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diff --git a/Documentation/iostats.txt b/Documentation/iostats.txt new file mode 100644 index 00000000..c76c21d8 --- /dev/null +++ b/Documentation/iostats.txt @@ -0,0 +1,162 @@ +I/O statistics fields +--------------- + +Since 2.4.20 (and some versions before, with patches), and 2.5.45, +more extensive disk statistics have been introduced to help measure disk +activity. Tools such as sar and iostat typically interpret these and do +the work for you, but in case you are interested in creating your own +tools, the fields are explained here. + +In 2.4 now, the information is found as additional fields in +/proc/partitions. In 2.6, the same information is found in two +places: one is in the file /proc/diskstats, and the other is within +the sysfs file system, which must be mounted in order to obtain +the information. Throughout this document we'll assume that sysfs +is mounted on /sys, although of course it may be mounted anywhere. +Both /proc/diskstats and sysfs use the same source for the information +and so should not differ. + +Here are examples of these different formats: + +2.4: + 3 0 39082680 hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160 + 3 1 9221278 hda1 35486 0 35496 38030 0 0 0 0 0 38030 38030 + + +2.6 sysfs: + 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160 + 35486 38030 38030 38030 + +2.6 diskstats: + 3 0 hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160 + 3 1 hda1 35486 38030 38030 38030 + +On 2.4 you might execute "grep 'hda ' /proc/partitions". On 2.6, you have +a choice of "cat /sys/block/hda/stat" or "grep 'hda ' /proc/diskstats". +The advantage of one over the other is that the sysfs choice works well +if you are watching a known, small set of disks. /proc/diskstats may +be a better choice if you are watching a large number of disks because +you'll avoid the overhead of 50, 100, or 500 or more opens/closes with +each snapshot of your disk statistics. + +In 2.4, the statistics fields are those after the device name. In +the above example, the first field of statistics would be 446216. +By contrast, in 2.6 if you look at /sys/block/hda/stat, you'll +find just the eleven fields, beginning with 446216. If you look at +/proc/diskstats, the eleven fields will be preceded by the major and +minor device numbers, and device name. Each of these formats provides +eleven fields of statistics, each meaning exactly the same things. +All fields except field 9 are cumulative since boot. Field 9 should +go to zero as I/Os complete; all others only increase (unless they +overflow and wrap). Yes, these are (32-bit or 64-bit) unsigned long +(native word size) numbers, and on a very busy or long-lived system they +may wrap. Applications should be prepared to deal with that; unless +your observations are measured in large numbers of minutes or hours, +they should not wrap twice before you notice them. + +Each set of stats only applies to the indicated device; if you want +system-wide stats you'll have to find all the devices and sum them all up. + +Field 1 -- # of reads completed + This is the total number of reads completed successfully. +Field 2 -- # of reads merged, field 6 -- # of writes merged + Reads and writes which are adjacent to each other may be merged for + efficiency. Thus two 4K reads may become one 8K read before it is + ultimately handed to the disk, and so it will be counted (and queued) + as only one I/O. This field lets you know how often this was done. +Field 3 -- # of sectors read + This is the total number of sectors read successfully. +Field 4 -- # of milliseconds spent reading + This is the total number of milliseconds spent by all reads (as + measured from __make_request() to end_that_request_last()). +Field 5 -- # of writes completed + This is the total number of writes completed successfully. +Field 7 -- # of sectors written + This is the total number of sectors written successfully. +Field 8 -- # of milliseconds spent writing + This is the total number of milliseconds spent by all writes (as + measured from __make_request() to end_that_request_last()). +Field 9 -- # of I/Os currently in progress + The only field that should go to zero. Incremented as requests are + given to appropriate struct request_queue and decremented as they finish. +Field 10 -- # of milliseconds spent doing I/Os + This field increases so long as field 9 is nonzero. +Field 11 -- weighted # of milliseconds spent doing I/Os + This field is incremented at each I/O start, I/O completion, I/O + merge, or read of these stats by the number of I/Os in progress + (field 9) times the number of milliseconds spent doing I/O since the + last update of this field. This can provide an easy measure of both + I/O completion time and the backlog that may be accumulating. + + +To avoid introducing performance bottlenecks, no locks are held while +modifying these counters. This implies that minor inaccuracies may be +introduced when changes collide, so (for instance) adding up all the +read I/Os issued per partition should equal those made to the disks ... +but due to the lack of locking it may only be very close. + +In 2.6, there are counters for each CPU, which make the lack of locking +almost a non-issue. When the statistics are read, the per-CPU counters +are summed (possibly overflowing the unsigned long variable they are +summed to) and the result given to the user. There is no convenient +user interface for accessing the per-CPU counters themselves. + +Disks vs Partitions +------------------- + +There were significant changes between 2.4 and 2.6 in the I/O subsystem. +As a result, some statistic information disappeared. The translation from +a disk address relative to a partition to the disk address relative to +the host disk happens much earlier. All merges and timings now happen +at the disk level rather than at both the disk and partition level as +in 2.4. Consequently, you'll see a different statistics output on 2.6 for +partitions from that for disks. There are only *four* fields available +for partitions on 2.6 machines. This is reflected in the examples above. + +Field 1 -- # of reads issued + This is the total number of reads issued to this partition. +Field 2 -- # of sectors read + This is the total number of sectors requested to be read from this + partition. +Field 3 -- # of writes issued + This is the total number of writes issued to this partition. +Field 4 -- # of sectors written + This is the total number of sectors requested to be written to + this partition. + +Note that since the address is translated to a disk-relative one, and no +record of the partition-relative address is kept, the subsequent success +or failure of the read cannot be attributed to the partition. In other +words, the number of reads for partitions is counted slightly before time +of queuing for partitions, and at completion for whole disks. This is +a subtle distinction that is probably uninteresting for most cases. + +More significant is the error induced by counting the numbers of +reads/writes before merges for partitions and after for disks. Since a +typical workload usually contains a lot of successive and adjacent requests, +the number of reads/writes issued can be several times higher than the +number of reads/writes completed. + +In 2.6.25, the full statistic set is again available for partitions and +disk and partition statistics are consistent again. Since we still don't +keep record of the partition-relative address, an operation is attributed to +the partition which contains the first sector of the request after the +eventual merges. As requests can be merged across partition, this could lead +to some (probably insignificant) inaccuracy. + +Additional notes +---------------- + +In 2.6, sysfs is not mounted by default. If your distribution of +Linux hasn't added it already, here's the line you'll want to add to +your /etc/fstab: + +none /sys sysfs defaults 0 0 + + +In 2.6, all disk statistics were removed from /proc/stat. In 2.4, they +appear in both /proc/partitions and /proc/stat, although the ones in +/proc/stat take a very different format from those in /proc/partitions +(see proc(5), if your system has it.) + +-- ricklind@us.ibm.com |