[Dovecot] Migration questions...

[Timo Sirainen]

On Mon, 2009-05-11 at 15:05 +0000, Richard Hobbs wrote:
> Hi All,
> 
> We are soon to migrate our mail server from one piece of hardware to
> another and we would like to take this opportunity to optimize things.
> As a result, we would like to replace "uw-imapd" and "qpopper" with
> "dovecot". The version we will be installing is 1.1.13-2, as this is
> what is available through the latest Debian stable backports. We will
> also be using exim to deliver mail (through dovecot's deliver mechanism,
> of course).
> 
> So... We are currently using the mbox format with uw-imapd, and would
> like to migrate to the fastest solution possible with dovecot on the new
> hardware. My understanding is that "multi-dbox" is not an option in this
> version anyway, maildir is OK, but not great, and "single-dbox" is
> therefore going to be the highest performing solution. Is single-dbox
> the fastest way to go, considering we're going to be using email in the
> following ways:

Single-dbox is the highest performing, but note that it's not as much
tested as mbox and Maildir code. I think it should work ok, but I'm not
aware of any larger installations using dbox currently. So in case you
find a problem, you might have to upgrade/patch Dovecot to get it fixed
and that would require compiling from sources.

> One problem that may arise is the fact that when we migrate, all msg
> UIDs will be lost. If i'm not mistaken, this means that all emails will
> be treated by the mail client as brand new, and if through IMAP, will
> all go bold, and if through POP3, will all be downloaded again (if still
> on the server) and therefore duplicated in the mail client. If this is
> the case, is there anything we can do to stop this happening? Does the
> "Convert" plugin does this job well?

mbox -> Maildir conversion can preserve both IMAP and POP3 UIDLs using
an external script. Maildir -> dbox conversion can also preserve both,
but that causes Dovecot to use this "hybrid Maildir-dbox format", which
is slower than the full native dbox.

> 8. Configure the "convert" plugin to begin converting the mail to dbox
> format.
> 
> 9. Run something manually (if possible) to convert mailboxes before
> people connect, so the task is already done by the time the outage is over.

There's convert-tool that you could use. I don't know if Debian packages
it.

> Basically, i suppose i'm asking, with the highest performing mailbox
> option, is dovecot going to run faster with 2 individual arrays each
> made from 2 disks, or a single 4-disk array (in which case we'd go
> RAID
> 0+1)?

My guess is that two RAID-1s would be faster, but I haven't really done
any benchmarking. Anyway index files are 10-30% of the mailbox size, so
the index-disks would be using a lot less disk space.

> Also... would it be useful to turn off "atime" when we mount the
> volume(s) or does dovecot rely on this?

Dovecot doesn't rely on atime updates, so turn them off.
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