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.
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.
20 Here are examples of these different formats::
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
27 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
28 35486 38030 38030 38030
31 3 0 hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
32 3 1 hda1 35486 38030 38030 38030
35 3 0 hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160 0 0 0 0
37 On 2.4 you might execute ``grep 'hda ' /proc/partitions``. On 2.6+, you have
38 a choice of ``cat /sys/block/hda/stat`` or ``grep 'hda ' /proc/diskstats``.
40 The advantage of one over the other is that the sysfs choice works well
41 if you are watching a known, small set of disks. ``/proc/diskstats`` may
42 be a better choice if you are watching a large number of disks because
43 you'll avoid the overhead of 50, 100, or 500 or more opens/closes with
44 each snapshot of your disk statistics.
46 In 2.4, the statistics fields are those after the device name. In
47 the above example, the first field of statistics would be 446216.
48 By contrast, in 2.6+ if you look at ``/sys/block/hda/stat``, you'll
49 find just the 15 fields, beginning with 446216. If you look at
50 ``/proc/diskstats``, the 15 fields will be preceded by the major and
51 minor device numbers, and device name. Each of these formats provides
52 15 fields of statistics, each meaning exactly the same things.
53 All fields except field 9 are cumulative since boot. Field 9 should
54 go to zero as I/Os complete; all others only increase (unless they
55 overflow and wrap). Wrapping might eventually occur on a very busy
56 or long-lived system; so applications should be prepared to deal with
57 it. Regarding wrapping, the types of the fields are either unsigned
58 int (32 bit) or unsigned long (32-bit or 64-bit, depending on your
59 machine) as noted per-field below. Unless your observations are very
60 spread in time, these fields should not wrap twice before you notice it.
62 Each set of stats only applies to the indicated device; if you want
63 system-wide stats you'll have to find all the devices and sum them all up.
65 Field 1 -- # of reads completed (unsigned long)
66 This is the total number of reads completed successfully.
68 Field 2 -- # of reads merged, field 6 -- # of writes merged (unsigned long)
69 Reads and writes which are adjacent to each other may be merged for
70 efficiency. Thus two 4K reads may become one 8K read before it is
71 ultimately handed to the disk, and so it will be counted (and queued)
72 as only one I/O. This field lets you know how often this was done.
74 Field 3 -- # of sectors read (unsigned long)
75 This is the total number of sectors read successfully.
77 Field 4 -- # of milliseconds spent reading (unsigned int)
78 This is the total number of milliseconds spent by all reads (as
79 measured from __make_request() to end_that_request_last()).
81 Field 5 -- # of writes completed (unsigned long)
82 This is the total number of writes completed successfully.
84 Field 6 -- # of writes merged (unsigned long)
85 See the description of field 2.
87 Field 7 -- # of sectors written (unsigned long)
88 This is the total number of sectors written successfully.
90 Field 8 -- # of milliseconds spent writing (unsigned int)
91 This is the total number of milliseconds spent by all writes (as
92 measured from __make_request() to end_that_request_last()).
94 Field 9 -- # of I/Os currently in progress (unsigned int)
95 The only field that should go to zero. Incremented as requests are
96 given to appropriate struct request_queue and decremented as they finish.
98 Field 10 -- # of milliseconds spent doing I/Os (unsigned int)
99 This field increases so long as field 9 is nonzero.
101 Since 5.0 this field counts jiffies when at least one request was
102 started or completed. If request runs more than 2 jiffies then some
103 I/O time might be not accounted in case of concurrent requests.
105 Field 11 -- weighted # of milliseconds spent doing I/Os (unsigned int)
106 This field is incremented at each I/O start, I/O completion, I/O
107 merge, or read of these stats by the number of I/Os in progress
108 (field 9) times the number of milliseconds spent doing I/O since the
109 last update of this field. This can provide an easy measure of both
110 I/O completion time and the backlog that may be accumulating.
112 Field 12 -- # of discards completed (unsigned long)
113 This is the total number of discards completed successfully.
115 Field 13 -- # of discards merged (unsigned long)
116 See the description of field 2
118 Field 14 -- # of sectors discarded (unsigned long)
119 This is the total number of sectors discarded successfully.
121 Field 15 -- # of milliseconds spent discarding (unsigned int)
122 This is the total number of milliseconds spent by all discards (as
123 measured from __make_request() to end_that_request_last()).
