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We recently had a raid array fail on a SQL 2008 server with 8 x 73GB drives. (Dell 2950). There were two drives that went bad (one had been bad for a few months it seems and was not caught...). To top it off log shipping was not working on this server and the last back up was 3 days old…

So I have these 8 drives on hand, which ones (if any) have specific completed DB files on them if I choose to send them out for recovery?

My programmer says HDD2 and HDD3. Is there a consistent logic used by Raid 5 or SQL to write data to a specific drive in these cases? HDD0 died first, and then HDD4 was the last drive that failed and crashed the array.

On a side note, we will no longer be using Raid 5 and moving to Raid 10 now on all new servers.

Thanks in advance.

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    Switching away from RAID5 is good, but as you noticed, it's much more important to have working, tested and reliable backup methods in place.
    – Sven
    Apr 14, 2015 at 20:41

2 Answers 2

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I've had to deal with 2 failed drive RAID 5 arrays, and the short answer is you have to send all drives which were part of the array out for recovery if you want to get that data back. It WILL cost you thousands of dollars, so it will be up to management to decide whether the lost productivity is worth the cost.

In general RAID 5 will stripe data across all disks and have a built in parity drive for each stripe. Files aren't ever stored in contiguous blocks on a specific drive of the array. See here for an explanation of how RAID 5 works: http://en.wikipedia.org/wiki/Standard_RAID_levels#RAID_5

I agree with Sven, it's much better to put your time and money toward reliable backups and hardware monitoring. Those will give you two (better IMHO) layers of defense against data loss than switching to RAID 10 (which will cost you in $ if you don't need the IOPS).

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  • how did your recovery go in your case? and how did you know which disks to open? the bad discs or the good discs or all 8?
    – Alex
    Apr 14, 2015 at 21:03
  • @Alex: You don't need to care about "which drives to open" - this is the business of the recovery firm.
    – Sven
    Apr 14, 2015 at 21:05
  • Like Sven said "If you told them this contains a RAID5, they will not ask this question. If they do, fear for the worst". They will open all the drives and transplant the platters as they deem necessary. Then they will read from the entire array and copy all the data onto a new drive and ship it back to you. The recovery was successful in my case. Once we got the data back we loaded it onto a new server with a new RAID 5 array (and better monitoring).
    – Cubano
    Apr 14, 2015 at 21:05
  • does this mean they will know which disc to open and it is actually recoverable? I am confused as on one hand people say "all data is lost in raid 5" then on the other hand they say "send it out for recovery"..how can you recover data that is "lost" if that is the case? Forgive my newbie questions.
    – Alex
    Apr 14, 2015 at 21:10
  • Got it. That makes more sense now..
    – Alex
    Apr 14, 2015 at 21:11
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In an n-disk RAID5, you need n-1 disks to read back the data. If two drives are dead, no other disks will contain a full set of data and you have to resort to your backup.

For speed purposes, data is striped over the disks to allow for faster reads.

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  • What is the definition of a "full" set of data? Is it possible transaction log files for one day are still on one of the working 6 drives? Or are you saying the data is in small chunks over all eight drives and opening one good drive is a waste of time?
    – Alex
    Apr 14, 2015 at 20:45
  • Yes, the data is striped in small chunks. Read about how RAID5 works.
    – Sven
    Apr 14, 2015 at 20:46
  • Just to clarify, I don't need all the data, just transaction logs from the last 3 days. Maybe 1MB of data total?
    – Alex
    Apr 14, 2015 at 20:46
  • It might be possible to rescue it, but this will need (very expensive) specialists.
    – Sven
    Apr 14, 2015 at 20:47
  • I have already sent all 8 drives out for recovery, so I am sure the next logical question back is "which drives do you wish to open to recover which data?" That's why the original question. :-)
    – Alex
    Apr 14, 2015 at 20:50

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