linux/Documentation/block/stat.rst
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   1===============================================
   2Block layer statistics in /sys/block/<dev>/stat
   3===============================================
   4
   5This file documents the contents of the /sys/block/<dev>/stat file.
   6
   7The stat file provides several statistics about the state of block
   8device <dev>.
   9
  10Q.
  11   Why are there multiple statistics in a single file?  Doesn't sysfs
  12   normally contain a single value per file?
  13
  14A.
  15   By having a single file, the kernel can guarantee that the statistics
  16   represent a consistent snapshot of the state of the device.  If the
  17   statistics were exported as multiple files containing one statistic
  18   each, it would be impossible to guarantee that a set of readings
  19   represent a single point in time.
  20
  21The stat file consists of a single line of text containing 11 decimal
  22values separated by whitespace.  The fields are summarized in the
  23following table, and described in more detail below.
  24
  25
  26=============== ============= =================================================
  27Name            units         description
  28=============== ============= =================================================
  29read I/Os       requests      number of read I/Os processed
  30read merges     requests      number of read I/Os merged with in-queue I/O
  31read sectors    sectors       number of sectors read
  32read ticks      milliseconds  total wait time for read requests
  33write I/Os      requests      number of write I/Os processed
  34write merges    requests      number of write I/Os merged with in-queue I/O
  35write sectors   sectors       number of sectors written
  36write ticks     milliseconds  total wait time for write requests
  37in_flight       requests      number of I/Os currently in flight
  38io_ticks        milliseconds  total time this block device has been active
  39time_in_queue   milliseconds  total wait time for all requests
  40discard I/Os    requests      number of discard I/Os processed
  41discard merges  requests      number of discard I/Os merged with in-queue I/O
  42discard sectors sectors       number of sectors discarded
  43discard ticks   milliseconds  total wait time for discard requests
  44flush I/Os      requests      number of flush I/Os processed
  45flush ticks     milliseconds  total wait time for flush requests
  46=============== ============= =================================================
  47
  48read I/Os, write I/Os, discard I/0s
  49===================================
  50
  51These values increment when an I/O request completes.
  52
  53flush I/Os
  54==========
  55
  56These values increment when an flush I/O request completes.
  57
  58Block layer combines flush requests and executes at most one at a time.
  59This counts flush requests executed by disk. Not tracked for partitions.
  60
  61read merges, write merges, discard merges
  62=========================================
  63
  64These values increment when an I/O request is merged with an
  65already-queued I/O request.
  66
  67read sectors, write sectors, discard_sectors
  68============================================
  69
  70These values count the number of sectors read from, written to, or
  71discarded from this block device.  The "sectors" in question are the
  72standard UNIX 512-byte sectors, not any device- or filesystem-specific
  73block size.  The counters are incremented when the I/O completes.
  74
  75read ticks, write ticks, discard ticks, flush ticks
  76===================================================
  77
  78These values count the number of milliseconds that I/O requests have
  79waited on this block device.  If there are multiple I/O requests waiting,
  80these values will increase at a rate greater than 1000/second; for
  81example, if 60 read requests wait for an average of 30 ms, the read_ticks
  82field will increase by 60*30 = 1800.
  83
  84in_flight
  85=========
  86
  87This value counts the number of I/O requests that have been issued to
  88the device driver but have not yet completed.  It does not include I/O
  89requests that are in the queue but not yet issued to the device driver.
  90
  91io_ticks
  92========
  93
  94This value counts the number of milliseconds during which the device has
  95had I/O requests queued.
  96
  97time_in_queue
  98=============
  99
 100This value counts the number of milliseconds that I/O requests have waited
 101on this block device.  If there are multiple I/O requests waiting, this
 102value will increase as the product of the number of milliseconds times the
 103number of requests waiting (see "read ticks" above for an example).
 104