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Documentation / iostats.txt

Based on kernel version 2.6.26. Page generated on 2008-07-16 21:13 EST.

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 completed
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 struct request_queue 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	More significant is the error induced by counting the numbers of
136	reads/writes before merges for partitions and after for disks. Since a
137	typical workload usually contains a lot of successive and adjacent requests,
138	the number of reads/writes issued can be several times higher than the
139	number of reads/writes completed.
140	
141	In 2.6.25, the full statistic set is again available for partitions and
142	disk and partition statistics are consistent again. Since we still don't
143	keep record of the partition-relative address, an operation is attributed to
144	the partition which contains the first sector of the request after the
145	eventual merges. As requests can be merged across partition, this could lead
146	to some (probably insignificant) innacuracy.
147	
148	Additional notes
149	----------------
150	
151	In 2.6, sysfs is not mounted by default.  If your distribution of
152	Linux hasn't added it already, here's the line you'll want to add to
153	your /etc/fstab:
154	
155	none /sys sysfs defaults 0 0
156	
157	
158	In 2.6, all disk statistics were removed from /proc/stat.  In 2.4, they
159	appear in both /proc/partitions and /proc/stat, although the ones in
160	/proc/stat take a very different format from those in /proc/partitions
161	(see proc(5), if your system has it.)
162	
163	-- ricklind[AT]us.ibm[DOT]com
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