zfs discuss - Mar 2010 - zpool iostat / how to tell if your iop bound

What is the best way to tell if your bound by the number of individual
operations per second / random io? "zpool iostat" has an
"operations" column
but this doesn''t really tell me if my disks are saturated. Traditional
"iostat" doesn''t seem to be the greatest place to look when
utilizing zfs.

Thanks,
Chris
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On Mar 10, 2010, at 4:18 PM, Chris Banal wrote:
> What is the best way to tell if your bound by the number of individual
operations per second / random io?
If no other resource is the bottleneck :-)
> "zpool iostat" has an "operations" column but this
doesn''t really tell me if my disks are saturated. Traditional
"iostat" doesn''t seem to be the greatest place to look when
utilizing zfs.
Observe the relationship between iops and asvc_t. You want your 
asvc_t to be as low as possible.  HDDs are not well modeled by a
simple queue, which is what iostat shows.  There are some SSDs,
like the DDRdrive X1, which are well modeled by a simple queue
(and are blazing fast :-).  For HDDs look for asvc_t well below 15 ms
and for SSDs look for asvc_t less than 1 ms.  Note: iostat will only
show 0.1 ms resolution, which doesn''t work so well for fast SSDs :-)

If you want to dig farther, there are some dtrace tools which will show
spatial distribution, size, queue depths, and pretty much anything 
else you can think of.
 -- richard

ZFS storage and performance consulting at http://www.RichardElling.com
ZFS training on deduplication, NexentaStor, and NAS performance
http://nexenta-atlanta.eventbrite.com (March 16-18, 2010)