Linked by Thom Holwerda on Mon 18th Jan 2010 16:57 UTC, submitted by wanker90210
Hardware, Embedded Systems ACM's latest journal had an interesting article about RAID which suggested it might be time for triple parity raid. "How much longer will current RAID techniques persevere? The RAID levels were codified in the late 1980s; double-parity RAID, known as RAID-6, is the current standard for high-availability, space-efficient storage. The incredible growth of hard-drive capacities, however, could impose serious limitations on the reliability even of RAID-6 systems. Recent trends in hard drives show that triple-parity RAID must soon become pervasive."
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RE[2]: RAID Z
by Kebabbert on Tue 19th Jan 2010 09:51 UTC in reply to "RE: RAID Z"
Kebabbert
Member since:
2007-07-27

C) You can mirror your ZFS system disk. That makes it less likely that you loose your production system.

Sure, ZFS has a different license than Linux, so what? ZFS is still open and other OSes use it. If Linux has problem with the CDDL license, so what? Does the whole world revolve around Linux? Why? "If Linux does not support it, it sucks"? Why so ego centric? Use FreeBSD instead than Linux, then. You will get better quality than Linux, and you will get ZFS.

Regarding standards, ZFS is not a standard, but it is open. And you can import your zfs raid into other OSes and export them. Even to CPUs with different endian! Can you do that with an ordinary raid?

And, zfs protects against silent corruption. CERN did a study on 3000 hardware Linux racks, it turned out that 152 of the Linux racks corrupted the data, without even noticing it! CERN noticed this, because they wrote a prespecified bit pattern, and after a short while they noticed differences in the expected bit patterns and the actual raid data. Had they ran the test for longer, they would have seen even more corrupted data! As the CERN guy concludes: ordinary checksums (which raid does) is not enough! You need end-to-end checksums to detect these errors so you can correct them - he suggests zfs as a solution to this problem.
http://storagemojo.com/2007/09/19/cerns-data-corruption-research/

Triple parity is needed because, as disks get bigger, the raids take longer time to repair if a disk breaks. With large disks, it can take over a week to repair the raid! During that time, more stress is on the other drives, so they also break. So two disk parity is not enough - for big drives. If you use small disks, then 1 or 2 disk parity is enough. Triple parity is only needed for large disks > 2TB.

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