125 Field 16 -- # of flush requests completed
126 This is the total number of flush requests completed successfully.
128 Block layer combines flush requests and executes at most one at a time.
129 This counts flush requests executed by disk. Not tracked for partitions.
131 Field 17 -- # of milliseconds spent flushing
132 This is the total number of milliseconds spent by all flush requests.
134 To avoid introducing performance bottlenecks, no locks are held while
135 modifying these counters. This implies that minor inaccuracies may be
136 introduced when changes collide, so (for instance) adding up all the
137 read I/Os issued per partition should equal those made to the disks ...
138 but due to the lack of locking it may only be very close.
140 In 2.6+, there are counters for each CPU, which make the lack of locking
141 almost a non-issue. When the statistics are read, the per-CPU counters
142 are summed (possibly overflowing the unsigned long variable they are
143 summed to) and the result given to the user. There is no convenient
144 user interface for accessing the per-CPU counters themselves.
146 Since 4.19 request times are measured with nanoseconds precision and
147 truncated to milliseconds before showing in this interface.
152 There were significant changes between 2.4 and 2.6+ in the I/O subsystem.
153 As a result, some statistic information disappeared. The translation from
154 a disk address relative to a partition to the disk address relative to
155 the host disk happens much earlier. All merges and timings now happen
156 at the disk level rather than at both the disk and partition level as
157 in 2.4. Consequently, you'll see a different statistics output on 2.6+ for
158 partitions from that for disks. There are only *four* fields available
159 for partitions on 2.6+ machines. This is reflected in the examples above.
161 Field 1 -- # of reads issued
162 This is the total number of reads issued to this partition.
164 Field 2 -- # of sectors read
165 This is the total number of sectors requested to be read from this
168 Field 3 -- # of writes issued
169 This is the total number of writes issued to this partition.
171 Field 4 -- # of sectors written
172 This is the total number of sectors requested to be written to
175 Note that since the address is translated to a disk-relative one, and no
176 record of the partition-relative address is kept, the subsequent success
177 or failure of the read cannot be attributed to the partition. In other
178 words, the number of reads for partitions is counted slightly before time
179 of queuing for partitions, and at completion for whole disks. This is
180 a subtle distinction that is probably uninteresting for most cases.
182 More significant is the error induced by counting the numbers of
183 reads/writes before merges for partitions and after for disks. Since a
184 typical workload usually contains a lot of successive and adjacent requests,
185 the number of reads/writes issued can be several times higher than the
186 number of reads/writes completed.
188 In 2.6.25, the full statistic set is again available for partitions and
189 disk and partition statistics are consistent again. Since we still don't
190 keep record of the partition-relative address, an operation is attributed to
191 the partition which contains the first sector of the request after the
192 eventual merges. As requests can be merged across partition, this could lead
193 to some (probably insignificant) inaccuracy.
198 In 2.6+, sysfs is not mounted by default. If your distribution of
199 Linux hasn't added it already, here's the line you'll want to add to
200 your ``/etc/fstab``::
202 none /sys sysfs defaults 0 0
205 In 2.6+, all disk statistics were removed from ``/proc/stat``. In 2.4, they
206 appear in both ``/proc/partitions`` and ``/proc/stat``, although the ones in
207 ``/proc/stat`` take a very different format from those in ``/proc/partitions``
208 (see proc(5), if your system has it.)
210 -- ricklind@us.ibm.com