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  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