I am confused about my
caching problem. I’ll try to keep this as
straightforward as possible and include the basic
details...
I have a sixteen node
distributed volume, one brick per node, XFS isize=512,
Debian 7/Wheezy, 32GB RAM minimally. Every brick node
is also a gluster client, and also importantly an HTTP
server. We use a back-end 1GbE network for gluster
traffic (eth1). There are a couple dozen gluster
client-only systems accessing this volume, as well.
We had a really hot spot
on one brick due to an oft-requested file, and every
time any httpd process on any gluster client was asked
to deliver the file, it was physically fetching it (we
could see this traffic using, say, ‘iftop -i eth1’,) so
we thought to increase the volume cache timeout and
cache size. We set the following values for testing:
performance.cache-size
16GB
performance.cache-refresh-timeout:
30
This test was run from a node that didn’t have the
requested file on the local brick:
while(true); do cat /path/to/file > /dev/null;
done
and what had been very high traffic on the gluster
backend network, delivering the data repeatedly to my
requesting node, dropped to nothing visible.
I thought good, problem fixed. Caching works. My
colleague had run a test early on to show this perf
issue, so he ran it again to sign off.
His testing used curl, because all the real front end
traffic is HTTP, and all the gluster nodes are web
servers, which are of course using the fuse mount to
access the document root. Even with our performance
tuning, the traffic on the gluster backend subnet was
continuous and undiminished. I saw no evidence of cache
(again using ‘iftop -i eth1’, which showed a steady 75+%
of line rate on a 1GbE link.
Does that make sense at all? We had theorized that
we wouldn’t get to use VFS/kernel page cache on any node
except maybe the one which held the data in the local
brick. That’s what drove us to setting the gluster
performance cache. But it doesn’t seem to come into
play with http access.
Volume info:
Volume Name: DOCROOT
Type: Distribute
Volume ID: 3aecd277-4d26-44cd-879d-cffbb1fec6ba
Status: Started
Number of Bricks: 16
Transport-type: tcp
Bricks:
<snipped list of bricks>
Options Reconfigured:
performance.cache-refresh-timeout: 30
performance.cache-size: 16GB
The net result of being overwhelmed by a hot spot is
all the gluster client nodes lose access to the gluster
volume—it becomes so busy it hangs. When the traffic
goes away (failing health checks by load balancers
causes requests to be redirected elsewhere), the volume
eventually unfreezes and life goes on.
I wish I could type ALL that into a google query and
get a lucid answer :)
Regards,
Christian
