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authorLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 18:20:36 -0400
committerLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 18:20:36 -0400
commit1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (patch)
tree0bba044c4ce775e45a88a51686b5d9f90697ea9d /Documentation/iostats.txt
Linux-2.6.12-rc2v2.6.12-rc2
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
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1I/O statistics fields
2---------------
3
4Last modified Sep 30, 2003
5
6Since 2.4.20 (and some versions before, with patches), and 2.5.45,
7more extensive disk statistics have been introduced to help measure disk
8activity. Tools such as sar and iostat typically interpret these and do
9the work for you, but in case you are interested in creating your own
10tools, the fields are explained here.
11
12In 2.4 now, the information is found as additional fields in
13/proc/partitions. In 2.6, the same information is found in two
14places: one is in the file /proc/diskstats, and the other is within
15the sysfs file system, which must be mounted in order to obtain
16the information. Throughout this document we'll assume that sysfs
17is mounted on /sys, although of course it may be mounted anywhere.
18Both /proc/diskstats and sysfs use the same source for the information
19and so should not differ.
20
21Here are examples of these different formats:
22
232.4:
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
282.6 sysfs:
29 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
30 35486 38030 38030 38030
31
322.6 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
36On 2.4 you might execute "grep 'hda ' /proc/partitions". On 2.6, you have
37a choice of "cat /sys/block/hda/stat" or "grep 'hda ' /proc/diskstats".
38The advantage of one over the other is that the sysfs choice works well
39if you are watching a known, small set of disks. /proc/diskstats may
40be a better choice if you are watching a large number of disks because
41you'll avoid the overhead of 50, 100, or 500 or more opens/closes with
42each snapshot of your disk statistics.
43
44In 2.4, the statistics fields are those after the device name. In
45the above example, the first field of statistics would be 446216.
46By contrast, in 2.6 if you look at /sys/block/hda/stat, you'll
47find just the eleven fields, beginning with 446216. If you look at
48/proc/diskstats, the eleven fields will be preceded by the major and
49minor device numbers, and device name. Each of these formats provide
50eleven fields of statistics, each meaning exactly the same things.
51All fields except field 9 are cumulative since boot. Field 9 should
52go to zero as I/Os complete; all others only increase. Yes, these are
5332 bit unsigned numbers, and on a very busy or long-lived system they
54may wrap. Applications should be prepared to deal with that; unless
55your observations are measured in large numbers of minutes or hours,
56they should not wrap twice before you notice them.
57
58Each set of stats only applies to the indicated device; if you want
59system-wide stats you'll have to find all the devices and sum them all up.
60
61Field 1 -- # of reads issued
62 This is the total number of reads completed successfully.
63Field 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.
68Field 3 -- # of sectors read
69 This is the total number of sectors read successfully.
70Field 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()).
73Field 5 -- # of writes completed
74 This is the total number of writes completed successfully.
75Field 7 -- # of sectors written
76 This is the total number of sectors written successfully.
77Field 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()).
80Field 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.
83Field 10 -- # of milliseconds spent doing I/Os
84 This field is increases so long as field 9 is nonzero.
85Field 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
93To avoid introducing performance bottlenecks, no locks are held while
94modifying these counters. This implies that minor inaccuracies may be
95introduced when changes collide, so (for instance) adding up all the
96read I/Os issued per partition should equal those made to the disks ...
97but due to the lack of locking it may only be very close.
98
99In 2.6, there are counters for each cpu, which made the lack of locking
100almost a non-issue. When the statistics are read, the per-cpu counters
101are summed (possibly overflowing the unsigned 32-bit variable they are
102summed to) and the result given to the user. There is no convenient
103user interface for accessing the per-cpu counters themselves.
104
105Disks vs Partitions
106-------------------
107
108There were significant changes between 2.4 and 2.6 in the I/O subsystem.
109As a result, some statistic information disappeared. The translation from
110a disk address relative to a partition to the disk address relative to
111the host disk happens much earlier. All merges and timings now happen
112at the disk level rather than at both the disk and partition level as
113in 2.4. Consequently, you'll see a different statistics output on 2.6 for
114partitions from that for disks. There are only *four* fields available
115for partitions on 2.6 machines. This is reflected in the examples above.
116
117Field 1 -- # of reads issued
118 This is the total number of reads issued to this partition.
119Field 2 -- # of sectors read
120 This is the total number of sectors requested to be read from this
121 partition.
122Field 3 -- # of writes issued
123 This is the total number of writes issued to this partition.
124Field 4 -- # of sectors written
125 This is the total number of sectors requested to be written to
126 this partition.
127
128Note that since the address is translated to a disk-relative one, and no
129record of the partition-relative address is kept, the subsequent success
130or failure of the read cannot be attributed to the partition. In other
131words, the number of reads for partitions is counted slightly before time
132of queuing for partitions, and at completion for whole disks. This is
133a subtle distinction that is probably uninteresting for most cases.
134
135Additional notes
136----------------
137
138In 2.6, sysfs is not mounted by default. If your distribution of
139Linux hasn't added it already, here's the line you'll want to add to
140your /etc/fstab:
141
142none /sys sysfs defaults 0 0
143
144
145In 2.6, all disk statistics were removed from /proc/stat. In 2.4, they
146appear 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