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 100Field 11 -- weighted # of milliseconds spent doing I/Os 101 This field is incremented at each I/O start, I/O completion, I/O 102 merge, or read of these stats by the number of I/Os in progress 103 (field 9) times the number of milliseconds spent doing I/O since the 104 last update of this field. This can provide an easy measure of both 105 I/O completion time and the backlog that may be accumulating. 106 107Field 12 -- # of discards completed 108 This is the total number of discards completed successfully. 109 110Field 13 -- # of discards merged 111 See the description of field 2 112 113Field 14 -- # of sectors discarded 114 This is the total number of sectors discarded successfully. 115 116Field 15 -- # of milliseconds spent discarding 117 This is the total number of milliseconds spent by all discards (as 118 measured from __make_request() to end_that_request_last()). 119 120To avoid introducing performance bottlenecks, no locks are held while 121modifying these counters. This implies that minor inaccuracies may be 122introduced when changes collide, so (for instance) adding up all the 123read I/Os issued per partition should equal those made to the disks ... 124but due to the lack of locking it may only be very close. 125 126In 2.6+, there are counters for each CPU, which make the lack of locking 127almost a non-issue. When the statistics are read, the per-CPU counters 128are summed (possibly overflowing the unsigned long variable they are 129summed to) and the result given to the user. There is no convenient 130user interface for accessing the per-CPU counters themselves. 131 132Disks vs Partitions 133------------------- 134 135There were significant changes between 2.4 and 2.6+ in the I/O subsystem. 136As a result, some statistic information disappeared. The translation from 137a disk address relative to a partition to the disk address relative to 138the host disk happens much earlier. All merges and timings now happen 139at the disk level rather than at both the disk and partition level as 140in 2.4. Consequently, you'll see a different statistics output on 2.6+ for 141partitions from that for disks. There are only *four* fields available 142for partitions on 2.6+ machines. This is reflected in the examples above. 143 144Field 1 -- # of reads issued 145 This is the total number of reads issued to this partition. 146 147Field 2 -- # of sectors read 148 This is the total number of sectors requested to be read from this 149 partition. 150 151Field 3 -- # of writes issued 152 This is the total number of writes issued to this partition. 153 154Field 4 -- # of sectors written 155 This is the total number of sectors requested to be written to 156 this partition. 157 158Note that since the address is translated to a disk-relative one, and no 159record of the partition-relative address is kept, the subsequent success 160or failure of the read cannot be attributed to the partition. In other 161words, the number of reads for partitions is counted slightly before time 162of queuing for partitions, and at completion for whole disks. This is 163a subtle distinction that is probably uninteresting for most cases. 164 165More significant is the error induced by counting the numbers of 166reads/writes before merges for partitions and after for disks. Since a 167typical workload usually contains a lot of successive and adjacent requests, 168the number of reads/writes issued can be several times higher than the 169number of reads/writes completed. 170 171In 2.6.25, the full statistic set is again available for partitions and 172disk and partition statistics are consistent again. Since we still don't 173keep record of the partition-relative address, an operation is attributed to 174the partition which contains the first sector of the request after the 175eventual merges. As requests can be merged across partition, this could lead 176to some (probably insignificant) inaccuracy. 177 178Additional notes 179---------------- 180 181In 2.6+, sysfs is not mounted by default. If your distribution of 182Linux hasn't added it already, here's the line you'll want to add to 183your ``/etc/fstab``:: 184 185 none /sys sysfs defaults 0 0 186 187 188In 2.6+, all disk statistics were removed from ``/proc/stat``. In 2.4, they 189appear in both ``/proc/partitions`` and ``/proc/stat``, although the ones in 190``/proc/stat`` take a very different format from those in ``/proc/partitions`` 191(see proc(5), if your system has it.) 192 193-- ricklind@us.ibm.com 194
