[Dovecot] Overly long email of miscellaneous Dovecot migration questions

[Mark Moseley]

Hello to the list! I've been asked to spec out the feasibility and, if
feasible, plan a migration from Courier to Dovecot for both POP3 and
IMAP for about 4 million mailboxes. I've been trying to absorb all the
dovecot-related info I could over the past couple of weeks from the
docs and from the list and I'm sure that I've muddled at least some of
it, so apologies in advance if I make grossly incorrect assumptions.

BTW, sorry for the giant email but I've got a bunch of questions. This
is the sort of overly wordy email that makes my eyes glaze over in
mailing lists. But I've got things configured to where (I think)
everything will 'just work' if we migrate (barring running POP3
mailbox conversions), so I'm not looking for configuration help, just
gotchas and assumption-checking mostly. Me breaking 4 million
mailboxes isn't really a good move for my job security ;)

* Since Dovecot 2.0 seems like it's just around the corner, that's all
I've been testing, and indeed all I've even looked at.

* This is a pretty fantastic piece of software. I love the neat little
details that make admins' lives easier, e.g. how easy it is to use
multiple SSL certs in the same server. That made me smile. I hadn't
looked at dovecot in a number of years, so I was really impressed at
the full feature set. It's been a while since I've played with a new
server application that I've enjoyed this much.

* Background: All of our mail is stored on NFS and will be for the
foreseeable future, all in maildir format. Courier POP3/IMAP runs on
load balanced Linux servers, so clients will be hitting multiple
servers, though stickiness based on client IP address is currently
done. Webmail as well is done on top of a local Courier IMAP server
accessing the same NFS servers but on an entirely different pool of
servers. All authentication is done against SQL. Courier has been
very, very good to us over the past 9 years, but our NFS servers (12
Netapps) are unfortunately being beaten to death. We've got lots of
users with thousands of emails in their inbox (i.e. in ./Maildir/cur/
alone) and I've seen plenty of mailboxes with 10's of thousands in
their inbox and I've even seen some really scary ones approaching 100k
emails just in cur/. Thanks to an endless cycle of industry raising
mailbox limits, we actually have to support this (though not so much
the 100k message ones, at least).

* Our #1 main motivation for looking Dovecot is relief for our
currently overtaxed NFS servers, mostly in the form of the index
files. Benchmarking dovecot looks great, even with the index files in
the maildir. I know the cool thing to do would be to move the index
files to a separate NFS server running entirely on SSDs for the
fastest accesses possible. Building a separate index NFS cluster
obviously adds complexity and performance with the index files in the
maildir seems pretty great already. But I also know that real life
often makes assumptions made during benchmarks look very foolish. My
inclination is to try things out with the indexes in the maildir
directory (and then start looking at moving to a dedicated NFS server
for the indexes in a subsequent phase) but I don't want to regret that
choice later. Anybody with large-scale NFS maildir stores have any
advice either way?

* Exim: We currently deliver all of our mail via Exim on separate
servers. Our POP3/IMAP servers only do POP3/IMAP and the Exim mail
servers delivering to maildirs only do Exim. From what I've seen in
the docs and various threads, from what I can gather, the best thing
to do in that case would be to use Exim's built-in maildir handling,
instead of using 'deliver'. That would be my preference anyway, but I
wanted to make sure I didn't misinterpret things.

* Any problems running Courier POP3 and Dovecot IMAP for a while,
possibly Courier IMAP and Dovecot IMAP concurrently?: Since migrating
POP mailboxes is going to be a mighty task, our gameplan would
probably be to start migrating *just* IMAP services at first and
likely on just a few servers at a time. Being load balanced, there are
several scenarios where a user could theoretically access a mailbox on
different servers and therefore end up hitting dovecot at one point in
time and then later hitting courier or vice versa. Or they could use
IMAP from one location and POP3 (during the span of time before we
migrated our POP3 services) from another. I've not yet seen any side
effects from switching mail servers like that (though I'm aware of
what would happen if both dovecot and courier *POP3* were in service
at the same time -- redownloaded mail, bleh). Has anyone seen
otherwise? I'm not worried as much by the 0.1% of IMAP users who might
be using advanced features but rather the other 99.9% who are using it
pretty vanilla-ishly.

