MD1000-PERC5/E Performance issues

[Mike McGrath]

On Fri, 2 Jan 2009, Stephen Dowdy wrote:
>
> I am curious about the LVM overhead, but everywhere i look i see handwave
> statements like "LVM adds NO, ZERO, NADA overhead", which sounds like utter
> BS to me.  Now, i doubt it would be larger than 3-5%, though.
>

I've always suspected lvm added only a little bit of overhead but I'd
always worried about alignment isues.

> >>> Ok, upon further testing I don't think the majority of my issue is related
> >>> to the MD1000 or the raid controller.  I'm seeing decent read speeds from
> >>> large files.  So I'll get back to debugging unless someone has a tip.  I
> >>> suspect another list would be better.
>
> > https://www.redhat.com/archives/fedora-infrastructure-list/2008-December/msg00142.html
> > https://www.redhat.com/archives/fedora-infrastructure-list/2009-January/msg00000.html
>
> In one of those threads you performed a 'dd', but w/o a 'bs' option.
> I think 'dd' uses a 512byte default buffer.  Try using 'bs=4096k' or larger.
>

<nod> the 4096 byte size proved much better performance.

> Also, since this is a RedHat related issue (apparently), i definitely
> noticed a 15-20% improvement in I/O going from RHEL4 to RHEL5.
>
> It's always possible that an errant drive is slowing the whole process down.
> Use 'megasasctl -t long a0' (or whatever controller) to initiate some SMART
> diagnostic tests on the drives.  'megasasctl -svv a0' will report success/fail.
> (megactl is on sourceforge)
>
> That wouldn't necessarily jive with your finding that large files are doing
> okay for you, though.
>
> Also, dump the TTYLOG from the PERC controller to make sure it's not reporting
> timeouts and the like.  (megacli -eventlog -getevents -a0  -- or something like
> that)
>
> FWIW, i easily get 500-600MB/s streaming IOZone read/write on MD1000s.
>

Thanks for the tips, I'm hoping to get some downtime scheduled soon to
bring the array down long enough to run some of these tests.  I'll report
back what I find.

	-Mike