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Updated iostats docs 

Previous docs mentioned 11 unsigned long fields, when the reality is that
we have 15 fields with a mix of unsigned long and unsigned int.

Signed-off-by: Albert Vaca Cintora <albertvaka@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
This commit is contained in:
Albert Vaca Cintora 2019-10-16 22:13:37 +02:00 committed by Jonathan Corbet
parent d8fb03e1ea
commit d94cdae138
1 changed files with 24 additions and 23 deletions
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47
Documentation/admin-guide/iostats.rst
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@ -46,78 +46,79 @@ each snapshot of your disk statistics.
In 2.4, the statistics fields are those after the device name. In In 2.4, the statistics fields are those after the device name. In
the above example, the first field of statistics would be 446216. 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 find just the 15 fields, beginning with 446216. If you look at
``/proc/diskstats``, the eleven fields will be preceded by the major and ``/proc/diskstats``, the 15 fields will be preceded by the major and
minor device numbers, and device name. Each of these formats provides minor device numbers, and device name. Each of these formats provides
eleven fields of statistics, each meaning exactly the same things. 15 fields of statistics, each meaning exactly the same things.
All fields except field 9 are cumulative since boot. Field 9 should 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 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 overflow and wrap). Wrapping might eventually occur on a very busy
(native word size) numbers, and on a very busy or long-lived system they or long-lived system; so applications should be prepared to deal with
may wrap. Applications should be prepared to deal with that; unless it. Regarding wrapping, the types of the fields are either unsigned
your observations are measured in large numbers of minutes or hours, int (32 bit) or unsigned long (32-bit or 64-bit, depending on your
they should not wrap twice before you notice them. machine) as noted per-field below. Unless your observations are very
spread in time, these fields should not wrap twice before you notice it.
Each set of stats only applies to the indicated device; if you want 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. system-wide stats you'll have to find all the devices and sum them all up.
Field 1 -- # of reads completed Field 1 -- # of reads completed (unsigned long)
This is the total number of reads completed successfully. This is the total number of reads completed successfully.
Field 2 -- # of reads merged, field 6 -- # of writes merged Field 2 -- # of reads merged, field 6 -- # of writes merged (unsigned long)
Reads and writes which are adjacent to each other may be merged for 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 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) 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. as only one I/O. This field lets you know how often this was done.
Field 3 -- # of sectors read Field 3 -- # of sectors read (unsigned long)
This is the total number of sectors read successfully. This is the total number of sectors read successfully.
Field 4 -- # of milliseconds spent reading Field 4 -- # of milliseconds spent reading (unsigned int)
This is the total number of milliseconds spent by all reads (as This is the total number of milliseconds spent by all reads (as
measured from __make_request() to end_that_request_last()). measured from __make_request() to end_that_request_last()).
Field 5 -- # of writes completed Field 5 -- # of writes completed (unsigned long)
This is the total number of writes completed successfully. This is the total number of writes completed successfully.
Field 6 -- # of writes merged Field 6 -- # of writes merged (unsigned long)
See the description of field 2. See the description of field 2.
Field 7 -- # of sectors written Field 7 -- # of sectors written (unsigned long)
This is the total number of sectors written successfully. This is the total number of sectors written successfully.
Field 8 -- # of milliseconds spent writing Field 8 -- # of milliseconds spent writing (unsigned int)
This is the total number of milliseconds spent by all writes (as This is the total number of milliseconds spent by all writes (as
measured from __make_request() to end_that_request_last()). measured from __make_request() to end_that_request_last()).
Field 9 -- # of I/Os currently in progress Field 9 -- # of I/Os currently in progress (unsigned int)
The only field that should go to zero. Incremented as requests are The only field that should go to zero. Incremented as requests are
given to appropriate struct request_queue and decremented as they finish. given to appropriate struct request_queue and decremented as they finish.
Field 10 -- # of milliseconds spent doing I/Os Field 10 -- # of milliseconds spent doing I/Os (unsigned int)
This field increases so long as field 9 is nonzero. This field increases so long as field 9 is nonzero.
Since 5.0 this field counts jiffies when at least one request was Since 5.0 this field counts jiffies when at least one request was
started or completed. If request runs more than 2 jiffies then some started or completed. If request runs more than 2 jiffies then some
I/O time will not be accounted unless there are other requests. I/O time will not be accounted unless there are other requests.
Field 11 -- weighted # of milliseconds spent doing I/Os Field 11 -- weighted # of milliseconds spent doing I/Os (unsigned int)
This field is incremented at each I/O start, I/O completion, I/O 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 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 (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 last update of this field. This can provide an easy measure of both
I/O completion time and the backlog that may be accumulating. I/O completion time and the backlog that may be accumulating.
Field 12 -- # of discards completed Field 12 -- # of discards completed (unsigned long)
This is the total number of discards completed successfully. This is the total number of discards completed successfully.
Field 13 -- # of discards merged Field 13 -- # of discards merged (unsigned long)
See the description of field 2 See the description of field 2
Field 14 -- # of sectors discarded Field 14 -- # of sectors discarded (unsigned long)
This is the total number of sectors discarded successfully. This is the total number of sectors discarded successfully.
Field 15 -- # of milliseconds spent discarding Field 15 -- # of milliseconds spent discarding (unsigned int)
This is the total number of milliseconds spent by all discards (as This is the total number of milliseconds spent by all discards (as
measured from __make_request() to end_that_request_last()). measured from __make_request() to end_that_request_last()).