[Linux-cluster] Problem with write performance

[Arturo Gonzalez Ferrer]

Hello,

We've set up recently a rhel 5.4 cluster of 3 nodes for a Moodle
high-availability website, where the sessions and data are share in a GFS2
volume.

We found that, while the read performance have been constantly good, there
is a problem with writes, as the system decrease its peformance after some
conditions. We think that it can be related with our backup procedure:

We do an NFS export of the GFS2 volume from one of the nodes, so that we can
backup the volume every night externally, from a veritas backup client.
After that, we find next morning that the write performance has decreased a
lot, so that it is practically unusable for some big files and for the
operation of cloning an existing course (zip, unzip the data of the course
in a new folder). After some experiments with the writes and clone
operations, we have found a way that improve the issue, but we think that
there should be a better way. What we did was to add the next entry to
crontab in every node:

0-59/10 * * * * sync; echo 3 > /proc/sys/vm/drop_caches

So that the lock caches are cleaned every ten minutes, as we didn't notice
that it affects badly to the performance of the system, and effectivily, it
improves the write performance somehow, at least making it usable.

Do you think this could be an option? Do you have a better explanation for
it? Any other ideas what could we do?

We have been since then having problems with apache service, being stopped
sometimes (not very offen) in one of the nodes. I think that it could be
related to this maintenance of the vm caches... but I'm not sure.

Best regards,
Arturo.
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