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.
Configure the "convert" plugin to begin converting the mail to dbox format.
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.