[Dovecot] performance of maildir on ocfs2

[karavelov]

On 26.04.2010 14:37, mailinglists at belfin.ch wrote:
> Hi,
>
> I would like to run my imap service on a active-active cluster. I wonder
> how well OCFS2 performs in read and write with millions of smallest files
> involved. Has anybody got any experience?
>
> Thanks
> John
>    

I have some experience.

I have done some test and benchmarks here for OCFS2 and NFS4. My
understanding is that GFS2 limitations and performance are similar to
OCFS2's, so I have not included it in the tests. My benckmarks showed
that OCFS2 has an order of magnitude better performance for IMAP
server (for benchmarks I have used imaptest).

Though, there are some concerns:

1. For better performance you would like to stick one account to one
server. If one account is checked concurrently from different nodes
of OCFS2 cluster they invalidates each-other the direcotory cache for
the Maildir. It result in a lot more IO. I use nginx for load balancing
incomming pop3/imap connections if front of dovecot servers.

2. OCFS2 has a limit for 32k files per directory. There is directory index
support in recent kernels (> 2.6.33) that will remove this limit, but the
userland tools are not yet ready for production (not in the master
branch of  ocfs2-tools)

3. My understanding is that OCFS2 uses a global lock for move/rename.
As you know, Maildir format uses a lot of such operations. I think that
dbox format (dovecot native) will be better choice, because there are
no file moves/renames. I am planning migration to dbox now. If I have
to start the service now, I would choose dbox for mail storage.

4. Filesystem quota. OCFS2 has support in recent kernels. It has 
integrated in
ocfs2-tools master branch but there is not yet release of the tools. So I
do not hurry to push it in production.

I hope it hepls
Best regards


-- 
Luben Karavelov
Research and development
Spectrum Net JSC

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