1 I/O statistics fields
2 ---------------
3
4 Last modified Sep 30, 2003
5
6 Since 2.4.20 (and some versions before, with patches), and 2.5.45,
7 more extensive disk statistics have been introduced to help measure disk
8 activity. Tools such as sar and iostat typically interpret these and do
9 the work for you, but in case you are interested in creating your own
10 tools, the fields are explained here.
11
12 In 2.4 now, the information is found as additional fields in
13 /proc/partitions. In 2.6, the same information is found in two
14 places: one is in the file /proc/diskstats, and the other is within
15 the sysfs file system, which must be mounted in order to obtain
16 the information. Throughout this document we'll assume that sysfs
17 is mounted on /sys, although of course it may be mounted anywhere.
18 Both /proc/diskstats and sysfs use the same source for the information
19 and so should not differ.
20
21 Here are examples of these different formats:
22
23 2.4:
24 3 0 39082680 hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
25 3 1 9221278 hda1 35486 0 35496 38030 0 0 0 0 0 38030 38030
26
27
28 2.6 sysfs:
29 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
30 35486 38030 38030 38030
31
32 2.6 diskstats:
33 3 0 hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
34 3 1 hda1 35486 38030 38030 38030
35
36 On 2.4 you might execute "grep 'hda ' /proc/partitions". On 2.6, you have
37 a choice of "cat /sys/block/hda/stat" or "grep 'hda ' /proc/diskstats".
38 The advantage of one over the other is that the sysfs choice works well
39 if you are watching a known, small set of disks. /proc/diskstats may
40 be a better choice if you are watching a large number of disks because
41 you'll avoid the overhead of 50, 100, or 500 or more opens/closes with
42 each snapshot of your disk statistics.
43
44 In 2.4, the statistics fields are those after the device name. In
45 the above example, the first field of statistics would be 446216.
46 By contrast, in 2.6 if you look at /sys/block/hda/stat, you'll
47 find just the eleven fields, beginning with 446216. If you look at
48 /proc/diskstats, the eleven fields will be preceded by the major and
49 minor device numbers, and device name. Each of these formats provide
50 eleven fields of statistics, each meaning exactly the same things.
51 All fields except field 9 are cumulative since boot. Field 9 should
52 go to zero as I/Os complete; all others only increase. Yes, these are
53 32 bit unsigned numbers, and on a very busy or long-lived system they
54 may wrap. Applications should be prepared to deal with that; unless
55 your observations are measured in large numbers of minutes or hours,
56 they should not wrap twice before you notice them.
57
58 Each set of stats only applies to the indicated device; if you want
59 system-wide stats you'll have to find all the devices and sum them all up.
60
61 Field 1 -- # of reads issued
62 This is the total number of reads completed successfully.
63 Field 2 -- # of reads merged, field 6 -- # of writes merged
64 Reads and writes which are adjacent to each other may be merged for
65 efficiency. Thus two 4K reads may become one 8K read before it is
66 ultimately handed to the disk, and so it will be counted (and queued)
67 as only one I/O. This field lets you know how often this was done.
68 Field 3 -- # of sectors read
69 This is the total number of sectors read successfully.
70 Field 4 -- # of milliseconds spent reading
71 This is the total number of milliseconds spent by all reads (as
72 measured from __make_request() to end_that_request_last()).
73 Field 5 -- # of writes completed
74 This is the total number of writes completed successfully.
75 Field 7 -- # of sectors written
76 This is the total number of sectors written successfully.
77 Field 8 -- # of milliseconds spent writing
78 This is the total number of milliseconds spent by all writes (as
79 measured from __make_request() to end_that_request_last()).
80 Field 9 -- # of I/Os currently in progress
81 The only field that should go to zero. Incremented as requests are
82 given to appropriate request_queue_t and decremented as they finish.
83 Field 10 -- # of milliseconds spent doing I/Os
84 This field is increases so long as field 9 is nonzero.
85 Field 11 -- weighted # of milliseconds spent doing I/Os
86 This field is incremented at each I/O start, I/O completion, I/O
87 merge, or read of these stats by the number of I/Os in progress
88 (field 9) times the number of milliseconds spent doing I/O since the
89 last update of this field. This can provide an easy measure of both
90 I/O completion time and the backlog that may be accumulating.
91
92
93 To avoid introducing performance bottlenecks, no locks are held while
94 modifying these counters. This implies that minor inaccuracies may be
95 introduced when changes collide, so (for instance) adding up all the
96 read I/Os issued per partition should equal those made to the disks ...
97 but due to the lack of locking it may only be very close.
98
99 In 2.6, there are counters for each cpu, which made the lack of locking
100 almost a non-issue. When the statistics are read, the per-cpu counters
101 are summed (possibly overflowing the unsigned 32-bit variable they are
102 summed to) and the result given to the user. There is no convenient
103 user interface for accessing the per-cpu counters themselves.
104
105 Disks vs Partitions
106 -------------------
107
108 There were significant changes between 2.4 and 2.6 in the I/O subsystem.
109 As a result, some statistic information disappeared. The translation from
110 a disk address relative to a partition to the disk address relative to
111 the host disk happens much earlier. All merges and timings now happen
112 at the disk level rather than at both the disk and partition level as
113 in 2.4. Consequently, you'll see a different statistics output on 2.6 for
114 partitions from that for disks. There are only *four* fields available
115 for partitions on 2.6 machines. This is reflected in the examples above.
116
117 Field 1 -- # of reads issued
118 This is the total number of reads issued to this partition.
119 Field 2 -- # of sectors read
120 This is the total number of sectors requested to be read from this
121 partition.
122 Field 3 -- # of writes issued
123 This is the total number of writes issued to this partition.
124 Field 4 -- # of sectors written
125 This is the total number of sectors requested to be written to
126 this partition.
127
128 Note that since the address is translated to a disk-relative one, and no
129 record of the partition-relative address is kept, the subsequent success
130 or failure of the read cannot be attributed to the partition. In other
131 words, the number of reads for partitions is counted slightly before time
132 of queuing for partitions, and at completion for whole disks. This is
133 a subtle distinction that is probably uninteresting for most cases.
134
135 Additional notes
136 ----------------
137
138 In 2.6, sysfs is not mounted by default. If your distribution of
139 Linux hasn't added it already, here's the line you'll want to add to
140 your /etc/fstab:
141
142 none /sys sysfs defaults 0 0
143
144
145 In 2.6, all disk statistics were removed from /proc/stat. In 2.4, they
146 appear in both /proc/partitions and /proc/stat, although the ones in
147 /proc/stat take a very different format from those in /proc/partitions
148 (see proc(5), if your system has it.)
149
150 -- ricklind@us.ibm.com
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