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
34discard I/Os    requests      number of discard I/Os processed
35discard merges  requests      number of discard I/Os merged with in-queue I/O
36discard sectors sectors       number of sectors discarded
37discard ticks   milliseconds  total wait time for discard requests
38
39read I/Os, write I/Os, discard I/0s
40===================================
41
42These values increment when an I/O request completes.
43
44read merges, write merges, discard merges
45=========================================
46
47These values increment when an I/O request is merged with an
48already-queued I/O request.
49
50read sectors, write sectors, discard_sectors
51============================================
52
53These values count the number of sectors read from, written to, or
54discarded from this block device.  The "sectors" in question are the
55standard UNIX 512-byte sectors, not any device- or filesystem-specific
56block size.  The counters are incremented when the I/O completes.
57
58read ticks, write ticks, discard ticks
59======================================
60
61These values count the number of milliseconds that I/O requests have
62waited on this block device.  If there are multiple I/O requests waiting,
63these values will increase at a rate greater than 1000/second; for
64example, if 60 read requests wait for an average of 30 ms, the read_ticks
65field will increase by 60*30 = 1800.
66
67in_flight
68=========
69
70This value counts the number of I/O requests that have been issued to
71the device driver but have not yet completed.  It does not include I/O
72requests that are in the queue but not yet issued to the device driver.
73
74io_ticks
75========
76
77This value counts the number of milliseconds during which the device has
78had I/O requests queued.
79
80time_in_queue
81=============
82
83This value counts the number of milliseconds that I/O requests have waited
84on this block device.  If there are multiple I/O requests waiting, this
85value will increase as the product of the number of milliseconds times the
86number of requests waiting (see "read ticks" above for an example).
87