With exchange, we had all of this expensive, reliable SAN storage that would be perfect for a low requirement glacier like solution. Unfortunately, we lacked the ops mojo to pull it off.
Archive is not about iops. its about streaming bandwidth.
for example I used to look after a quantum iScaler 24 drive robot, each drive was capable of kicking out ~100 megabytes a second. It was more than capable of saturating a 40 gig pipe.
However random IO was shite, it could take up to 20 minutes to get to random file. (Each tape is stored in a caddy of (from memory) 10 tapes, There is contention on the drives, and then spooling to the right place on the tape.)
Email is essentially random IO on a long tale. So, unless your users want a 20 minute delay in accessing last year's emails, I doubt its the right fit.
The same applies to Optical disk packs (although the spool time is much less.)
I think that's the point - the e-mail is using up all of the IOPs. There would be a small amount of IOPs left over that could deal with streaming data. The data is unlikely to be accessed on a regular basis. The data not used by e-mail would then be used for the archive - data that's pretty much write-only.
Why do you care how fast you can read it back when you're storing it for regulatory purposes? Isn't that a sunk cost? Buy high capacity, high reliability and don't care for the read speed?
With exchange, we had all of this expensive, reliable SAN storage that would be perfect for a low requirement glacier like solution. Unfortunately, we lacked the ops mojo to pull it off.