stat.rst

  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 17 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).