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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.

1	=====================
2	I/O statistics fields
3	=====================
4	
5	Since 2.4.20 (and some versions before, with patches), and 2.5.45,
6	more extensive disk statistics have been introduced to help measure disk
7	activity. Tools such as ``sar`` and ``iostat`` typically interpret these and do
8	the work for you, but in case you are interested in creating your own
9	tools, the fields are explained here.
10	
11	In 2.4 now, the information is found as additional fields in
12	``/proc/partitions``.  In 2.6 and upper, the same information is found in two
13	places: one is in the file ``/proc/diskstats``, and the other is within
14	the sysfs file system, which must be mounted in order to obtain
15	the information. Throughout this document we'll assume that sysfs
16	is mounted on ``/sys``, although of course it may be mounted anywhere.
17	Both ``/proc/diskstats`` and sysfs use the same source for the information
18	and so should not differ.
19	
20	Here are examples of these different formats::
21	
22	   2.4:
23	      3     0   39082680 hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
24	      3     1    9221278 hda1 35486 0 35496 38030 0 0 0 0 0 38030 38030
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	
37	The advantage of one over the other is that the sysfs choice works well
38	if you are watching a known, small set of disks.  ``/proc/diskstats`` may
39	be a better choice if you are watching a large number of disks because
40	you'll avoid the overhead of 50, 100, or 500 or more opens/closes with
41	each snapshot of your disk statistics.
42	
43	In 2.4, the statistics fields are those after the device name. In
44	the above example, the first field of statistics would be 446216.
45	By contrast, in 2.6+ if you look at ``/sys/block/hda/stat``, you'll
46	find just the eleven fields, beginning with 446216.  If you look at
47	``/proc/diskstats``, the eleven fields will be preceded by the major and
48	minor device numbers, and device name.  Each of these formats provides
49	eleven fields of statistics, each meaning exactly the same things.
50	All fields except field 9 are cumulative since boot.  Field 9 should
51	go to zero as I/Os complete; all others only increase (unless they
52	overflow and wrap).  Yes, these are (32-bit or 64-bit) unsigned long
53	(native word size) 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	
64	Field  2 -- # of reads merged, field 6 -- # of writes merged
65	    Reads and writes which are adjacent to each other may be merged for
66	    efficiency.  Thus two 4K reads may become one 8K read before it is
67	    ultimately handed to the disk, and so it will be counted (and queued)
68	    as only one I/O.  This field lets you know how often this was done.
69	
70	Field  3 -- # of sectors read
71	    This is the total number of sectors read successfully.
72	
73	Field  4 -- # of milliseconds spent reading
74	    This is the total number of milliseconds spent by all reads (as
75	    measured from __make_request() to end_that_request_last()).
76	
77	Field  5 -- # of writes completed
78	    This is the total number of writes completed successfully.
79	
80	Field  6 -- # of writes merged
81	    See the description of field 2.
82	
83	Field  7 -- # of sectors written
84	    This is the total number of sectors written successfully.
85	
86	Field  8 -- # of milliseconds spent writing
87	    This is the total number of milliseconds spent by all writes (as
88	    measured from __make_request() to end_that_request_last()).
89	
90	Field  9 -- # of I/Os currently in progress
91	    The only field that should go to zero. Incremented as requests are
92	    given to appropriate struct request_queue and decremented as they finish.
93	
94	Field 10 -- # of milliseconds spent doing I/Os
95	    This field increases so long as field 9 is nonzero.
96	
97	Field 11 -- weighted # of milliseconds spent doing I/Os
98	    This field is incremented at each I/O start, I/O completion, I/O
99	    merge, or read of these stats by the number of I/Os in progress
100	    (field 9) times the number of milliseconds spent doing I/O since the
101	    last update of this field.  This can provide an easy measure of both
102	    I/O completion time and the backlog that may be accumulating.
103	
104	
105	To avoid introducing performance bottlenecks, no locks are held while
106	modifying these counters.  This implies that minor inaccuracies may be
107	introduced when changes collide, so (for instance) adding up all the
108	read I/Os issued per partition should equal those made to the disks ...
109	but due to the lack of locking it may only be very close.
110	
111	In 2.6+, there are counters for each CPU, which make the lack of locking
112	almost a non-issue.  When the statistics are read, the per-CPU counters
113	are summed (possibly overflowing the unsigned long variable they are
114	summed to) and the result given to the user.  There is no convenient
115	user interface for accessing the per-CPU counters themselves.
116	
117	Disks vs Partitions
118	-------------------
119	
120	There were significant changes between 2.4 and 2.6+ in the I/O subsystem.
121	As a result, some statistic information disappeared. The translation from
122	a disk address relative to a partition to the disk address relative to
123	the host disk happens much earlier.  All merges and timings now happen
124	at the disk level rather than at both the disk and partition level as
125	in 2.4.  Consequently, you'll see a different statistics output on 2.6+ for
126	partitions from that for disks.  There are only *four* fields available
127	for partitions on 2.6+ machines.  This is reflected in the examples above.
128	
129	Field  1 -- # of reads issued
130	    This is the total number of reads issued to this partition.
131	
132	Field  2 -- # of sectors read
133	    This is the total number of sectors requested to be read from this
134	    partition.
135	
136	Field  3 -- # of writes issued
137	    This is the total number of writes issued to this partition.
138	
139	Field  4 -- # of sectors written
140	    This is the total number of sectors requested to be written to
141	    this partition.
142	
143	Note that since the address is translated to a disk-relative one, and no
144	record of the partition-relative address is kept, the subsequent success
145	or failure of the read cannot be attributed to the partition.  In other
146	words, the number of reads for partitions is counted slightly before time
147	of queuing for partitions, and at completion for whole disks.  This is
148	a subtle distinction that is probably uninteresting for most cases.
149	
150	More significant is the error induced by counting the numbers of
151	reads/writes before merges for partitions and after for disks. Since a
152	typical workload usually contains a lot of successive and adjacent requests,
153	the number of reads/writes issued can be several times higher than the
154	number of reads/writes completed.
155	
156	In 2.6.25, the full statistic set is again available for partitions and
157	disk and partition statistics are consistent again. Since we still don't
158	keep record of the partition-relative address, an operation is attributed to
159	the partition which contains the first sector of the request after the
160	eventual merges. As requests can be merged across partition, this could lead
161	to some (probably insignificant) inaccuracy.
162	
163	Additional notes
164	----------------
165	
166	In 2.6+, sysfs is not mounted by default.  If your distribution of
167	Linux hasn't added it already, here's the line you'll want to add to
168	your ``/etc/fstab``::
169	
170		none /sys sysfs defaults 0 0
171	
172	
173	In 2.6+, all disk statistics were removed from ``/proc/stat``.  In 2.4, they
174	appear in both ``/proc/partitions`` and ``/proc/stat``, although the ones in
175	``/proc/stat`` take a very different format from those in ``/proc/partitions``
176	(see proc(5), if your system has it.)
177	
178	-- ricklind@us.ibm.com
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