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