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Someone knows what happens if the parity disk fails simultaneously with a data disk? It is supposed to handle double disk failure but since one parity disk fails it seems impossible. Does anybody have an answer? Thanks

G.Bar
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    You need to provide a bit more info: What kind of RAID setup? Number of drives, RAID level, controller, etc. Different RAID configurations can survive different numbers of drive failures. – Michael Kohne Jul 06 '17 at 16:08
  • If you need to read up on RAID a bit, [Wikipedia's article on the subject](https://en.wikipedia.org/wiki/RAID) isn't a bad starting point. – Michael Kohne Jul 06 '17 at 16:09
  • Let's say its raid dp with five disks. Disks 1-3 are data , 4 is parity and 5 is dp. So if disks 5 and 1 fail, can I retrieve my data? Since one of the parities is down it seems impossible ,unless parity info excludes the parity disks themselves, meaning that disk 4 doesn't include disk 5 in the parity calculation and vice versa. – G.Bar Jul 06 '17 at 16:15
  • RAID DP is a NetAPP specific technonolgy. http://gestaltit.com/exclusive/devang/netapps-raiddp-enhanced-raid-6 has more information on how it's implemented. If you dedicate two disks from the set to parity, you can lose any two. – quadruplebucky Jul 06 '17 at 20:50

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The 'DP' bit means Double Parity - and in this case will generally help to ensure your data is safe.

In the most basic Aggregate you need;

1 x Data disk 1 x Parity disk 1 x Double-Parity disk 1 x Spare (these can be on a per-head basis but either way you one of them minimum)

So in this case, with one data disk, both Parity and DParity will hold all of the data too - so if you lost the data and Parity disk you still have the DParity disk and the Spare will start rebuilding the data disk from it to ensure you have another set of data once it's finished.

If you have 2 or more data disks then again the DParity disk will carry on just fine in the event of losing a data disk and the Parity disk - and the spare will jump in to replace the failed data disk again.

Hope this helps.

Chopper3
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