1===================== 2I/O statistics fields 3===================== 4 5Since 2.4.20 (and some versions before, with patches), and 2.5.45, 6more extensive disk statistics have been introduced to help measure disk 7activity. Tools such as ``sar`` and ``iostat`` typically interpret these and do 8the work for you, but in case you are interested in creating your own 9tools, the fields are explained here. 10 11In 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 13places: one is in the file ``/proc/diskstats``, and the other is within 14the sysfs file system, which must be mounted in order to obtain 15the information. Throughout this document we'll assume that sysfs 16is mounted on ``/sys``, although of course it may be mounted anywhere. 17Both ``/proc/diskstats`` and sysfs use the same source for the information 18and so should not differ. 19 20Here 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 4.18+ diskstats: 35 3 0 hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160 0 0 0 0 36 37On 2.4 you might execute ``grep 'hda ' /proc/partitions``. On 2.6+, you have 38a choice of ``cat /sys/block/hda/stat`` or ``grep 'hda ' /proc/diskstats``. 39 40The advantage of one over the other is that the sysfs choice works well 41if you are watching a known, small set of disks. ``/proc/diskstats`` may 42be a better choice if you are watching a large number of disks because 43you'll avoid the overhead of 50, 100, or 500 or more opens/closes with 44each snapshot of your disk statistics. 45 46In 2.4, the statistics fields are those after the device name. In 47the above example, the first field of statistics would be 446216. 48By contrast, in 2.6+ if you look at ``/sys/block/hda/stat``, you'll 49find just the eleven fields, beginning with 446216. If you look at 50``/proc/diskstats``, the eleven fields will be preceded by the major and 51minor device numbers, and device name. Each of these formats provides 52eleven fields of statistics, each meaning exactly the same things. 53All fields except field 9 are cumulative since boot. Field 9 should 54go to zero as I/Os complete; all others only increase (unless they 55overflow and wrap). Yes, these are (32-bit or 64-bit) unsigned long 56(native word size) numbers, and on a very busy or long-lived system they 57may wrap. Applications should be prepared to deal with that; unless 58your observations are measured in large numbers of minutes or hours, 59they should not wrap twice before you notice them. 60 61Each set of stats only applies to the indicated device; if you want 62system-wide stats you'll have to find all the devices and sum them all up. 63 64Field 1 -- # of reads completed 65 This is the total number of reads completed successfully. 66 67Field 2 -- # of reads merged, field 6 -- # of writes merged 68 Reads and writes which are adjacent to each other may be merged for 69 efficiency. Thus two 4K reads may become one 8K read before it is 70 ultimately handed to the disk, and so it will be counted (and queued) 71 as only one I/O. This field lets you know how often this was done. 72 73Field 3 -- # of sectors read 74 This is the total number of sectors read successfully. 75 76Field 4 -- # of milliseconds spent reading 77 This is the total number of milliseconds spent by all reads (as 78 measured from __make_request() to end_that_request_last()). 79 80Field 5 -- # of writes completed 81 This is the total number of writes completed successfully. 82 83Field 6 -- # of writes merged 84 See the description of field 2. 85 86Field 7 -- # of sectors written 87 This is the total number of sectors written successfully. 88 89Field 8 -- # of milliseconds spent writing 90 This is the total number of milliseconds spent by all writes (as 91 measured from __make_request() to end_that_request_last()). 92 93Field 9 -- # of I/Os currently in progress 94 The only field that should go to zero. Incremented as requests are 95 given to appropriate struct request_queue and decremented as they finish. 96 97Field 10 -- # of milliseconds spent doing I/Os 98 This field increases so long as field 9 is nonzero. 99 100 Since 5.0 this field counts jiffies when at least one request was 101 started or completed. If request runs more than 2 jiffies then some 102 I/O time will not be accounted unless there are other requests. 103 104Field 11 -- weighted # of milliseconds spent doing I/Os 105 This field is incremented at each I/O start, I/O completion, I/O 106 merge, or read of these stats by the number of I/Os in progress 107 (field 9) times the number of milliseconds spent doing I/O since the 108 last update of this field. This can provide an easy measure of both 109 I/O completion time and the backlog that may be accumulating. 