[Dovecot] keeping indexes in tmpfs

[Timo Sirainen]

On Mon, 2007-04-16 at 09:37 -0400, Justin McAleer wrote:
> While doing some testing with converting accounts while simulating 
> incoming mail load (no other pop/imap processes going but 4 processes 
> converting users), we found that we were maxing out the local disk in 
> the server with the index activity. To find out that it was the index 
> activity, I mounted a tmpfs for dovecot to keep indexes on, and the 
> system load dropped from 70 to 3 :)

How exactly were you converting the users? I guess if the system is
building new index files for tons of users that could take a while.
fsync_disable=yes could help a lot with reducing the disk writes, and
maybe also mmap_disable=yes.

For Dovecot v2.0 I'm planning on reducing at least dovecot.index.log
file sizes. I think currently it uses way too much space when building
the initial indexes.

> Anyway, Timo and others, do you have any thoughts about keeping indexes 
> on a tmpfs partition? I realize the obvious issues with this: running 
> out of space, and the fact that the partition is lost on reboot. 

I think memory would be better used for keeping mailbox data cached
that's actually useful at the time.

Also if you've POP3 users who keep messages in the server and
dovecot.index.cache file is lost, all the messages are read to calculate
the messages' virtual sizes when the user logs in the next time. That
could be expensive.

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