In an effort to catch up on links.
Adam talks about triple parity RAID (raidz3) in an ACM queue article.
When RAID systems were developed in the 1980s and 1990s, reconstruction times were measured in minutes. The trend for the past 10 years is quite clear regardless of the drive speed or its market segment: the time to perform a RAID reconstruction is increasing exponentially as capacity far outstrips throughput. At the extreme, rebuilding a fully populated 2-TB 7200-RPM SATA disk—today’s capacity champ—after a failure would take four hours operating at the theoretical optimal throughput. It is rare to achieve those data rates in practice; in the context of a heavily used system the full bandwidth can’t be dedicated exclusively to RAID repair without adversely affecting performance.
Fifteen years ago, RAID-5 reached a threshold at which it no longer provided adequate protection. The answer then was RAID-6. Today RAID-6 is quickly approaching that same threshold. In about 10 years, RAID-6 will provide only the level of protection that we get from RAID-5 today. It is again time to create a new RAID level to accommodate the realities of disk reliability, capacity, and throughput merely to maintain that same level of data protection.