[Dovecot] Performance of Maildir vs sdbox/mdbox

[Lee Standen]

Out of interest, has the NFS issue been tested on NFS4?  My 
understanding is that NFS4 has a lot of fixes for the locking/caching 
problems that plague NFS3, and we were planning to use NFS4 from day 
one.

If this hasn't been tested, is there some kind of load simulator that 
we could run to see if the issue does occur in our environment?


On 18.01.2012 21:54, Timo Sirainen wrote:
> On Wed, 2012-01-18 at 20:44 +0800, Lee Standen wrote:
>
>> I've been desperately trying to find some comparative performance
>> information about the different mailbox formats supported by Dovecot 
>> in
>> order to make an assessment on which format is right for our 
>> environment.
>
> Unfortunately there aren't really any. Everyone who seems to switch 
> to
> sdbox/mdbox usually also change their hardware at the same time, so
> there aren't really any before/after metrics. I've of course some
> unrealistic synthetic benchmarks, but I don't think they are very
> useful.
>
> So, I would also be very interested in seeing some before/after 
> graphs
> of disk IO, CPU and memory usage of Maildir -> dbox switch in same
> hardware.
>
> Maildir is anyway definitely worse performance then sdbox or mdbox.
> mdbox also uses less NFS operations, but I don't know how much faster
> (if any) it is with Netapps.
>
>> * All mail storage presented via NFS over 10Gbps Ethernet (Jumbo 
>> Frames)
>>
>> * Postfix will feed new email to Dovecot via LMTP
>>
>> * Dovecot servers have been split based on their role
>>
>>   - Dovecot LDA Servers (running LMTP protocol)
>>
>>   - Dovecot POP/IMAP servers (running POP/IMAP protocols)
>
> You're going to run into NFS caching troubles with the above split
> setup. I don't recommend it. You will see error messages about index
> corruption with it, and with dbox it can cause metadata loss.
> http://wiki2.dovecot.org/NFS http://wiki2.dovecot.org/Director
>
>>   - LDA & POP/IMAP servers are segmented into geographically split 
>> groups
>> (so no server sees every single mailbox)
>>
>>   - Nginx proxy used to terminate customer connections, connections 
>> are
>> redirected to the appropriate geographic servers
>
> Can the same mailbox still be accessed via multiple geographic 
> servers?
> I've had some plans for doing this kind of access/replication using
> dsync..
>
>> * Apache Lucene indexes will be used to accelerate IMAP search for 
>> users
>
> Dovecot's fts-solr or fts-lucene?
>
>> Our closest current live configuration (Qmail SMTP, Courier IMAP, 
>> Maildir)
>> has 600K mailboxes and pushes ~ 35,000 NFS operations per second at 
>> peak
>>
>> Some of the things I would like to know:
>>
>> * Are we likely to see a reduction in IOPS/User by using Maildir 
>> alone under
>> Dovecot?
>
> If you have webmail type of clients, definitely. For 
> Outlook/Thunderbird
> you should still see improvement, but not necessarily as much.
>
> You didn't mention POP3. That isn't Dovecot's strong point. Its
> performance should be about the same as Courier-POP3, but could be 
> less
> than QMail-POP3. Although if many of your POP3 users keep a lot of 
> mails
> on server it
>
>> * If someone can give some technical reasoning behind why mdbox does 
>> less
>> IOPS than Maildir?
>
> Maildir renames files a lot. From new/ -> to cur/ and then every time
> message flag changes. That's why sdbox is faster. Why mdbox should be
> faster than sdbox is because mdbox puts (or should put) more mail 
> data
> physically closer in disks to make reading it faster.
>
>> I understand some of the reasons for the mdbox IOPS question, but I 
>> need
>> some more information so we can discuss internally and make a 
>> decision as to
>> whether we're comfortable going with mdbox from day one.  We're very
>> familiar with Maidlir, and there's just some uneasiness internally 
>> around
>> going to a new mail storage format.
>
> It's at least safer to first switch to Dovecot+Maildir to make sure 
> that
> any problems you might find aren't related to the mailbox format..