[Dovecot] Dovecot write activity (mostly 1.1.x)

[mikkel at euro123.dk]

On Sun, November 4, 2007 4:32 pm, Timo Sirainen wrote:
>>
>> I didn't know that mail_nfs_index=yes resulted in a forced chown.
>> How come that's necessary with NFS but not on local disks?
>>
>
> It's used to flush NFS attribute cache. Enabling it allows you to use
> multiple servers to access the same maildir at the same time while still
> having attribute cache enabled (v1.0 required actime=0). If you don't need
> this (and it's better if you don't), then just set the mail_nfs_* to "no"
> and it works faster.
>
>> By the way I misinformed you about fsync_disable=yes.
>> It was like that before i upgraded to v1.1, but v1.1 requires
>> fsync_disable=no when mail_nfs_index=true so I had to disable it.
>
> So you use ZFS on the NFS server, but Dovecot is using NFS on client
> side? The fsyncs then just mean that the data is sent to NFS server, not a
> real fsync on ZFS.
>

Thanks a lot for the help - this changed a lot!
Dist writes fell to about 1/3 of before.
I guess the reason is that ZFS can now make use if it's caching capabilities.


Deliver’s activity is completely random since it's impossible to load
balance a connection based on the e-mail recipient, since only the ip is
known at the load balancing point.
Therefore I have fsync_disable=no for deliver.

It's easy to force the clients using imap/pop3 to the same server since it
can be based on the ip only.
Therefore I have fsync_disable=yes for imap/pop3.


This changed everything. Now there's a real performance gain upgrading
from 1.0.x to 1.1.x. About two or three times less disk activity overall
(reads were already improved) for both reads and writes.
That’s pretty neat!