[DRBD-user] Significant write performance degredation LVM vs DRBD

Mark Watts m.watts at eris.qinetiq.com
Mon Jan 26 12:28:34 CET 2009


On Friday 23 January 2009 18:13:55 Florian Haas wrote:
> Mark,
>
> On 01/23/2009 05:01 PM, Mark Watts wrote:
> > The following test was used to obtain a write average:
> > # for loop in `seq 1 10`; do echo "Writing to /empty, loop $loop"; dd
> > if=/dev/zero of=/empty bs=$(( 1024 * 1024 * 1024 )) count=4; done
>
> You're not writing to your disk, but to your page/buffer cache. Add
> "oflag=direct".

Ok. We've realised that we don't have the controller batteries either, so 
we're going to get some and re-test.

> Firstly, I repeat, because you hadn't been writing to your disk, but to
> your page cache (i.e. memory). But:
> >     syncer {
> >         al-extents       257;
> >     }
>
> ... is probably way too low for your setup. Try upping that one to 1801,
> maybe as far as 3389.

3389 seems to help a fair bit - its taken the numbers up to ~90MB/sec

> Still, this:
> >     syncer {
> >         rate             100M;
> >     }
>
> ... is likely to kill your foreground I/O performance during any resync,
> even when you've upped your al-extents to the values suggested above.
>
> And of course, you'll never reach your anywhere near your 150 MB/s if
> your replication link is Gigabit Ethernet, as that tops out at 110 MB/s.

Yep, GigE here. Having said that, if everything else is capable/tuned of going 
faster then we should be hitting 100M almost all the time, right?

> These resources may provide additional insights:
> http://www.linbit.com/en/education/on-demand-webinars/drbd-performance-tuni
>ng/ http://www.drbd.org/users-guide/s-configure-syncer-rate.html

Looks like I've found my lunch-time reading :)

Cheers,

Mark.

-- 
Mark Watts BSc RHCE MBCS
Senior Systems Engineer
QinetiQ Applied Technologies
GPG Key: http://www.linux-corner.info/mwatts.gpg
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