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diff --git a/Documentation/iostats.txt b/Documentation/iostats.txt deleted file mode 100644 index 65f694f2d1c9..000000000000 --- a/Documentation/iostats.txt +++ /dev/null @@ -1,164 +0,0 @@ -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 6 -- # of writes merged - See the description of field 2. -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 |