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What's the best way to convert Linux software RAID 1 array to RAID 5? ( I have already bought the 3rd Hard Disc !)

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Before doing anything of this sort back up your data to separate media and verify the backup via sha1sum.

The process from there would look like

  • Break the RAID1 mirroring so that one of the drives is free
  • Add the third drive to your system
  • Create a degraded RAID5 out of the new drive and the one freed from the RAID1
  • Copy the data over to the RAID5 volume
  • Add the RAID1 disk to the RAID5 volume, and give it plenty of time to synchronize itself properly.
  • Verify that the data on the new volume matches the backup

I've assumed that you're using Linux's software RAID support, in which case all of this would be managed by the mdadm command.

  • 3
    You may also want to make offerings to whatever deity you believe in before beginning this process. – Matt Simmons Aug 11 '10 at 2:10
  • So it's possible to do without a 4th hard drive, thanks. – Chris Huang-Leaver Aug 15 '10 at 7:22
  • Didn't realise you could create a degraded raid5 with only 2 drives! – barrymac Nov 1 '11 at 1:32
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Many nice RAID controllers can take a RAID 1 array, and when you add another drive, allow you to convert it on the fly to RAID5, of course, now twice the size.

The AMI/LSI based controllers have been doing this for a few years (is it a decade yet?). Dell PERC's, Compaq/HP's Smart Array controllers have often used these chipsets.

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