I had a 4 disk software based raid5. Two drives failed at the same time, so I'm at 2/4 operational drives. I have run raid5 with 2 drives before, but this doesn't seem to want to mount. Can you add a spare and rebuild the raid or am I hosed?

Why can raid5 run on two drives, but if two fail from a 4 drive array you can no longer use the array?

  • You have NEVER run a RAID5 with 2 drives, the minimum is 3. If you are under the thought that 2 should work, then you should probably shut everything down, label the drives of their position in the RAID array if you can, and send it out for recovery. – DanBig Oct 26 '10 at 12:13
  • You can run Raid5 w/2drives, in a degraded state. Which is different than what happened to me, raid across 4 drives and one failed. Ironically I had a 5th drive as a spare, but it did not prevent me from losing all data. – Drew Oct 26 '10 at 14:25
  • Degraded, yes. The way you phrased it made it sound like you were running in an optimal state. – DanBig Oct 26 '10 at 18:40
  • Optimal is overkill, I just want to have my data at this point :/ – Drew Oct 26 '10 at 18:56

Sorry, but in RAID5, when two or more drives fail, your array is dead.

To answer your second question: If you start with a 3-drive RAID5 array, and one drive dies, then you've got a degraded (but still alive) array.

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If it is a 4-drive RAID5 (not a 3-drive RAID5 with +1 spare), you are hosed.

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A good description of RAID-levels is on Wikipedia: http://en.wikipedia.org/wiki/RAID#Standard_levels An even better one is in the german Wikipedia, so if you understand german watch there.

For RAID5 as others stated here one drive may fail. Two drives failing destroys the complete RAID and you 're unlucky. And remember: RAID doesn't really replace a Backup anyway!

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Understand that the capacity of one drive (not the physical disk) is given up for parity, checksums if you will to be used for rebuild in case of a failure. So it doesn't matter if you have 3 disks or 10 in your RAID5 set, if you lose two drives simultaneously your array has failed. RAID6 dedicates the capacity of TWO disks to do this, and can suffer 2 failures. A third failure will array will fail.

I believe RAID4 to be very similar to RAID5, except that instead of distributing the parity data across all drives, it uses a dedicated drive. Still, RAID4 cannot suffer two failues.

I myself, prefer RAID10 :)

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  • I prefer RAID10 too, but we just dropped $60k on a 9.6TB Equallogic PS6010VX and it's depressing to get a usable space of < 3TB in RAID10! – Mark Henderson Oct 26 '10 at 5:04
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    3TB? You're going for triple redundancy? I would expect that in most cases, that third level might better be served by a separate appliance with lower cost lower performance backup hardware... You must have some serious availability and performance requirements. – Paul McMillan Oct 26 '10 at 7:47
  • Yeah raid10 is becoming more and more a standard as drive sizes increase at a faster rate than MTBF. raid6 is a good measure upto certain sizes but many controllers do not support it – JamesRyan Oct 26 '10 at 10:00
  • A PS6010XV should give you plenty of options... RAID50 is a good compromise. – SpacemanSpiff Oct 26 '10 at 20:08

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