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Which level of RAID will continue to operate correctly even after 2 drives in the array fail? RAID5, RAID6 or RAID10?

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3 Answers 3

up vote 10 down vote accepted

RAID 6 is the only RAID level that will work working under all possible two-drive failure scenarios. RAID 10 can SOMETIMES handle two failures, except when the 2 failed drives are in the same mirrored pair.

So, RAID 6 it is.

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Thanks TomTom. If I have a 4-disk RAID0 system already and I decide to I need better redundancy, would RAID5 or RAID 10 apply in addition to RAID6? –  Barney May 1 '10 at 15:52
    
Not in addition, instead of- Expect significant performance degradation, obviously. –  TomTom May 1 '10 at 16:08
    
@Barney: Not on the same four drives, though you can nest RAID arrays (so a RAID0 of RAID6 if you had 8 drives - RAID 10 is often implemented as a nest with a RAID0 of RAID1s). The choice between 6 and 10 comes down to redundancy and write performance as both would give two drives worth of space. The write performance of RAID6 can be bad for many applications (you don't say what you are using the machine for) but it will survive any two drives being dead at once. RAID10 will perform better for writes but can only survive four of the six possible two-drives-dead-at-once situations. –  David Spillett May 1 '10 at 16:14
    
So given my current 4 disk RAID0 setup, if I decide to use add additional disks (more than my current 4 disks), then RAID6 and RAID10 would continue correct operation in case of two disk failure. Correct? RAID5 won't. Do I understand correctly? –  Barney May 1 '10 at 16:38
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Then head over to en.wikipedia.org/wiki/Standard_RAID_levels#RAID_4. Read it, then you understand all options. And no, rRAID 5 does not work - RAID 5 specifically handles only ONE failure. RAID 6 specifically handles two. –  TomTom May 1 '10 at 16:50

RAID6, or RAID5 with a Hot Spare. I recommend 6

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On of our clients runs Raid 5 with a 6 disk setup. The 6th being a hot swap, that way two drives can fail, the hot swap does it's job, leaving time for the two "failed" disks to be replaced. Rare that you would have two disks fail, but it can happen.

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On large-capacity disks, it is shockingly common for a failure to occur during an attempted RAID 5 rebuild. –  Skyhawk Dec 1 '11 at 4:37

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