dovecot replication (active-active) - server specs

[Martin Schmidt]

Hello,
 
i have some questions about the new dovecot replication and mdbox format.
 
my company has currently 3 old dovecot 2.0.x fileserver/backend with ca. 120k mailboxes and ca. 6 TB data used.
They are synchronised per drbd/corosync.
Each fileserver/backend have ca. 40k mailboxes im Maildir format.
 
Our MX server is delivering ca. 30 GB new mails per day.
Two IMAP proxy server get the connections from the users. Atm. without dovecot director.
We've got around 700k connections per day (imap 200k / pop3 500k)
 
The system is getting issues because the fileserver still have old slow HDDs.
Users sometime get connection timeouts, because the fileserver can not answer fast enough due to I/O waiting lag.
 
So we want to make a new system.
We desire the new system to use mdbox format ( bigger files, less I/O)
and replication through dovecot replication (active/active) instead of drbd.
Each fileserver should know every mailbox/user and for the time being 2 dovecot proxies for the user connections (IMAP/POP).
(later after the migration from the old system to the new, dovecot director instead of proxies, for caching reasons).

we've got 2 new fileservers, they have each SSD HDDs for "new-storage"
and 7200rpm SATA HDDs on RAID 5 with 10 TB for "alt-storage"
32 GB RAM per Server

Do you have some tips for the system?
Do you believe 32 GB RAM are enough for one fileserver each and have you experience with the I/O Waiting problem with huge amounts of Data on the alt-storage?
Could there be issues with the RAM, if one fileserver has a downtime, so the second one has to take over all mailboxes for a short amount of time?
 
In general are only 2 new fileserver enough or should we think in bigger dimensions, like 4 fileserver
Storage expansion in the new servers should not be a problem (bigger HDDs and a few slots free, so we can expand the raid 5).
 
 
thank you
kind regards
 
Martin Schmidt