 | | | I/O Utilization Statistic Strangeness in U5 | I/O Utilization Statistic Strangeness in U5 2005-06-13 - By Ed Greshko
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Tom Sightler wrote:
> Over the weekend we upgraded several of our RHEL3 systems to U5 and
> since then we have noticed that our network monitoring started alerting
> us to high IO utilization values on multiple servers. Upon looking into
> this problem we've found that running 'iostat -x 1' does show 100%
> utilization even though nothing is really going on on the disk. It
> would appear that this might be a bug in iostat, or that something
> changes in /proc/partitions that iostat was using because older versions
> of iostat report even stranger numbers.
>
> Also, if we reboot a system it initially reports normal, but continues
> to grow as IO occurs. This would imply that iostat might be reading the
> wrong information from /proc/partitions.
>
> Anyone else seeing this or is it some uniqueness on our systems?
>
> Running a quick 'iostat -x 1' and see if any of your drives show 100%
> utilization would be an easy way to show it.
Seeing the same thing. It should be noted however that only the drive
itself shows 100%. e.g. /dev/hdc Individual partitions seem to
reporting just fine. In reading the man page for iostat I see:
Because of what seems to be a Linux kernel bug, iostat -x may
display huge I/O response times (svctm) and a bandwidth utilization
(%util) of 100% for some devices. Indeed these devices have a value for
the field #9 in /proc/{partitions,diskstats} which is always
different from 0, and even negative sometimes. Yet this field should go
to zero, since it gives the number of I/Os currently in progress (it
is incremented as requests are submitted, and decremented as they finish).
So, it appears you have stumbled upon a known issue?
--
What if everything is an illusion and nothing exists? In that case, I
definitely overpaid for my carpet.
-- Woody Allen, "Without Feathers"
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