mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-11-01 17:08:10 +00:00
877b638ff8
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>
178 lines
8.1 KiB
Text
178 lines
8.1 KiB
Text
=====================
|
|
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
|
|
|
|
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
|