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Based on kernel version 3.9. Page generated on 2013-05-02 23:09 EST.

1	I/O statistics fields
2	---------------
3	
4	Since 2.4.20 (and some versions before, with patches), and 2.5.45,
5	more extensive disk statistics have been introduced to help measure disk
6	activity. Tools such as sar and iostat typically interpret these and do
7	the work for you, but in case you are interested in creating your own
8	tools, the fields are explained here.
9	
10	In 2.4 now, the information is found as additional fields in
11	/proc/partitions.  In 2.6, the same information is found in two
12	places: one is in the file /proc/diskstats, and the other is within
13	the sysfs file system, which must be mounted in order to obtain
14	the information. Throughout this document we'll assume that sysfs
15	is mounted on /sys, although of course it may be mounted anywhere.
16	Both /proc/diskstats and sysfs use the same source for the information
17	and so should not differ.
18	
19	Here are examples of these different formats:
20	
21	2.4:
22	   3     0   39082680 hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
23	   3     1    9221278 hda1 35486 0 35496 38030 0 0 0 0 0 38030 38030
24	
25	
26	2.6 sysfs:
27	   446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
28	   35486    38030    38030    38030
29	
30	2.6 diskstats:
31	   3    0   hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
32	   3    1   hda1 35486 38030 38030 38030
33	
34	On 2.4 you might execute "grep 'hda ' /proc/partitions". On 2.6, you have
35	a choice of "cat /sys/block/hda/stat" or "grep 'hda ' /proc/diskstats".
36	The advantage of one over the other is that the sysfs choice works well
37	if you are watching a known, small set of disks.  /proc/diskstats may
38	be a better choice if you are watching a large number of disks because
39	you'll avoid the overhead of 50, 100, or 500 or more opens/closes with
40	each snapshot of your disk statistics.
41	
42	In 2.4, the statistics fields are those after the device name. In
43	the above example, the first field of statistics would be 446216.
44	By contrast, in 2.6 if you look at /sys/block/hda/stat, you'll
45	find just the eleven fields, beginning with 446216.  If you look at
46	/proc/diskstats, the eleven fields will be preceded by the major and
47	minor device numbers, and device name.  Each of these formats provides
48	eleven fields of statistics, each meaning exactly the same things.
49	All fields except field 9 are cumulative since boot.  Field 9 should
50	go to zero as I/Os complete; all others only increase (unless they
51	overflow and wrap).  Yes, these are (32-bit or 64-bit) unsigned long
52	(native word size) numbers, and on a very busy or long-lived system they
53	may wrap. Applications should be prepared to deal with that; unless
54	your observations are measured in large numbers of minutes or hours,
55	they should not wrap twice before you notice them.
56	
57	Each set of stats only applies to the indicated device; if you want
58	system-wide stats you'll have to find all the devices and sum them all up.
59	
60	Field  1 -- # of reads completed
61	    This is the total number of reads completed successfully.
62	Field  2 -- # of reads merged, field 6 -- # of writes merged
63	    Reads and writes which are adjacent to each other may be merged for
64	    efficiency.  Thus two 4K reads may become one 8K read before it is
65	    ultimately handed to the disk, and so it will be counted (and queued)
66	    as only one I/O.  This field lets you know how often this was done.
67	Field  3 -- # of sectors read
68	    This is the total number of sectors read successfully.
69	Field  4 -- # of milliseconds spent reading
70	    This is the total number of milliseconds spent by all reads (as
71	    measured from __make_request() to end_that_request_last()).
72	Field  5 -- # of writes completed
73	    This is the total number of writes completed successfully.
74	Field  7 -- # of sectors written
75	    This is the total number of sectors written successfully.
76	Field  8 -- # of milliseconds spent writing
77	    This is the total number of milliseconds spent by all writes (as
78	    measured from __make_request() to end_that_request_last()).
79	Field  9 -- # of I/Os currently in progress
80	    The only field that should go to zero. Incremented as requests are
81	    given to appropriate struct request_queue and decremented as they finish.
82	Field 10 -- # of milliseconds spent doing I/Os
83	    This field increases so long as field 9 is nonzero.
84	Field 11 -- weighted # of milliseconds spent doing I/Os
85	    This field is incremented at each I/O start, I/O completion, I/O
86	    merge, or read of these stats by the number of I/Os in progress
87	    (field 9) times the number of milliseconds spent doing I/O since the
88	    last update of this field.  This can provide an easy measure of both
89	    I/O completion time and the backlog that may be accumulating.
90	
91	
92	To avoid introducing performance bottlenecks, no locks are held while
93	modifying these counters.  This implies that minor inaccuracies may be
94	introduced when changes collide, so (for instance) adding up all the
95	read I/Os issued per partition should equal those made to the disks ...
96	but due to the lack of locking it may only be very close.
97	
98	In 2.6, there are counters for each CPU, which make the lack of locking
99	almost a non-issue.  When the statistics are read, the per-CPU counters
100	are summed (possibly overflowing the unsigned long variable they are
101	summed to) and the result given to the user.  There is no convenient
102	user interface for accessing the per-CPU counters themselves.
103	
104	Disks vs Partitions
105	-------------------
106	
107	There were significant changes between 2.4 and 2.6 in the I/O subsystem.
108	As a result, some statistic information disappeared. The translation from
109	a disk address relative to a partition to the disk address relative to
110	the host disk happens much earlier.  All merges and timings now happen
111	at the disk level rather than at both the disk and partition level as
112	in 2.4.  Consequently, you'll see a different statistics output on 2.6 for
113	partitions from that for disks.  There are only *four* fields available
114	for partitions on 2.6 machines.  This is reflected in the examples above.
115	
116	Field  1 -- # of reads issued
117	    This is the total number of reads issued to this partition.
118	Field  2 -- # of sectors read
119	    This is the total number of sectors requested to be read from this
120	    partition.
121	Field  3 -- # of writes issued
122	    This is the total number of writes issued to this partition.
123	Field  4 -- # of sectors written
124	    This is the total number of sectors requested to be written to
125	    this partition.
126	
127	Note that since the address is translated to a disk-relative one, and no
128	record of the partition-relative address is kept, the subsequent success
129	or failure of the read cannot be attributed to the partition.  In other
130	words, the number of reads for partitions is counted slightly before time
131	of queuing for partitions, and at completion for whole disks.  This is
132	a subtle distinction that is probably uninteresting for most cases.
133	
134	More significant is the error induced by counting the numbers of
135	reads/writes before merges for partitions and after for disks. Since a
136	typical workload usually contains a lot of successive and adjacent requests,
137	the number of reads/writes issued can be several times higher than the
138	number of reads/writes completed.
139	
140	In 2.6.25, the full statistic set is again available for partitions and
141	disk and partition statistics are consistent again. Since we still don't
142	keep record of the partition-relative address, an operation is attributed to
143	the partition which contains the first sector of the request after the
144	eventual merges. As requests can be merged across partition, this could lead
145	to some (probably insignificant) inaccuracy.
146	
147	Additional notes
148	----------------
149	
150	In 2.6, sysfs is not mounted by default.  If your distribution of
151	Linux hasn't added it already, here's the line you'll want to add to
152	your /etc/fstab:
153	
154	none /sys sysfs defaults 0 0
155	
156	
157	In 2.6, all disk statistics were removed from /proc/stat.  In 2.4, they
158	appear in both /proc/partitions and /proc/stat, although the ones in
159	/proc/stat take a very different format from those in /proc/partitions
160	(see proc(5), if your system has it.)
161	
162	-- ricklind@us.ibm.com
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