They're going Google's route, which is fine, it's just... a whole different direction than a Sun Thumper. I'd be interested to see how their costs per petabyte stack against Google's.
Google's operation has at most half the disks-per-rack density of Backblaze, but with 4 or 8 times the server density. Google does almost all their computation on the same nodes where the data is stored. Google is also storing each tablet a minimum of 3 times within each cluster, with most systems having multiple replica clusters.
Backblaze's system is going to have several orders of magnitude less bandwidth:
way too many drives in one server (some of them on the PCI bus!)
use of port multipliers (causing 5 drives to share one SATA cable's 300MB/s)
RAID6 with too many drives per array (15-way XOR is no fun)
why use JFS?
only access is via HTTPS, not clear if SSL is done on the pod