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This is the exact scenario I am looking to solve:

I currently have 2x 4TB drives set up as a two-way mirror. I am now just about out of space and would like to be able to add a third 4TB drive and switch to parity instead (leaving me with 8TB of usable space instead of 4). Is there any way to do this without losing all of the data on my current drives?

I don't have enough extra hard drives lying around to offload everything to so that I can format, set up the parity pool with the 3x 4TB drives and then copy them back.

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  • If time / bandwidth isn't an issue, you could just totally abuse things like Drive / Drop box / Skydrive or similar - create accounts, chuck everything up their, do your thing and then re-download. It's free at least!
    – Dave
    Commented Aug 2, 2013 at 8:15

2 Answers 2

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Yes, you can with a trick thanks to the thin provisioning feature.

  1. Add the new disk to your storage pool.
  2. Create another storage space with parity setup.
  3. Copy/move all your files from the old storage space to the new one.
  4. Delete the old, now empty storage space.
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Logically, with MDADM, you convert to a 2 drive RAID 5 array. Then you grow the array to add the 3rd disk.

This is link of someone doing it

Although I would find a way to backup data when messing with RAID arrays; any wrong move or accident can and usually will mean complete data loss.

If you are using something else like a hardware RAID controller, you will be limited by its built in functionality.

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