* Union mailboxes: I'm pretty sure in a fairly recent thread that Timo
said that something like a 'union' mailbox (at least with maildir)
wasn't possible. I tried messing with multiple 'private' namespaces
(i.e. a namespace called "ARCHIVE" with a "location" different than
the INBOX location, ideally placed on slower but denser NFS servers)
but even with 'hidden=no' and 'list=yes', only the main INBOX folders
would show up, so I'm guessing that's not going to work. That would be
a killer feature, to be able to serve an alternate namespace that
would show up in a mail client's subscribable list that could be on
slower storage than the main inbox (though I'm not sure a mail client
can even handle multiple namespaces).

* Any problems with keeping only quota limits in db and not current
quota numbers? Our limits come out of a SQL table but  the current
counts just live in the maildir file. Trying to update quota counts in
SQL for 4 million mailboxes is a non-starter for us at this point.
>From what I've done, dovecot seems pretty happy to maintain the quota
using the 'maildir' quota plugin but let it be overridden by the
userdb lookup. So I think I'm fine on that count, but I'd love to hear
from anyone knowing of extreme gotchas doing it that way.

* Any problem with leaving the namespace in "Courier compatibility"
mode? I.e. in namespace 'private', leaving "prefix = INBOX.". With 4
million mailboxes, FAQs all over the place, and support reps trained
in a particular way of doing things in IMAP, it'd be hellish to try to
change the prefix (I know I could leave the courier namespace around
with 'hidden=yes' but retraining support staff is perhaps better left
to phase #2). Do I lose anything besides tidiness by not changing it
to "prefix =" as if I was deploying dovecot from scratch? Does it hurt
performance in any significant way? Benchmarking doesn't look any
different, so I'm guessing not.

* One thing that threw me and might be good for a FAQ (unless it was
just me misconfiguring things) was when I started playing with putting
the index files in an alternate location. I was utterly perplexed why
it'd create the directories for the indexes but they'd be empty. Based
on their location and names when they're in the maildir, I was just
looking for the same dovecot.index* files right in the alternate
directory. It wasn't till I started strace'ing that I noticed that the
index files were indeed getting created but in a subdirectory called
.INBOX (and with me just doing 'ls').

* The courier-dovecot-migrate.pl does a fine job at converting our
POP3 mailboxes. We'll definitely be doing a mass run of it across all
of our mailboxes. But with this many mailboxes, there's no way to get
around the time lag between the script getting run and new mail
showing up. Which means that there'll be some number of new messages
that will re-download in POP3 when the switchover to Dovecot POP3 is
done. To keep from getting too many complaints from customers, I'm
thinking that I might have to write a wrapper script to do something
like compare the mtime of ./Maildir/new/ against dovecot-uidlist and
overwrite if ./Maildir/new/ is newer or perhaps see if the mtime of
courierpop3dsizelist is newer than dovecot-uidlist and only if that's
the case execute the conversion script with --overwrite. Considering
we get between 500-2000 POP3 logins/sec, running any script after a
POP3 login terrifies me--even a 6 line bash script. Anybody have any
opinions to share on that? With a pooled architecture, it's near
impossible to only have some mailboxes hit Courier and some hit
Dovecot and keep slowly incrementing. At best my available units of
increments to migrate are a few thousand at a time and more
realistically 10-20k at a time. If we're going to have to live with
users complaining about a one-time redownload of just post-conversion
mail, I'll need to get started convincing the higher-ups that that's
life.

A big thanks to anyone who even actually reads this entire tedious
email and a tremendous thanks to anyone who actually replies to any of
it!