Dovecot + Ceph Cluster Sizing

[Michele Soragni]

Hi all. We are searching for information about how to size dovecot servers
in a clustered architecture (dovecot + postfix + roundcube + mysql).
We need to migrate mailboxes from an old Exchange installation:

60k mailbox
900 GB total storage
14 MB/mailbox
About 133 email/mailbox
Low usage: about 2k simultaneous imap/pop3 connections


This is how we are thinking to build our new architecture
3 x Dell R630 or R530 servers with 6x300SAS disks for Ceph storage with 1
replica . Total storage = 2,5 TB for mailboxes

3 x Dell R630 or R530 with 2x CPU 6core,64GB RAM, 4x 300SAS disks hosting
the following VMs in a VMware cluster:
 - 3 x VM Dovecot
 - 3 x VM Postfix
 - 3 x VM Roundcube webmail
 - 3 x VM MySQL

All VMs are behind a balancer
All OS disks are on local storage
Dovecot VMs share Ceph storage with OCFS2 filesystem for the mailboxes.

I'd like to have some hints about CPU and memory for Dovecot VMs. 2 vCPU
and 8 GB RAM each sould be enough? Is there a way to calculate this?
Do you think the storage performance could be a problem using Ceph + OCFS2?
Is Ceph + XFS + Dsync replication a better choice?

Thanks!

Michele