Message Store design without Solaris+ZFS (Linux)
I would imagine that the vast majority of large sites using Messaging Server (specifically the Message Store) are on some flavor of Solaris and are using ZFS. I would consider a large site to be >10,000 active mailboxes with all access services available, and at least 30% of mailboxes being accessed at any given time.
There are lots of reasons Solaris+ZFS are a nice fit for the store. We've enjoyed years stability and data integrity on various Solaris 10 iterations, both SPARC and x86, with ZFS. Disk IO is not a problem as the SAN supporting the message store is very much over spec'd for the workload.