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

[Tim Chen]

Dear All,

My system has about 30,000 accounts and these accounts are
created as unix system users. Each account has its own entry
in /etc/passwd with different UID and home directory.

We plan to deploy an Layer 4 switch as a load balancer. Proposed
architecure is as the following:

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

Will INDEX stored on disk outperform INDEX 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 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?

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

Looking for your valuable suggestions!
Thanks very much.

Tim Chen