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Warren Young
Asked: 2011-08-31 09:43:50 +0800 CST

RAID-6: better to replace two dead drives at the same time, or one at a time?

  • 772

We have a 16-drive RAID-6 that has three problem drives. Two are already dead, and the third is giving SMART warnings. (Nevermind how it got in such a bad state.)

Obviously we want to replace the dead drives before the one that is still working, but is it better to:

  1. replace one dead drive, let the RAID rebuild, then replace the other, and let it rebuild again; or

  2. replace both drives at once and let it rebuild both in parallel?

To put it another way, will we get back to a state of redundancy faster by reintroducing one drive or two? Does rebuilding two drives in parallel slow the rebuild process?

In case it matters, the controller is a 3ware 9650SE-16ML.

hard-drive raid replace 3ware

4 Answers

  1. Best Answer

    !!!!! ONE !!!!!

    Do one at a time, seriously dude, don't think of doing this ANY other way ok.

    Anything else will test your full system restoration skills.

    • 28

  2. Do you have good, recent backups? If not do you think you can get them in reasonable time?

    I'd honestly be more concerned about tripping the bad drive offline during a rebuild than anything else - If you're already throwing SMART errors you're more than halfway there.

    My suggestion would be to confirm your backups, then rebuild one drive at a time to try to recover to a state where you can replace the one throwing SMART errors (dead drives first, soft-errors last).

    If you have no backups it's a crap shoot: Backing up may create enough soft errors to mark the marginal drive as failed, as may trying to do a rebuild.

    • 14

  3. I see no point in changing it as "one disk a time".

    Obviously, if RAID is capable of "resilvering" both disks simultaneously (that are failed anyways) you only win allowing the whole RAID to regain its ability to sustain up to 2 failures faster.

    • 1

  4. My 0.02.$

    Since the server is already offline, run ddrescue on the drive that is about to fail, to clone it to another, sane drive.

    Then put the new, sane drive in the array instead. If the cloning is successful, you'll avoid the risk of seeing that drive fail during 2 rebuilds.

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