[Gluster-users] Gluster 3.6.3 performance.cache-size not working as expected in some cases

[Christian Rice]

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
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