From: Rick Lindsley Here is a file to add to the Documentation/ directory which describes the disk statistics fields. 25-akpm/Documentation/iostats.txt | 148 ++++++++++++++++++++++++++++++++++++++ 1 files changed, 148 insertions(+) diff -puN /dev/null Documentation/iostats.txt --- /dev/null Thu Apr 11 07:25:15 2002 +++ 25-akpm/Documentation/iostats.txt Mon May 19 15:51:06 2003 @@ -0,0 +1,148 @@ +I/O statistics fields +--------------- + +Last modified 5/15/03 + +In 2.4.20 (and some versions before, with patches), and 2.5.45, +more extensive disk statistics were 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 most versions of the 2.4 patch, the information is found as additional +fields in /proc/partitions. In 2.5, the same information is found in +two places: one is in the file /proc/diskstats (appears in 2.5.69 and +beyond), 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. In 2.5, 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 + + +2.5 sysfs: + 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160 + 35486 38030 38030 38030 + +2.5 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 2.5, 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 2.5 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 provide +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. Yes, these are +32 bit unsigned 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 issued + 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 request_queue_t and decremented as they finish. +Field 10 -- # of milliseconds spent doing I/Os + This field is 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 release 2.5.65 the 2.5 counters were made per-cpu, which made the lack +of locking almost a non-issue. When the statistics are read, the per-cpu +counters are summed (possibly overflowing the unsigned 32-bit 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 2.5 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 2.5 for +partitions from that for disks. There are only *four* fields available +for partitions on 2.5 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 reads issued + This is the total number of writes issued to this partition. +Field 4 -- # of sectors read + 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. + +Additional notes +---------------- + +In 2.5, sysfs is not mounted by default. Here's the line you'll want +to add to your /etc/fstab: + +none /sys sysfs defaults 0 0 + + +In 2.5, at the same time that disk statistics appeared in sysfs, they were +removed from /proc/stat. In 2.4, they appear in both /proc/partitions +and /proc/stat. + +-- ricklind@us.ibm.com _