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 15 fields, beginning with 446216. If you look at 50``/proc/diskstats``, the 15 fields will be preceded by the major and 51minor device numbers, and device name. Each of these formats provides 5215 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). Wrapping might eventually occur on a very busy 56or long-lived system; so applications should be prepared to deal with 57it. Regarding wrapping, the types of the fields are either unsigned 58int (32 bit) or unsigned long (32-bit or 64-bit, depending on your 59machine) as noted per-field below. Unless your observations are very 60spread in time, these fields should not wrap twice before you notice it. 61 62Each set of stats only applies to the indicated device; if you want 63system-wide stats you'll have to find all the devices and sum them all up. 64 65Field 1 -- # of reads completed (unsigned long) 66 This is the total number of reads completed successfully. 67 68Field 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. 73 74Field 3 -- # of sectors read (unsigned long) 75 This is the total number of sectors read successfully. 76 77Field 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()). 80 81Field 5 -- # of writes completed (unsigned long) 82 This is the total number of writes completed successfully. 83 84Field 6 -- # of writes merged (unsigned long) 85 See the description of field 2. 86 87Field 7 -- # of sectors written (unsigned long) 88 This is the total number of sectors written successfully. 89 90Field 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()). 93 94Field 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. 97 98Field 10 -- # of milliseconds spent doing I/Os (unsigned int) 99 This field increases so long as field 9 is nonzero. 100 101 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 will not be accounted unless there are other requests. 104 105Field 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. 111 112Field 12 -- # of discards completed (unsigned long) 113 This is the total number of discards completed successfully. 114 115Field 13 -- # of discards merged (unsigned long) 116 See the description of field 2 117 118Field 14 -- # of sectors discarded (unsigned long) 119 This is the total number of sectors discarded successfully. 120 121Field 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()). 124 125To avoid introducing performance bottlenecks, no locks are held while 126modifying these counters. This implies that minor inaccuracies may be 127introduced when changes collide, so (for instance) adding up all the 128read I/Os issued per partition should equal those made to the disks ... 129but due to the lack of locking it may only be very close. 130 131In 2.6+, there are counters for each CPU, which make the lack of locking 132almost a non-issue. When the statistics are read, the per-CPU counters 133are summed (possibly overflowing the unsigned long variable they are 134summed to) and the result given to the user. There is no convenient 135user interface for accessing the per-CPU counters themselves. 136 137Disks vs Partitions 138------------------- 139 140There were significant changes between 2.4 and 2.6+ in the I/O subsystem. 141As a result, some statistic information disappeared. The translation from 142a disk address relative to a partition to the disk address relative to 143the host disk happens much earlier. All merges and timings now happen 144at the disk level rather than at both the disk and partition level as 145in 2.4. Consequently, you'll see a different statistics output on 2.6+ for 146partitions from that for disks. There are only *four* fields available 147for partitions on 2.6+ machines. This is reflected in the examples above. 148 149Field 1 -- # of reads issued 150 This is the total number of reads issued to this partition. 151 152Field 2 -- # of sectors read 153 This is the total number of sectors requested to be read from this 154 partition. 155 156Field 3 -- # of writes issued 157 This is the total number of writes issued to this partition. 158 159Field 4 -- # of sectors written 160 This is the total number of sectors requested to be written to 161 this partition. 162 163Note that since the address is translated to a disk-relative one, and no 164record of the partition-relative address is kept, the subsequent success 165or failure of the read cannot be attributed to the partition. In other 166words, the number of reads for partitions is counted slightly before time 167of queuing for partitions, and at completion for whole disks. This is 168a subtle distinction that is probably uninteresting for most cases. 169 170More significant is the error induced by counting the numbers of 171reads/writes before merges for partitions and after for disks. Since a 172typical workload usually contains a lot of successive and adjacent requests, 173the number of reads/writes issued can be several times higher than the 174number of reads/writes completed. 175 176In 2.6.25, the full statistic set is again available for partitions and 177disk and partition statistics are consistent again. Since we still don't 178keep record of the partition-relative address, an operation is attributed to 179the partition which contains the first sector of the request after the 180eventual merges. As requests can be merged across partition, this could lead 181to some (probably insignificant) inaccuracy. 182 183Additional notes 184---------------- 185 186In 2.6+, sysfs is not mounted by default. If your distribution of 187Linux hasn't added it already, here's the line you'll want to add to 188your ``/etc/fstab``:: 189 190 none /sys sysfs defaults 0 0 191 192 193In 2.6+, all disk statistics were removed from ``/proc/stat``. In 2.4, they 194appear in both ``/proc/partitions`` and ``/proc/stat``, although the ones in 195``/proc/stat`` take a very different format from those in ``/proc/partitions`` 196(see proc(5), if your system has it.) 197 198-- ricklind@us.ibm.com 199