[Gluster-users] IO-Cache does not cache

Olivier Le Cam Olivier.LeCam at crdp.ac-versailles.fr
Sun Apr 11 09:47:09 UTC 2010

Hi -

Michael Schmid wrote:
> When I now clear the debug file, make a pagerefresh, the debug file contains about:
> ~4800 Lines
> Now I remove the IO-Cache and make the same again:
> ~4800 Lines
> So the same count of files are requested over the network. Even if I request the same files again after 10 secounds, there are 4800 new lines. So no Cache at all.
> When I'm now using performance/quick-read it looks like this:
> With Quick-Read:
> ~2500 LInes
> Without Quick-Read:
> ~4800 Lines

Number of lines in the debug file is not (IMHO) a proof of concept. You 
should instead tcpdump the network traffic or monitor the bandwidth usage.

tcpdump -XX -s 0 -i <device> host <one.of.the.server> (for example)

You will see that file content is not fetched the second time you access 
it when io-cache is enabled.

Note that the client will still request some (lots of) information to 
the servers even if the file is actually available in io-cache. This 
unfortunately makes (IMHO) io-cache of little interest when you have to 
deal with small files performance access. There is a talk on the 
glusterfs-devel mailing about this you should have a look to:
(see the "lookup caching" thread).

Stats lookups are mandatory in order not to lose integrity in case 
multiple clients "concurrently" modify the same file.

In my particular use case, the http reverse-proxy in front of my webfarm 
always forwards http requests to the same webserver (except in case of 
failover) so I'm not really concerned by that fact and would rather take 
best advantage of local caches. This is why I have started to make some 
experiments and development around!


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