[Dovecot] Performance of Maildir vs sdbox/mdbox

[Mark Moseley]

On Wed, Jan 18, 2012 at 9:58 AM, Timo Sirainen <tss at iki.fi> wrote:
> On 18.1.2012, at 19.54, Mark Moseley wrote:
>
>> I'm in the middle of working on a Maildir->mdbox migration as well,
>> and likewise, over NFS (all Netapps but moving to Sun), and likewise
>> with split LDA and IMAP/POP servers (and both of those served out of
>> pools). I was hoping doing things like setting "mail_nfs_index = yes"
>> and "mmap_disable = yes" and "mail_fsync = always/optimized" would
>> mitigate most of the risks of index corruption,
>
> They help, but aren't 100% effective and they also make the performance worse.

In testing, it seemed very much like the benefits of reducing IOPS by
up to a couple orders of magnitude outweighed having to use those
settings. Both in scripted testing and just using a mail UI, with the
NFS-ish settings, I didn't notice any lag and doing things like
checking a good-sized mailbox were at least as quick as Maildir. And
I'm hoping that reducing IOPS across the entire set of NFS servers
will compound the benefits quite a bit.


>> as well as probably
>> turning indexing off on the LDA side of things
>
> You can't turn off indexing with dbox.

Ah, too bad. I was hoping I could get away with the LDA not updating
the index but just dropping the message into storage/m.# but it'd
still be seen on the IMAP/POP side--but hadn't tested that. Guess
that's not the case.


>> --i.e. all the
>> suggestions at http://wiki2.dovecot.org/NFS. Is that definitely not
>> the case? Is there anything else (beyond moving to a director-based
>> architecture) that can mitigate the risk of index corruption? In our
>> case, incoming IMAP/POP are 'stuck' to servers based on IP persistence
>> for a given amount of time, but incoming LDA is randomly distributed.
>
> What's the problem with director-based architecture?

Nothing, per se. It's just that migrating to mdbox *and* to a director
architecture is quite a bit more added complexity than simply
migrating to mdbox alone.

Hopefully, I'm not hijacking this thread. This seems pretty pertinent
as well to the OP.