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Forum Discussion
Raid5 Failures HDD MTBF XRaid2 Raid6 Dual Redundancy
Why Raid5(Single Redundancy) is not enough and why Raid6(Dual Redundancy) is better. I was reading this with interest some time ago ..http://www.usenix.org/events/fast07/tech/schroeder/schroeder_ht...
The analysis as presented is misleading. Note the scenario that is graphed is the probability of data loss in a 7 drive array after a single drive failure.
So the RAID-5 protection is already gone in the graph, it is simply attempting to compute the odds that there is another undetected bad sector on one of the remaining drives. There are some assumptions in the data model - particularly that unrecoverable read errors are simply random events that occur every X reads (and are therefore not related to the disk's condition). If that were true, scrubbing a 12 TB raid-5 array would result in (on average) one read failure every time it was done. That is not my experience.
Graphing by year is also attention-getting, but rather silly.
The main takeaways are ones already noted in the forum:
So the RAID-5 protection is already gone in the graph, it is simply attempting to compute the odds that there is another undetected bad sector on one of the remaining drives. There are some assumptions in the data model - particularly that unrecoverable read errors are simply random events that occur every X reads (and are therefore not related to the disk's condition). If that were true, scrubbing a 12 TB raid-5 array would result in (on average) one read failure every time it was done. That is not my experience.
Graphing by year is also attention-getting, but rather silly.
The main takeaways are ones already noted in the forum:
- (a) RAID is not a substitute for backup.
(b) In larger arrays you should consider RAID-6
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