Documentation/iostats.txt - maze/linux - Git at Google

 I/O statistics fields
 ---------------

 Since 2.4.20 (and some versions before, with patches), and 2.5.45,
 more extensive disk statistics have been introduced to help measure disk
 activity. Tools such as sar and iostat typically interpret these and do
 the work for you, but in case you are interested in creating your own
 tools, the fields are explained here.

 In 2.4 now, the information is found as additional fields in
 /proc/partitions.  In 2.6, the same information is found in two
 places: one is in the file /proc/diskstats, and the other is within
 the sysfs file system, which must be mounted in order to obtain
 the information. Throughout this document we'll assume that sysfs
 is mounted on /sys, although of course it may be mounted anywhere.
 Both /proc/diskstats and sysfs use the same source for the information
 and so should not differ.

 Here are examples of these different formats:

 2.4:
    3     0   39082680 hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
    3     1    9221278 hda1 35486 0 35496 38030 0 0 0 0 0 38030 38030


 2.6 sysfs:
    446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
    35486    38030    38030    38030

 2.6 diskstats:
    3    0   hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
    3    1   hda1 35486 38030 38030 38030

 On 2.4 you might execute "grep 'hda ' /proc/partitions". On 2.6, you have
 a choice of "cat /sys/block/hda/stat" or "grep 'hda ' /proc/diskstats".
 The advantage of one over the other is that the sysfs choice works well
 if you are watching a known, small set of disks.  /proc/diskstats may
 be a better choice if you are watching a large number of disks because
 you'll avoid the overhead of 50, 100, or 500 or more opens/closes with
 each snapshot of your disk statistics.

 In 2.4, the statistics fields are those after the device name. In
 the above example, the first field of statistics would be 446216.
 By contrast, in 2.6 if you look at /sys/block/hda/stat, you'll
 find just the eleven fields, beginning with 446216.  If you look at
 /proc/diskstats, the eleven fields will be preceded by the major and
 minor device numbers, and device name.  Each of these formats provides
 eleven fields of statistics, each meaning exactly the same things.
 All fields except field 9 are cumulative since boot.  Field 9 should
 go to zero as I/Os complete; all others only increase (unless they
 overflow and wrap).  Yes, these are (32-bit or 64-bit) unsigned long
 (native word size) numbers, and on a very busy or long-lived system they
 may wrap. Applications should be prepared to deal with that; unless
 your observations are measured in large numbers of minutes or hours,
 they should not wrap twice before you notice them.

 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
     This is the total number of reads completed successfully.
 Field  2 -- # of reads merged, field 6 -- # of writes merged
     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
     This is the total number of sectors read successfully.
 Field  4 -- # of milliseconds spent reading
     This is the total number of milliseconds spent by all reads (as
     measured from __make_request() to end_that_request_last()).
 Field  5 -- # of writes completed
     This is the total number of writes completed successfully.
 Field  6 -- # of writes merged
     See the description of field 2.
 Field  7 -- # of sectors written
     This is the total number of sectors written successfully.
 Field  8 -- # of milliseconds spent writing
     This is the total number of milliseconds spent by all writes (as
     measured from __make_request() to end_that_request_last()).
 Field  9 -- # of I/Os currently in progress
     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
     This field increases so long as field 9 is nonzero.
 Field 11 -- weighted # of milliseconds spent doing I/Os
     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.


 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.

 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
	---------------

	Since 2.4.20 (and some versions before, with patches), and 2.5.45,
	more extensive disk statistics have been introduced to help measure disk
	activity. Tools such as sar and iostat typically interpret these and do
	the work for you, but in case you are interested in creating your own
	tools, the fields are explained here.

	In 2.4 now, the information is found as additional fields in
	/proc/partitions. In 2.6, the same information is found in two
	places: one is in the file /proc/diskstats, and the other is within
	the sysfs file system, which must be mounted in order to obtain
	the information. Throughout this document we'll assume that sysfs
	is mounted on /sys, although of course it may be mounted anywhere.
	Both /proc/diskstats and sysfs use the same source for the information
	and so should not differ.

	Here are examples of these different formats:

	2.4:
	3 0 39082680 hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
	3 1 9221278 hda1 35486 0 35496 38030 0 0 0 0 0 38030 38030


	2.6 sysfs:
	446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
	35486 38030 38030 38030

	2.6 diskstats:
	3 0 hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
	3 1 hda1 35486 38030 38030 38030

	On 2.4 you might execute "grep 'hda ' /proc/partitions". On 2.6, you have
	a choice of "cat /sys/block/hda/stat" or "grep 'hda ' /proc/diskstats".
	The advantage of one over the other is that the sysfs choice works well
	if you are watching a known, small set of disks. /proc/diskstats may
	be a better choice if you are watching a large number of disks because
	you'll avoid the overhead of 50, 100, or 500 or more opens/closes with
	each snapshot of your disk statistics.

	In 2.4, the statistics fields are those after the device name. In
	the above example, the first field of statistics would be 446216.
	By contrast, in 2.6 if you look at /sys/block/hda/stat, you'll
	find just the eleven fields, beginning with 446216. If you look at
	/proc/diskstats, the eleven fields will be preceded by the major and
	minor device numbers, and device name. Each of these formats provides
	eleven fields of statistics, each meaning exactly the same things.
	All fields except field 9 are cumulative since boot. Field 9 should
	go to zero as I/Os complete; all others only increase (unless they
	overflow and wrap). Yes, these are (32-bit or 64-bit) unsigned long
	(native word size) numbers, and on a very busy or long-lived system they
	may wrap. Applications should be prepared to deal with that; unless
	your observations are measured in large numbers of minutes or hours,
	they should not wrap twice before you notice them.

	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
	This is the total number of reads completed successfully.
	Field 2 -- # of reads merged, field 6 -- # of writes merged
	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
	This is the total number of sectors read successfully.
	Field 4 -- # of milliseconds spent reading
	This is the total number of milliseconds spent by all reads (as
	measured from __make_request() to end_that_request_last()).
	Field 5 -- # of writes completed
	This is the total number of writes completed successfully.
	Field 6 -- # of writes merged
	See the description of field 2.
	Field 7 -- # of sectors written
	This is the total number of sectors written successfully.
	Field 8 -- # of milliseconds spent writing
	This is the total number of milliseconds spent by all writes (as
	measured from __make_request() to end_that_request_last()).
	Field 9 -- # of I/Os currently in progress
	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
	This field increases so long as field 9 is nonzero.
	Field 11 -- weighted # of milliseconds spent doing I/Os
	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.


	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.

	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