Quoting Wojciech Puchar <wojtek@wojtek.tensor.gdynia.pl>:
is random seek latency. And the faster the spindle, the lower the latency. Thus 15k Seagate SAS drives are excellent candidates for mail store duty, as are any 10k or 15k drives. definitely not counting by $/IOPS rate. even worse looking with $/GB
which is more important unless you make <1GB mail account.
It largely depends what type of users your supporting.
One system, I have 300 users, and it have an load on the drives of
500 iops a lot, need 8disk raid10 to support the load, about 300gigs
worth of email.
Another system, has 10's of k of users, storage of about 2tb and
normal usage of about 7k iops.
Both of these systems have a 10gig limit per user account. I find most
of the time users don't max out the mailstore, unless they stop using
that account.
So in both of my cases, $/IOPS is much more important than $/GB, as
the GB just goes to waste.