support wrote:
> We have users whose folders are between 100 MB and 800 MB in size.
> Most of those users are using Outlook but some are using Thunderbird.
>
---
Not to negate anything Mark has stated about large email folder. I
generally keep my mboxes below 100MB except for archives, however,
comparing performance on the same machine, so I don't have a great deal
of experience with mboxes of such size. I've generally tuned my mboxes
for the size of my 'server machine'. On older machines, 10-20MB was the
limit of my comfort level mbox size. My current server can handle up to
100MB or more without much inconvenience, but I wouldn't want to go much
beyond that for my purposes.
What I can speak about is improving performance on a particular
piece of hardware. My current distro, SuSE11.2, sad to say, gave me a
large hiccup when I last upgraded from 11.1->11.2. One of aspects of
this hiccup was drop of inclusion of imap-gw as a default inclusion (not
a major hangup, I've built it before, could again). They tried to set
me up with cyrus -- which seemed rather unwieldy in comparison, not to
mention, I don't like the idea of giving up my 'mbox' format (works with
too many text searches when I'm logged into my server). So I looked
around. I know there could be those who might think me a traitor for
giving up so quickly, but not having tried anything other than umap-gw
for the past ...geez, 5-10 years, I might be a bit naive about the
landscape.
What I stumbled upon was 'dovecot' (goog: 'dovecot imap'). It
supports the same formats as the imap-gw(mbox, and mail format, the
differences between I never fully groked), as well as the format used by
cyrus. It includes built in search capabilities that have enabled my
mozilla-Tbird on Windows-client, -based searches, even on unretrieved
bodies, to go VERY fast -- near the speeds of me logged in using grep.
It's ability to handle multiple mboxes and connections is also
impressive as well as it's attention to security.
Under 'imap-gw', I generally used 2-4 connections max, to imap-gw,
othersize, I ended up with too many server-dropped messages (I have
~70-80 mboxes that are checked as active receive points for new mail --
not just an Inbox). So now I run with about 16 connections
concurrently, and my email checks don't begin to bog down in my imap
server, I could use more threads, but with my disk-subsystem, it doesn't
make sense (I'd need more 2nd-level-read spindles offered from
RAID50/60).
It uses separate threads for each connection, and seems to have no
problem putting the load on the disk. In short, it looks like 'dovecot'
might be able to be recommended, as a imap-gw compatible replacement if
imap-gw isn't working for you, as is. imap-gw is a workhorse with an
excellent track record, so I wouldn't move if you don't need to.
Dovecot, though, seems to handle larger numbers of concurrent
connections more reliably than I experienced with imap-gw (but I'm not a
typical email customer). That said, with 800MB mboxes, you need a good
wide first level RAID (0, 5, 6), and for multiple clients, or high
numbers of connections, a good RAID50/60 with which I believe the
2nd-level '0' goes straight to number of concurrent I/O's it can support
in parallel.
No matter the client -- the backend of the client is vital. I use xfs
(no-barriers with battery backed-up UPS for the server), to get the best
performance for read/write on large files.
Hope this isn't taken as stepping on anyone's fingers -- and maybe Mark
could look at dovecot and give a thumbs up/down for anyone else -- I'd
trust his judgment on imap servers/clients over mine in a heartbeat. My
experience is often 'unique', to my setup and workload. Just wanted to
share it as I sorta got pushed to try something else and didn't want to
give up my mbox format. Dovecot is what I ended up with.
Linda Walsh
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