110 111Field 12 -- # of discards completed 112 This is the total number of discards completed successfully. 113 114Field 13 -- # of discards merged 115 See the description of field 2 116 117Field 14 -- # of sectors discarded 118 This is the total number of sectors discarded successfully. 119 120Field 15 -- # of milliseconds spent discarding 121 This is the total number of milliseconds spent by all discards (as 122 measured from __make_request() to end_that_request_last()). 123 124Field 16 -- # of flush requests completed 125 This is the total number of flush requests completed successfully. 126 127 Block layer combines flush requests and executes at most one at a time. 128 This counts flush requests executed by disk. Not tracked for partitions. 129 130Field 17 -- # of milliseconds spent flushing 131 This is the total number of milliseconds spent by all flush requests. 132 133To avoid introducing performance bottlenecks, no locks are held while 134modifying these counters. This implies that minor inaccuracies may be 135introduced when changes collide, so (for instance) adding up all the 136read I/Os issued per partition should equal those made to the disks ... 137but due to the lack of locking it may only be very close. 138 139In 2.6+, there are counters for each CPU, which make the lack of locking 140almost a non-issue. When the statistics are read, the per-CPU counters 141are summed (possibly overflowing the unsigned long variable they are 142summed to) and the result given to the user. There is no convenient 143user interface for accessing the per-CPU counters themselves. 144 145Disks vs Partitions 146------------------- 147 148There were significant changes between 2.4 and 2.6+ in the I/O subsystem. 149As a result, some statistic information disappeared. The translation from 150a disk address relative to a partition to the disk address relative to 151the host disk happens much earlier. All merges and timings now happen 152at the disk level rather than at both the disk and partition level as 153in 2.4. Consequently, you'll see a different statistics output on 2.6+ for 154partitions from that for disks. There are only *four* fields available 155for partitions on 2.6+ machines. This is reflected in the examples above. 156 157Field 1 -- # of reads issued 158 This is the total number of reads issued to this partition. 159 160Field 2 -- # of sectors read 161 This is the total number of sectors requested to be read from this 162 partition. 163 164Field 3 -- # of writes issued 165 This is the total number of writes issued to this partition. 166 167Field 4 -- # of sectors written 168 This is the total number of sectors requested to be written to 169 this partition. 170 171Note that since the address is translated to a disk-relative one, and no 172record of the partition-relative address is kept, the subsequent success 173or failure of the read cannot be attributed to the partition. In other 174words, the number of reads for partitions is counted slightly before time 175of queuing for partitions, and at completion for whole disks. This is 176a subtle distinction that is probably uninteresting for most cases. 177 178More significant is the error induced by counting the numbers of 179reads/writes before merges for partitions and after for disks. Since a 180typical workload usually contains a lot of successive and adjacent requests, 181the number of reads/writes issued can be several times higher than the 182number of reads/writes completed. 183 184In 2.6.25, the full statistic set is again available for partitions and 185disk and partition statistics are consistent again. Since we still don't 186keep record of the partition-relative address, an operation is attributed to 187the partition which contains the first sector of the request after the 188eventual merges. As requests can be merged across partition, this could lead 189to some (probably insignificant) inaccuracy. 190 191Additional notes 192---------------- 193 194In 2.6+, sysfs is not mounted by default. If your distribution of 195Linux hasn't added it already, here's the line you'll want to add to 196your ``/etc/fstab``:: 197 198 none /sys sysfs defaults 0 0 199 200 201In 2.6+, all disk statistics were removed from ``/proc/stat``. In 2.4, they 202appear in both ``/proc/partitions`` and ``/proc/stat``, although the ones in 203``/proc/stat`` take a very different format from those in ``/proc/partitions`` 204(see proc(5), if your system has it.) 205 206-- ricklind@us.ibm.com 207