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
lxr.linux.no kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.