- # of sectors discarded (unsigned long)
     This is the total number of sectors discarded successfully.

 Field 15 -- # of milliseconds spent discarding (unsigned int)
     This is the total number of milliseconds spent by all discards (as
     measured from blk_mq_alloc_request() to __blk_mq_end_request()).

 Field 16 -- # of flush requests completed
     This is the total number of flush requests completed successfully.

     Block layer combines flush requests and executes at most one at a time.
     This counts flush requests executed by disk. Not tracked for partitions.

 Field 17 -- # of milliseconds spent flushing
     This is the total number of milliseconds spent by all flush requests.

 To avoid introducing performance bottlenecks, no locks are held while
 modifying these counters.  This implies that minor inaccuracies may be
 introduced when changes collide, so (for instance) adding up all the
 read I/Os issued per partition should equal those made to the disks ...
 but due to the lack of locking it may only be very close.

 In 2.6+, there are counters for each CPU, which make the lack of locking
 almost a non-issue.  When the statistics are read, the per-CPU counters
 are summed (possibly overflowing the unsigned long variable they are
 summed to) and the result given to the user.  There is no convenient
 user interface for accessing the per-CPU counters themselves.

 Since 4.19 request times are measured with nanoseconds precision and
 truncated to milliseconds before showing in this interface.

 Disks vs Partitions
 -------------------

 There were significant changes between 2.4 and 2.6+ in the I/O subsystem.
 As a result, some statistic information disappeared. The translation from
 a disk address relative to a partition to the disk address relative to
 the host disk happens much earlier.  All merges and timings now happen
 at the disk level rather than at both the disk and partition level as
 in 2.4.  Consequently, you'll see a different statistics output on 2.6+ for
 partitions from that for disks.  There are only *four* fields available
 for partitions on 2.6+ machines.  This is reflected in the examples above.

 Field  1 -- # of reads issued
     This is the total number of reads issued to this partition.

 Field  2 -- # of sectors read
     This is the total number of sectors requested to be read from this
     partition.

 Field  3 -- # of writes issued
     This is the total number of writes issued to this partition.

 Field  4 -- # of sectors written
     This is the total number of sectors requested to be written to
     this partition.

 Note that since the address is translated to a disk-relative one, and no
 record of the partition-relative address is kept, the subsequent success
 or failure of the read cannot be attributed to the partition.  In other
 words, the number of reads for partitions is counted slightly before time
 of queuing for partitions, and at completion for whole disks.  This is
 a subtle distinction that is probably uninteresting for most cases.

 More significant is the error induced by counting the numbers of
 reads/writes before merges for partitions and after for disks. Since a
 typical workload usually contains a lot of successive and adjacent requests,
 the number of reads/writes issued can be several times higher than the
 number of reads/writes completed.

 In 2.6.25, the full statistic set is again available for partitions and
 disk and partition statistics are consistent again. Since we still don't
 keep record of the partition-relative address, an operation is attributed to
 the partition which contains the first sector of the request after the
 eventual merges. As requests can be merged across partition, this could lead
 to some (probably insignificant) inaccuracy.

 Additional notes
 ----------------

 In 2.6+, sysfs is not mounted by default.  If your distribution of
 Linux hasn't added it already, here's the line you'll want to add to
 your ``/etc/fstab``::

 	none /sys sysfs defaults 0 0


 In 2.6+, all disk statistics were removed from ``/proc/stat``.  In 2.4, they
 appear in both ``/proc/partitions`` and ``/proc/stat``, although the ones in
 ``/proc/stat`` take a very different format from those in ``/proc/partitions``
 (see proc(5), if your system has it.)

 -- ricklind@us.ibm.com
	=====================
	I/O statistics fields
	=====================

	The kernel exposes disk statistics via ``/proc/diskstats`` and
	``/sys/block/<device>/stat``. These stats are usually accessed via tools
	such as ``sar`` and ``iostat``.

	Here are examples using a disk with two partitions::

	/proc/diskstats:
	259 0 nvme0n1 255999 814 12369153 47919 996852 81 36123024 425995 0 301795 580470 0 0 0 0 60602 106555
	259 1 nvme0n1p1 492 813 17572 96 848 81 108288 210 0 76 307 0 0 0 0 0 0
	259 2 nvme0n1p2 255401 1 12343477 47799 996004 0 36014736 425784 0 344336 473584 0 0 0 0 0 0

	/sys/block/nvme0n1/stat:
	255999 814 12369153 47919 996858 81 36123056 426009 0 301809 580491 0 0 0 0 60605 106562

	/sys/block/nvme0n1/nvme0n1p1/stat:
	492 813 17572 96 848 81 108288 210 0 76 307 0 0 0 0 0 0

	Both files contain the same 17 statistics. ``/sys/block/<device>/stat``
	contains the fields for ``<device>``. In ``/proc/diskstats`` the fields
	are prefixed with the major and minor device numbers and the device
	name. In the example above, the first stat value for ``nvme0n1`` is
	255999 in both files.

	The sysfs ``stat`` file is efficient for monitoring a small, known set
	of disks. If you're tracking a large number of devices,
	``/proc/diskstats`` is often the better choice since it avoids the
	overhead of opening and closing multiple files for each snapshot.

	All fields are cumulative, monotonic counters, except for field 9, which
	resets to zero as I/Os complete. The remaining fields reset at boot, on
	device reattachment or reinitialization, or when the underlying counter
	overflows. Applications reading these counters should detect and handle
	resets when comparing stat snapshots.

	Each set of stats only applies to the indicated device; if you want
	system-wide stats you'll have to find all the devices and sum them all up.

	Field 1 -- # of reads completed (unsigned long)
	This is the total number of reads completed successfully.

