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Hi Christian,<br>
<br>
I have been working on it since couple of days. I have not been
able to recreate the issue. I will continue to recreate and get
back to you in a day or two.<br>
<br>
Regards,<br>
Raghavendra Bhat<br>
<br>
<br>
On 09/02/2015 12:45 AM, Christian Rice wrote:<br>
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<div>This is still an issue for me, I don’t need anyone to tear
the code apart, but I’d be grateful if someone would even chime
in and say “yeah, we’ve seen that too."</div>
<div><br>
</div>
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<blockquote cite="mid:D20B470D.116F5B%25crice@pandora.com"
type="cite">
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<span style="font-weight:bold">From: </span>Christian Rice
<<a moz-do-not-send="true" href="mailto:crice@pandora.com">crice@pandora.com</a>><br>
<span style="font-weight:bold">Date: </span>Sunday, August
30, 2015 at 11:18 PM<br>
<span style="font-weight:bold">To: </span>"<a
moz-do-not-send="true"
href="mailto:gluster-users@gluster.org">gluster-users@gluster.org</a>"
<<a moz-do-not-send="true"
href="mailto:gluster-users@gluster.org">gluster-users@gluster.org</a>><br>
<span style="font-weight:bold">Subject: </span>[Gluster-users]
Gluster 3.6.3 performance.cache-size not working as expected
in some cases<br>
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<div style="color: rgb(0, 0, 0); font-family: Calibri,
sans-serif; font-size: 14px;">
I am confused about my caching problem. I’ll try to keep
this as straightforward as possible and include the basic
details...</div>
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<br>
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<div style="color: rgb(0, 0, 0); font-family: Calibri,
sans-serif; font-size: 14px;">
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.</div>
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<br>
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<div style="color: rgb(0, 0, 0); font-family: Calibri,
sans-serif; font-size: 14px;">
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:</div>
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<br>
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<div><font face="Calibri,sans-serif">performance.cache-size
16GB</font></div>
<div><font face="Calibri,sans-serif">performance.cache-refresh-timeout:
30</font></div>
<div><font face="Calibri,sans-serif"><br>
</font></div>
<div>This test was run from a node that didn’t have the
requested file on the local brick:</div>
<div><br>
</div>
<div>while(true); do cat /path/to/file > /dev/null; done</div>
<div><br>
</div>
<div>and what had been very high traffic on the gluster
backend network, delivering the data repeatedly to my
requesting node, dropped to nothing visible.</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div><br>
</div>
<div>Volume info:</div>
<div>
<div>Volume Name: DOCROOT</div>
<div>Type: Distribute</div>
<div>Volume ID: 3aecd277-4d26-44cd-879d-cffbb1fec6ba</div>
<div>Status: Started</div>
<div>Number of Bricks: 16</div>
<div>Transport-type: tcp</div>
<div>Bricks:</div>
<div><snipped list of bricks></div>
<div>Options Reconfigured:</div>
<div>performance.cache-refresh-timeout: 30</div>
<div>performance.cache-size: 16GB</div>
</div>
<div><br>
</div>
<div>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.</div>
<div><br>
</div>
<div>I wish I could type ALL that into a google query and
get a lucid answer :)</div>
<div><br>
</div>
<div>Regards,</div>
<div>Christian</div>
</div>
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