[Dovecot] Dovecot tuning for GFS2

[Stan Hoeppner]

On 8/23/2013 3:30 AM, Jan-Frode Myklebust wrote:
> On Thu, Aug 22, 2013 at 08:57:40PM -0500, Stan Hoeppner wrote:

>> 130m to 18m is 'only' a 7 fold decrease.  18m inodes is still rather
>> large for any filesystem, cluster or local.  A check on an 18m inode XFS
>> filesystem, even on fast storage, would take quite some time.  I'm sure
>> it would take quite a bit longer to check a GFS2 with 18m inodes.  
> 
> We use GPFS, not GFS2. 

Understood.  But it makes little difference.  None of the cluster
filesystems perform very well with high metadata workloads or extremely
high inode counts, whether using OCFS, GFS, GPFS, CXFS, etc.

> Luckily we've never needed to run fsck on it, but
> it has support for online fsck so hopefully it would be bareable (but
> please, lets not talk about such things, knock on wood).

I'm not that familiar with the GPFS tools.  It may be able to run an
online check but I'd bet you have to unmount it to do a destructive
repair, as with most filesystems, cluster or not.

>> Any reason you didn't go a little larger with your mdbox rotation
>> size?
> 
> Just that we didn't see any clear recommendation/documentation for
> why one would want to switch from the default 2MB. 2 MB should already
> be packing 50-100 messages/file, so why are we only seeing 7x decrease
> in number of files.. 
...
> Hmm, I see the m-files isn't really utilizing 2 MB.
> Looking at my own mdbox-storage I see 59 m-files, using a total of 34MB
> (avg. 576KB/file)-- with sizes ranging from ~100 KB to 2 MB.  Checking our
> quarantine mailbox I see 3045 files, using 2.6GB (avg. 850KB/file).

Apparently 2MB is approximate.  I'd guess if a new msg comes in that'll
put the m-file over the limit, the file is closed, a new one started,
and the new mail goes into the new file, leaving the current (previous)
file at less that the rotate size limit.  Timo will need to give the
definitive answer.

> Guess I should look into changing to a larger rotation size.
> 
> BTW, what happens if I change the mdbox_rotate_size from 2MB to 10MB?
> Will all the existing 2MB m-files grow to 10MB, or is it just new
> m-files that will use this new size? Can I get dovecot to migrate out of
> the 2MB files, and reorganize to 10MB files ?

I'd guess existing m-files will remain as they are.  The rotation logic
acts on the currently open and not yet full file.  This is a serial
operation, only forward, not back.  Again, Timo should have a definitive
answer.

-- 
Stan