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iostats.txt: update it to cover recent Kernels

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Everything there that it is said for 2.6 also applies on
current 4.x Kernels. So, update the information there.

While here, use ``foo`` for literals.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Mauro Carvalho Chehab 2017-05-14 15:08:22 -03:00 committed by Jonathan Corbet
parent 378012cf68
commit 877b638ff8
1 changed files with 22 additions and 22 deletions
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44
Documentation/iostats.txt
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@ -4,17 +4,17 @@ 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
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
``/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
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::
@ -23,28 +23,28 @@ Here are examples of these different formats::
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:
2.6+ sysfs:
446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160
35486 38030 38030 38030
2.6 diskstats:
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".
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
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
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
``/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
@ -108,7 +108,7 @@ 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
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
@ -117,14 +117,14 @@ 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.
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
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.
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.
@ -163,16 +163,16 @@ to some (probably insignificant) inaccuracy.
Additional notes
----------------
In 2.6, sysfs is not mounted by default. If your distribution of
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::
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
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