	Field 2 -- # of reads merged, field 6 -- # of writes merged (unsigned long)
	Reads and writes which are adjacent to each other may be merged for
	efficiency. Thus two 4K reads may become one 8K read before it is
	ultimately handed to the disk, and so it will be counted (and queued)
	as only one I/O. This field lets you know how often this was done.

	Field 3 -- # of sectors read (unsigned long)
	This is the total number of sectors read successfully.

	Field 4 -- # of milliseconds spent reading (unsigned int)
	This is the total number of milliseconds spent by all reads (as
	measured from blk_mq_alloc_request() to __blk_mq_end_request()).

	Field 5 -- # of writes completed (unsigned long)
	This is the total number of writes completed successfully.

	Field 6 -- # of writes merged (unsigned long)
	See the description of field 2.

	Field 7 -- # of sectors written (unsigned long)
	This is the total number of sectors written successfully.

	Field 8 -- # of milliseconds spent writing (unsigned int)
	This is the total number of milliseconds spent by all writes (as
	measured from blk_mq_alloc_request() to __blk_mq_end_request()).

	Field 9 -- # of I/Os currently in progress (unsigned int)
	The only field that should go to zero. Incremented as requests are
	given to appropriate struct request_queue and decremented as they finish.

	Field 10 -- # of milliseconds spent doing I/Os (unsigned int)
	This field increases so long as field 9 is nonzero.

	Since 5.0 this field counts jiffies when at least one request was
	started or completed. If request runs more than 2 jiffies then some
	I/O time might be not accounted in case of concurrent requests.

	Field 11 -- weighted # of milliseconds spent doing I/Os (unsigned int)
	This field is incremented at each I/O start, I/O completion, I/O
	merge, or read of these stats by the number of I/Os in progress
	(field 9) times the number of milliseconds spent doing I/O since the
	last update of this field. This can provide an easy measure of both
	I/O completion time and the backlog that may be accumulating.

	Field 12 -- # of discards completed (unsigned long)
	This is the total number of discards completed successfully.

	Field 13 -- # of discards merged (unsigned long)
	See the description of field 2

	Field 14 -- # of sectors discarded (unsigned long)
	This is the total number of sectors discarded successfully.

	Field 15 -- # of milliseconds spent discarding (unsigned int)
	This is the total number of milliseconds spent by all discards (as
	measured from blk_mq_alloc_request() to __blk_mq_end_request()).

	Field 16 -- # of flush requests completed
	This is the total number of flush requests completed successfully.

	Block layer combines flush requests and executes at most one at a time.
	This counts flush requests executed by disk. Not tracked for partitions.

	Field 17 -- # of milliseconds spent flushing
	This is the total number of milliseconds spent by all flush requests.

	To avoid introducing performance bottlenecks, no locks are held while
	modifying these counters. This implies that minor inaccuracies may be
	introduced when changes collide, so (for instance) adding up all the
	read I/Os issued per partition should equal those made to the disks ...
	but due to the lack of locking it may only be very close.

	In 2.6+, there are counters for each CPU, which make the lack of locking
	almost a non-issue. When the statistics are read, the per-CPU counters
	are summed (possibly overflowing the unsigned long variable they are
	summed to) and the result given to the user. There is no convenient
	user interface for accessing the per-CPU counters themselves.

	Since 4.19 request times are measured with nanoseconds precision and
	truncated to milliseconds before showing in this interface.

	Disks vs Partitions
	-------------------

	There were significant changes between 2.4 and 2.6+ in the I/O subsystem.
	As a result, some statistic information disappeared. The translation from
	a disk address relative to a partition to the disk address relative to
	the host disk happens much earlier. All merges and timings now happen
	at the disk level rather than at both the disk and partition level as
	in 2.4. Consequently, you'll see a different statistics output on 2.6+ for
	partitions from that for disks. There are only four fields available
	for partitions on 2.6+ machines. This is reflected in the examples above.

	Field 1 -- # of reads issued
	This is the total number of reads issued to this partition.

	Field 2 -- # of sectors read
	This is the total number of sectors requested to be read from this
	partition.

	Field 3 -- # of writes issued
	This is the total number of writes issued to this partition.

	Field 4 -- # of sectors written
	This is the total number of sectors requested to be written to
	this partition.

	Note that since the address is translated to a disk-relative one, and no
	record of the partition-relative address is kept, the subsequent success
	or failure of the read cannot be attributed to the partition. In other
	words, the number of reads for partitions is counted slightly before time
	of queuing for partitions, and at completion for whole disks. This is
	a subtle distinction that is probably uninteresting for most cases.

	More significant is the error induced by counting the numbers of
	reads/writes before merges for partitions and after for disks. Since a
	typical workload usually contains a lot of successive and adjacent requests,
	the number of reads/writes issued can be several times higher than the
	number of reads/writes completed.

	In 2.6.25, the full statistic set is again available for partitions and
	disk and partition statistics are consistent again. Since we still don't
	keep record of the partition-relative address, an operation is attributed to
	the partition which contains the first sector of the request after the
	eventual merges. As requests can be merged across partition, this could lead
	to some (probably insignificant) inaccuracy.

	Additional notes
	----------------

	In 2.6+, sysfs is not mounted by default. If your distribution of
	Linux hasn't added it already, here's the line you'll want to add to
	your ``/etc/fstab``::

	none /sys sysfs defaults 0 0


	In 2.6+, all disk statistics were removed from ``/proc/stat``. In 2.4, they
	appear in both ``/proc/partitions`` and ``/proc/stat``, although the ones in
	``/proc/stat`` take a very different format from those in ``/proc/partitions``
	(see proc(5), if your system has it.)

	-- ricklind@us.ibm.com