linux/Documentation/iostats.txt
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   1I/O statistics fields
   2---------------
   3
   4Since 2.4.20 (and some versions before, with patches), and 2.5.45,
   5more extensive disk statistics have been introduced to help measure disk
   6activity. Tools such as sar and iostat typically interpret these and do
   7the work for you, but in case you are interested in creating your own
   8tools, the fields are explained here.
   9
  10In 2.4 now, the information is found as additional fields in
  11/proc/partitions.  In 2.6, the same information is found in two
  12places: one is in the file /proc/diskstats, and the other is within
  13the sysfs file system, which must be mounted in order to obtain
  14the information. Throughout this document we'll assume that sysfs
  15is mounted on /sys, although of course it may be mounted anywhere.
  16Both /proc/diskstats and sysfs use the same source for the information
  17and so should not differ.
  18
  19Here are examples of these different formats:
  20
  212.4:
  22   3     0   39082680 hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
  23   3     1    9221278 hda1 35486 0 35496 38030 0 0 0 0 0 38030 38030
  24
  25
  262.6 sysfs:
  27   446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
  28   35486    38030    38030    38030
  29
  302.6 diskstats:
  31   3    0   hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
  32   3    1   hda1 35486 38030 38030 38030
  33
  34On 2.4 you might execute "grep 'hda ' /proc/partitions". On 2.6, you have
  35a choice of "cat /sys/block/hda/stat" or "grep 'hda ' /proc/diskstats".
  36The advantage of one over the other is that the sysfs choice works well
  37if you are watching a known, small set of disks.  /proc/diskstats may
  38be a better choice if you are watching a large number of disks because
  39you'll avoid the overhead of 50, 100, or 500 or more opens/closes with
  40each snapshot of your disk statistics.
  41
  42In 2.4, the statistics fields are those after the device name. In
  43the above example, the first field of statistics would be 446216.
  44By contrast, in 2.6 if you look at /sys/block/hda/stat, you'll
  45find just the eleven fields, beginning with 446216.  If you look at
  46/proc/diskstats, the eleven fields will be preceded by the major and
  47minor device numbers, and device name.  Each of these formats provides
  48eleven fields of statistics, each meaning exactly the same things.
  49All fields except field 9 are cumulative since boot.  Field 9 should
  50go to zero as I/Os complete; all others only increase (unless they
  51overflow and wrap).  Yes, these are (32-bit or 64-bit) unsigned long
  52(native word size) numbers, and on a very busy or long-lived system they
  53may wrap. Applications should be prepared to deal with that; unless
  54your observations are measured in large numbers of minutes or hours,
  55they should not wrap twice before you notice them.
  56
  57Each set of stats only applies to the indicated device; if you want
  58system-wide stats you'll have to find all the devices and sum them all up.
  59
  60Field  1 -- # of reads completed
  61    This is the total number of reads completed successfully.
  62Field  2 -- # of reads merged, field 6 -- # of writes merged
  63    Reads and writes which are adjacent to each other may be merged for
  64    efficiency.  Thus two 4K reads may become one 8K read before it is
  65    ultimately handed to the disk, and so it will be counted (and queued)
  66    as only one I/O.  This field lets you know how often this was done.
  67Field  3 -- # of sectors read
  68    This is the total number of sectors read successfully.
  69Field  4 -- # of milliseconds spent reading
  70    This is the total number of milliseconds spent by all reads (as
  71    measured from __make_request() to end_that_request_last()).
  72Field  5 -- # of writes completed
  73    This is the total number of writes completed successfully.
  74Field  6 -- # of writes merged
  75    See the description of field 2.
  76Field  7 -- # of sectors written
  77    This is the total number of sectors written successfully.
  78Field  8 -- # of milliseconds spent writing
  79    This is the total number of milliseconds spent by all writes (as
  80    measured from __make_request() to end_that_request_last()).
  81Field  9 -- # of I/Os currently in progress
  82    The only field that should go to zero. Incremented as requests are
  83    given to appropriate struct request_queue and decremented as they finish.
  84Field 10 -- # of milliseconds spent doing I/Os
  85    This field increases so long as field 9 is nonzero.
  86Field 11 -- weighted # of milliseconds spent doing I/Os
  87    This field is incremented at each I/O start, I/O completion, I/O
  88    merge, or read of these stats by the number of I/Os in progress
  89    (field 9) times the number of milliseconds spent doing I/O since the
  90    last update of this field.  This can provide an easy measure of both
  91    I/O completion time and the backlog that may be accumulating.
  92
  93
  94To avoid introducing performance bottlenecks, no locks are held while
  95modifying these counters.  This implies that minor inaccuracies may be
  96introduced when changes collide, so (for instance) adding up all the
  97read I/Os issued per partition should equal those made to the disks ...
  98but due to the lack of locking it may only be very close.
  99
 100In 2.6, there are counters for each CPU, which make the lack of locking
 101almost a non-issue.  When the statistics are read, the per-CPU counters
 102are summed (possibly overflowing the unsigned long variable they are
 103summed to) and the result given to the user.  There is no convenient
 104user interface for accessing the per-CPU counters themselves.
 105
 106Disks vs Partitions
 107-------------------
 108
 109There were significant changes between 2.4 and 2.6 in the I/O subsystem.
 110As a result, some statistic information disappeared. The translation from
 111a disk address relative to a partition to the disk address relative to
 112the host disk happens much earlier.  All merges and timings now happen
 113at the disk level rather than at both the disk and partition level as
 114in 2.4.  Consequently, you'll see a different statistics output on 2.6 for
 115partitions from that for disks.  There are only *four* fields available
 116for partitions on 2.6 machines.  This is reflected in the examples above.
 117
 118Field  1 -- # of reads issued
 119    This is the total number of reads issued to this partition.
 120Field  2 -- # of sectors read
 121    This is the total number of sectors requested to be read from this
 122    partition.
 123Field  3 -- # of writes issued
 124    This is the total number of writes issued to this partition.
 125Field  4 -- # of sectors written
 126    This is the total number of sectors requested to be written to
 127    this partition.
 128
 129Note that since the address is translated to a disk-relative one, and no
 130record of the partition-relative address is kept, the subsequent success
 131or failure of the read cannot be attributed to the partition.  In other
 132words, the number of reads for partitions is counted slightly before time
 133of queuing for partitions, and at completion for whole disks.  This is
 134a subtle distinction that is probably uninteresting for most cases.
 135
 136More significant is the error induced by counting the numbers of
 137reads/writes before merges for partitions and after for disks. Since a
 138typical workload usually contains a lot of successive and adjacent requests,
 139the number of reads/writes issued can be several times higher than the
 140number of reads/writes completed.
 141
 142In 2.6.25, the full statistic set is again available for partitions and
 143disk and partition statistics are consistent again. Since we still don't
 144keep record of the partition-relative address, an operation is attributed to
 145the partition which contains the first sector of the request after the
 146eventual merges. As requests can be merged across partition, this could lead
 147to some (probably insignificant) inaccuracy.
 148
 149Additional notes
 150----------------
 151
 152In 2.6, sysfs is not mounted by default.  If your distribution of
 153Linux hasn't added it already, here's the line you'll want to add to
 154your /etc/fstab:
 155
 156none /sys sysfs defaults 0 0
 157
 158
 159In 2.6, all disk statistics were removed from /proc/stat.  In 2.4, they
 160appear in both /proc/partitions and /proc/stat, although the ones in
 161/proc/stat take a very different format from those in /proc/partitions
 162(see proc(5), if your system has it.)
 163
 164-- ricklind@us.ibm.com
 165
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