linux/Documentation/block/stat.txt
<<
>>
Prefs
   1Block layer statistics in /sys/block/<dev>/stat
   2===============================================
   3
   4This file documents the contents of the /sys/block/<dev>/stat file.
   5
   6The stat file provides several statistics about the state of block
   7device <dev>.
   8
   9Q. Why are there multiple statistics in a single file?  Doesn't sysfs
  10   normally contain a single value per file?
  11A. By having a single file, the kernel can guarantee that the statistics
  12   represent a consistent snapshot of the state of the device.  If the
  13   statistics were exported as multiple files containing one statistic
  14   each, it would be impossible to guarantee that a set of readings
  15   represent a single point in time.
  16
  17The stat file consists of a single line of text containing 11 decimal
  18values separated by whitespace.  The fields are summarized in the
  19following table, and described in more detail below.
  20
  21Name            units         description
  22----            -----         -----------
  23read I/Os       requests      number of read I/Os processed
  24read merges     requests      number of read I/Os merged with in-queue I/O
  25read sectors    sectors       number of sectors read
  26read ticks      milliseconds  total wait time for read requests
  27write I/Os      requests      number of write I/Os processed
  28write merges    requests      number of write I/Os merged with in-queue I/O
  29write sectors   sectors       number of sectors written
  30write ticks     milliseconds  total wait time for write requests
  31in_flight       requests      number of I/Os currently in flight
  32io_ticks        milliseconds  total time this block device has been active
  33time_in_queue   milliseconds  total wait time for all requests
  34
  35read I/Os, write I/Os
  36=====================
  37
  38These values increment when an I/O request completes.
  39
  40read merges, write merges
  41=========================
  42
  43These values increment when an I/O request is merged with an
  44already-queued I/O request.
  45
  46read sectors, write sectors
  47===========================
  48
  49These values count the number of sectors read from or written to this
  50block device.  The "sectors" in question are the standard UNIX 512-byte
  51sectors, not any device- or filesystem-specific block size.  The
  52counters are incremented when the I/O completes.
  53
  54read ticks, write ticks
  55=======================
  56
  57These values count the number of milliseconds that I/O requests have
  58waited on this block device.  If there are multiple I/O requests waiting,
  59these values will increase at a rate greater than 1000/second; for
  60example, if 60 read requests wait for an average of 30 ms, the read_ticks
  61field will increase by 60*30 = 1800.
  62
  63in_flight
  64=========
  65
  66This value counts the number of I/O requests that have been issued to
  67the device driver but have not yet completed.  It does not include I/O
  68requests that are in the queue but not yet issued to the device driver.
  69
  70io_ticks
  71========
  72
  73This value counts the number of milliseconds during which the device has
  74had I/O requests queued.
  75
  76time_in_queue
  77=============
  78
  79This value counts the number of milliseconds that I/O requests have waited
  80on this block device.  If there are multiple I/O requests waiting, this
  81value will increase as the product of the number of milliseconds times the
  82number of requests waiting (see "read ticks" above for an example).
  83