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docs: iostats: Rewrite intro, remove outdated formats

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The introduction discussed stat file formats for very old kernel versions,
which obscured key information that readers may find useful. Additionally,
the example file contents and the reference to "15 fields" did not account
for the flush fields added in b6866318657 ("block: add iostat counters for
flush requests") [1].

Rewrite the introduction to focus only on the current kernel's disk I/O stat
file formats. Also, clean up wording for conciseness.

Link: https://lore.kernel.org/lkml/157433282607.7928.5202409984272248322.stgit@buzz/T/ [1]

Signed-off-by: David Reaver <me@davidreaver.com>
Link: https://lore.kernel.org/r/20250215180114.157948-1-me@davidreaver.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
David Reaver 2025-02-15 10:01:13 -08:00 committed by Jonathan Corbet
parent 15f738292b
commit 85df12c599
1 changed files with 26 additions and 49 deletions
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75
Documentation/admin-guide/iostats.rst
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@ -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.
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``.
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 using a disk with two partitions::
Here are examples of these different formats::
/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
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
/sys/block/nvme0n1/stat:
255999 814 12369153 47919 996858 81 36123056 426009 0 301809 580491 0 0 0 0 60605 106562
2.6+ sysfs:
446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
35486 38030 38030 38030
/sys/block/nvme0n1/nvme0n1p1/stat:
492 813 17572 96 848 81 108288 210 0 76 307 0 0 0 0 0 0
2.6+ diskstats:
3 0 hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
3 1 hda1 35486 38030 38030 38030
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.
4.18+ diskstats:
3 0 hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160 0 0 0 0
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.
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.
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.