[Gluster-users] Is read cache a file cache or a block cache?

[Jon Heese]

Anand,

Thanks for the response.

So am I correctly interpreting your answer as saying that it caches (statically sized) 128KB pages in the configured memory space?

In other words, is it accurate to say that if I configure performance.cache-size to be 1GB, it will store the eight most recently used 128KB pages in the cache?

Thanks again.

Regards,
Jon Heese

On Mar 12, 2015, at 1:20 PM, "Anand Avati" <avati at gluster.org<mailto:avati at gluster.org>> wrote:

The cache works by remembering 128KB "pages" within files. Effectively "blocks" in your terminology.

Thanks

On Wed, 11 Mar 2015 at 12:36 Jon Heese <jonheese at jonheese.com<mailto:jonheese at jonheese.com>> wrote:
Hello,

I have a two-server, two-brick (one brick per server) replicated Gluster
3.6.2 volume, and I'm interested in the 'performance.cache-size' option
and how that read cache works.

My volume currently stores a handful of ~500GB image files, which are
then fed to an iSCSI daemon to serve up datastores and other
miscellaneous iSCSI disks to servers over an iSCSI network.

I have about 14GB of unutilized (minus system cache/buffers) memory on
the gluster servers (which are also the gluster clients, in this case)
which I'd like to utilize to improve the read performance of this volume.

So since my files are are well over the "tens of GB" mark, I'm curious:
Does the Gluster read cache work at the block level -- i.e. caching
*blocks* that are likely to be read -- or does it work at the file level
-- caching *files* that are likely to be read?  Obviously, the latter
might work well for me, but the former is likely not very useful.

I've tried searching around for details on how this works, but short of
diving into the code itself (which is likely beyond my skill level and
time allowance), I haven't been able to find the answer to this question.

If I've misunderstood how any of this is supposed to work, please feel
free to correct me.  Thanks in advance!

Regards,
Jon Heese
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