Queue depth on QLogic/3par

Just wondering if anyone has done much with tweaking the queue depth on the
HBA.  I'm running some benchmarks with Oracle's Orion tool
(Redhat5/Opteron/QLogic/3par) and noticed that with the small random I/O
tests the bottleneck really seems to be the queue depth - even with hundreds
of threads iostat reports sub-millisecond service time yet shows long queues
and long wait times.  Since this database being migrated to this new system
is like 95% "sequential read" from the wait events I figure that there will
be a lot of these small I/O requests and it seems worth tuning.

There seems to be a option "ql2xmaxqdepth" on the qlogic driver that
controls the queue depth.  It defaults to 32 but I saw some chatter on the
VMware forums about increasing it to 64.  But I also saw a note for
Solaris/Symmetrix saying something about using a max queue depth less than
or equal to the max queue depth of the LUN (perhaps configured on the SAN?)
to avoid commands getting rejected by a full queue condition.

I'm not sure exactly how all of this works internally - can different HBA's
or SAN's only handle a certain max queue depth?  Has anyone else
experimented with different queue depths?  Does this change something on the
HBA or just in kernel-land?  I think I'm going to try increasing it and
re-run the Orion benchmark to see if I get any errors or performance
difference - but I'm curious what others' experience has been.

-Jeremy

-- 
Jeremy Schneider
Chicago, IL
http://www.ardentperf.com/category/technical

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