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 4.18+ diskstats: 35 3 0 hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160 0 0 0 0 36 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``. 39 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. 45 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. 61 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. 64 65 Field 1 -- # of reads completed (unsigned long) 66 This is the total number of reads completed successfully. 67 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. 73 74 Field 3 -- # of sectors read (unsigned long) 75 This is the total number of sectors read successfully. 76 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 blk_mq_alloc_request() to __blk_mq_end_request()). 80 81 Field 5 -- # of writes completed (unsigned long) 82 This is the total number of writes completed successfully. 83 84 Field 6 -- # of writes merged (unsigned long) 85 See the description of field 2. 86 87 Field 7 -- # of sectors written (unsigned long) 88 This is the total number of sectors written successfully. 89 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 blk_mq_alloc_request() to __blk_mq_end_request()). 93 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. 97 98 Field 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 might be not accounted in case of concurrent requests. 104 105 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. 111 112 Field 12 -- # of discards completed (unsigned long) 113 This is the total number of discards completed successfully. 114 115 Field 13 -- # of discards merged (unsigned long) 116 See the description of field 2 117 118 Field 14 -- # of sectors discarded (unsigned long) 119 This is the total number of sectors discarded successfully. 120 121 Field 15 -- # of milliseconds spent discarding (unsigned int) 122 This is the total number of milliseconds spent by all discards (as 123 measured from blk_mq_alloc_request() to __blk_mq_end_request()). 124 125 Field 16 -- # of flush requests completed 126 This is the total number of flush requests completed successfully. 127 128 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. 130 131 Field 17 -- # of milliseconds spent flushing 132 This is the total number of milliseconds spent by all flush requests. 133 134 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. 139 140 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. 145 146 Since 4.19 request times are measured with nanoseconds precision and 147 truncated to milliseconds before showing in this interface. 148 149 Disks vs Partitions 150 ------------------- 151 152 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. 160 161 Field 1 -- # of reads issued 162 This is the total number of reads issued to this partition. 163 164 Field 2 -- # of sectors read 165 This is the total number of sectors requested to be read from this 166 partition. 167 168 Field 3 -- # of writes issued 169 This is the total number of writes issued to this partition. 170 171 Field 4 -- # of sectors written 172 This is the total number of sectors requested to be written to 173 this partition. 174 175 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. 181 182 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. 187 188 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. 194 195 Additional notes 196 ---------------- 197 198 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``:: 201 202 none /sys sysfs defaults 0 0 203 204 205 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.) 209 210 -- ricklind@us.ibm.com
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