[Dovecot] v1.2 development tree started

Ed W lists at wildgooses.com
Fri Jun 20 15:39:42 EEST 2008

>> Can I make a very weak suggestion to look at that ZLIB compression 
>> extension I think you mentioned in the past?
> It would have to be done by proxying in imap-login similar to how SSL 
> connections are handled. But aren't you using SSL already, and why 
> not? Using that would give compression for free. Although I haven't 
> really looked at if it's already automatically enabled or if I or 
> clients should do something special..

I don't think that SSL in general has compression enabled?  Could be 
wrong, but I believe it's a option, but badly supported?  I'm not an 
expert though so I don't know that for sure...  I would be interested if 
someone had a recipe for enabling compression on TLS?

Also if you use SSL then you can no longer do after the fact 
compression.  By definition, encryption done well produces an output 
which cannot be compressed.  So it's even more important to precompress 
before encryption

>> Anyway, just a thought - I'm assuming that the probable 
>> implementation is going to be fairly simple.  I would think that zlib 
>> and/or lzo would be good compressors if there is a choice of 
>> implementations?  Certainly LZO would be a good choice for faster 
>> 100mbit connections
> http://www.ietf.org/rfc/rfc4978.txt specifies DEFLATE format that can 
> be implemented using zlib.

I think this is probably what you referenced before.

My own experience is using a very powerful (cpu hungry) compressor where 
it doesn't seem to matter all that much if stuff is base64 encoded or 
not.  Long shot is that whilst all that reflushing sounds really nice I 
think it's just icing compared with just doing blind compression of 

My guess is that with the replication stuff you are going to see a 
5x-10x speedup on exchanging long lists of guids to compare folders, 
etc.  Compression on the actual mailbodies may be much less.  In my case 
even with an incompressible jpg file which is base64 encoded we still 
knock off the expected 1/3 in file size due to the base64 encoding so 
it's a nice benefit

(My customers are on dialup connections of just 2,400 baud... ie 20KB 
per *minute*  http://www.mailasail.com )

For replication I would have thought support of an optional non RFC LZO 
compressor would be beneficial on anything under gigabit links..?

Ed W

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