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Diffstat (limited to 'Documentation/admin-guide/iostats.rst')
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diff --git a/Documentation/admin-guide/iostats.rst b/Documentation/admin-guide/iostats.rst index 609a3201fd4e..9453196ade51 100644 --- a/Documentation/admin-guide/iostats.rst +++ b/Documentation/admin-guide/iostats.rst @@ -2,62 +2,39 @@ 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 and upper, 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 - - 4.18+ diskstats: - 3 0 hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160 0 0 0 0 - -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 15 fields, beginning with 446216. If you look at -``/proc/diskstats``, the 15 fields will be preceded by the major and -minor device numbers, and device name. Each of these formats provides -15 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). Wrapping might eventually occur on a very busy -or long-lived system; so applications should be prepared to deal with -it. Regarding wrapping, the types of the fields are either unsigned -int (32 bit) or unsigned long (32-bit or 64-bit, depending on your -machine) as noted per-field below. Unless your observations are very -spread in time, these fields should not wrap twice before you notice it. +The kernel exposes disk statistics via ``/proc/diskstats`` and +``/sys/block/<device>/stat``. These stats are usually accessed via tools +such as ``sar`` and ``iostat``. + +Here are examples using a disk with two partitions:: + + /proc/diskstats: + 259 0 nvme0n1 255999 814 12369153 47919 996852 81 36123024 425995 0 301795 580470 0 0 0 0 60602 106555 + 259 1 nvme0n1p1 492 813 17572 96 848 81 108288 210 0 76 307 0 0 0 0 0 0 + 259 2 nvme0n1p2 255401 1 12343477 47799 996004 0 36014736 425784 0 344336 473584 0 0 0 0 0 0 + + /sys/block/nvme0n1/stat: + 255999 814 12369153 47919 996858 81 36123056 426009 0 301809 580491 0 0 0 0 60605 106562 + + /sys/block/nvme0n1/nvme0n1p1/stat: + 492 813 17572 96 848 81 108288 210 0 76 307 0 0 0 0 0 0 + +Both files contain the same 17 statistics. ``/sys/block/<device>/stat`` +contains the fields for ``<device>``. In ``/proc/diskstats`` the fields +are prefixed with the major and minor device numbers and the device +name. In the example above, the first stat value for ``nvme0n1`` is +255999 in both files. + +The sysfs ``stat`` file is efficient for monitoring a small, known set +of disks. If you're tracking a large number of devices, +``/proc/diskstats`` is often the better choice since it avoids the +overhead of opening and closing multiple files for each snapshot. + +All fields are cumulative, monotonic counters, except for field 9, which +resets to zero as I/Os complete. The remaining fields reset at boot, on +device reattachment or reinitialization, or when the underlying counter +overflows. Applications reading these counters should detect and handle +resets when comparing stat snapshots. 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. |