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Based on this question about RAID-5 for 5 disks: Two disks failed simultaneously?

Sorry if I butcher some of the terminology

Why is a double disk failure an issue for a 5 disk Raid 5 configuration? Even with 2 disks offline, I still can access the sections (terminology?) A, B, C, D and E. Assuming there is no disagreements (terminology?) between (D1 and D2) OR (E1 and E2), I don't see why a double disk failure couldn't continue to work and rebuild 2 new drives.

If you lose more than a single disk in a RAID 5, your array has been irreperably damaged in some way. In most cases, the data is entirely destroyed in your case if you're not an expert at recovery, or if you are unwilling to ship it off to a recovery outfit. - SmallLoanOf1M https://serverfault.com/a/834147/348076

enter image description here

Paul Totzke
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  • Yeah. You have 3 of 5. Sadly that is not enough to read the data. What is the question? Because you really need to read the basics of how the data is stored. – TomTom Apr 26 '17 at 17:39
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    Your misunderstanding is thinking that A1=A2=A3=A4. They don't, they're all separate pieces of data. Think of them (for example) as the 4 parts of a single file. You've lost the fourth part, plus the parity that would have let you rebuild the 4th part. – mfinni Apr 26 '17 at 18:10
  • Thank you @mfinni. That clears up it for me. I thought the data was replicated 4 times and then added parity but I now see why that doesn't make sense. If anyone wants to explain why this is a bad question, please let me know. – Paul Totzke Apr 26 '17 at 18:55
  • That would be RAID11113, right? – mfinni Apr 26 '17 at 19:03
  • It sounds like a joke but my coworkers just told my about RAID 50, so... I don't really know – Paul Totzke Apr 26 '17 at 20:13

2 Answers2

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You are not able to read the A4 data in the stripe in this instance. And you are also unable to read the Parity info to rebuild the data from the Parity information for stripe A.

Therefore, you have data loss and are not able to recover/read the data.

Rex
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Exactly, where the confusion here is how the data is saved.

Using the graphic above, if you were to save a block of data of "ABCDEFG", then you would have "ABCD_FG". Because the volume that the E has failed, the controller would need to "rebuild" the missing block of data from the parity drive. However, since the parity drive is also failed, there is no way to recover the 'E' piece of data.

This is where RAID 6 comes into play to better save your data.