Commit 9d2e157d authored by Randy Dunlap's avatar Randy Dunlap Committed by Jens Axboe

Documentation/iostats.txt: bit-size reference etc.

- correction that disk stats values are native-word-sized
  32-bit or 64-bit values, not always 32-bi values
- drop "Last modified" entry; use git for that
- fix a few typos
- change "cpu" to "CPU"
Reported-by: default avatarLinda Walsh <lkml@tlinx.org>
Signed-off-by: default avatarRandy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: default avatarJens Axboe <jaxboe@fusionio.com>
parent c4ade94f
I/O statistics fields
---------------
Last modified Sep 30, 2003
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
......@@ -46,11 +44,12 @@ 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 provide
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. Yes, these are
32 bit unsigned numbers, and on a very busy or long-lived system they
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.
......@@ -96,11 +95,11 @@ 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 made the lack of locking
almost a non-issue. When the statistics are read, the per-cpu counters
are summed (possibly overflowing the unsigned 32-bit variable they are
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.
user interface for accessing the per-CPU counters themselves.
Disks vs Partitions
-------------------
......
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