The disk groups tend to be 20 - 30 TB but they are typically sub grouped in sets of 6 - 11 disks. These sub groups are RAID 5 and then sets of these sets are then organized in a RAID 5 so you get a mesh of disks. You can tolerate several disk failures as you don't get two in the same sub group of disks. If properly laid out you can actually tolerate the failure of an entire shelf as long as no two disks in the shelf are in the same sub grouping. Add in the protection of RAID 6 and you are quite well protected.
Additionally SAN designers are NOT USING SATA except for temporary storage or dedupe backups. They are typically using 300/600 GB Fiber Channel disks in the big SANS. The spindle speed is typically 10,000 or 15,000 RPM for fast rebuilds, fast scrubbing, and fast access.
I assume, Robin, that you are targeting consumer RAID and very small business RAID. Even small business is typically purchasing 10K and 15K enterprise class SAS disks that are on the order of 146 - 300 GB with UREs every 10^15.
The situation is very different in the enterprise where people are typically not storing huge iTunes and movie libraries.
Discussion on:
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