[Dovecot] Need suggestion regarding index file and Quota settings.

[Timo Sirainen]

On Fri, 2008-07-18 at 15:11 +0800, Tim Chen wrote:
> ---- internet ---- Layer 4 switch ---- mail server1 ---- NFS storage
>                                            |                           |
>                                            |--- mail server2 -----|
> 
> Both mail servers will share the same storage which contains users'
> home with Maildir inside. Dovecot will be installed on the mail servers
> serving pop3 and imap4 protocols.
> 
> Since the layer 4 switch will direct connection to backend mail server
> in random(or some of its algorithm), users can not guarantee which mail
> server to use. 

Would be best if you at least could redirect users from the same IP to
the same server.

> My first question is which kind of INDEX settings should
> I apply?
> 1) INDEX=MEMORY
> 2) INDEX=/nfs/%u
> 3) INDEX=/var/spool/%u

What OS do you use? What kind of IMAP clients are there going to be?
With webmail clients you'll definitely want to use index files. With
Outlook/TB type of clients it might not matter that much.

In any case be sure to use v1.1 and read http://wiki.dovecot.org/NFS

> Will INDEX stored on disk outperform INDEX stored in memory?

Yes, definitely, because it's only temporarily stored in memory.

> If INDEX stored on NFS storage, we can be sure the consistency of INDEX
> file,
> but what about the performance and locking problem?

If fcntl locking is buggy, you can fallback to dotlock files, although
their performance is worse.

> If INDEX stored on local disk, since users may connect to different mail
> servers
> every time. Will that impose the problem of inconsistency? What kind of
> syndrome
> will happen if the INDEX became inconsistent to real maildir content?

Dovecot just updates the index if it's out-of-date. Although I've heard
there have been some issues here with v1.0, but hopefully they're fixed
in v1.1.

> My second question is about quota. We use maildir as mail storage format.
> Our users
> were divided into 3-4 groups, each group of users with different quota size
> ranging
> from 100MB - 1G. To my best knowledge, dovecot's quota setting in config
> file
> supports only "one global configuration". (We are not using db, we use only
> /etc/passwd).
> dovecot.conf
>    quota_rule = *:storage=10MB
> Since dovecot supports both fs and maildir++ quota, I would like to ask for
> your suggestion:
> 
> 1. Should I use system quota instead? How about the comparision of
> performance?
>    Is there any drawback/limitation using system quota?

See http://wiki.dovecot.org/Quota/FS

> 2. Is there any settings of dovecot that could fullfil my situation?

How about generating a passwd-file from /etc/passwd and your own quota
configuration file, and then using:

userdb passwd-file {
  args = /etc/dovecot.passwd
}
userdb passwd {
}

The dovecot.passwd could contain only those users who have non-default
quota value. If user isn't found from there, it fallbacks to passwd.

> 3. My thought is to modify dovecot.conf, comment out quota_rule =
> *:storage=10MB and leave
>    only the quota = maildir in the plugin { block. According to my
> experiment, dovecot
>    will read the maildirsize file under user's home. Then I have to set the
> maildirsize
>    file for every user individually! It seems to be a little crappy but
> works.

That'd work too.
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