[Dovecot] Dovecot performance under high load (vs. Courier)

Ed W lists at wildgooses.com
Fri Jun 22 00:55:04 EEST 2012

On 21/06/2012 21:37, René Neumann wrote:
> Am 21.06.2012 22:22, schrieb Timo Sirainen:
>> On Thu, 2012-06-21 at 13:05 -0700, email builder wrote:
>>> Do you know what webmails are caching vs. non-caching?
>> Nearly all of them are non-caching. (I don't know of any caching ones.)
> At least roundcube (v0.7.1 here) has some caching options:
> ------------------[excerpt from roundcubes main.inc.php]-------------
> // Type of IMAP indexes cache. Supported values: 'db', 'apc' and 'memcache'.
> $rcmail_config['imap_cache'] = null;
> // Enables messages cache. Only 'db' cache is supported.
> $rcmail_config['messages_cache'] = false;
> -------------------------[end]----------------------------------------
> But I don't know, whether this is the sort of caching you are referring to.
> - René

It is caching, but unless your mysql / memcache server is lower latency 
than your dovecot server, then the caching does very little.

I tested it very briefly and it added a lot of latency to my results 
when adding a mysql cache.  However, my setup has the 
mysql/dovecot/roundcube all on the same machine, so latency is minimal.

Roughly I found that the amount of caching is absolutely massive, eg 
roughly subject headers, message ids and more for every message in every 
folder.  This meant multiple seconds of latency on first login and then 
slight additional latency on every folder view.  I guess this might 
breakeven in the situation of a roundcube installation in an office and 
dovecot on the far end of an ADSL line with 60-100ms+ of latency and 
bandwidth constraints, but it's really, really hard to see it's sensible 
for two machines in the same datacenter with an uncontended network 
connection between them

This isn't to say that the caching isn't sensible for use with other 
mail servers, but I don't see it offers any benefit for most Dovecot 

However, very clever and full featured webmail client!

Ed W

P.S.  Sogo has a kind of caching in that it has a clientside javascript 
cache.  Not what was meant, but for all practical purposes much more 

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