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Short: Can I share a disk between two separate zpools?

Long: I want a raidz2 array but I can't afford it all at once. My possible solution is to start with two drives. I would create a mirror using only half the capacity of each disk. When I can afford a more drives I would would create a new zpool with a single raidz2 vdev using the new drives and the 50% unused space on the initial disks. I then plan to copy the data from one zpool to the other, delete the zpool containing the mirror and expand the zpool with the raidz2 vdev.

Would this work?

Links and references would be appreciated.

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1 Answer

  1. Yes, you can create a pool which consists of a mirrored vdev between two partitions.
  2. Yes, you can have two pools each which share a single disk each using their own partition.
  3. Yes, when increasing all the sizes of the disks in a raid-z2 vdev it will increase the available capacity.

But you shouldn't do it. ZFS is best when you give it the entire disk and although partitions 'work', your world is much easier/better with whole disks. If you really wanna do the mirror -> raid-z2 pool transition down the road and are prepared to live within the bounds of 50% use of your mirror I have an alternate idea:

  1. Buy 2 disks (e.g. 2x2TB)
  2. Create a mirrored zpool: zpool create yourPool mirror cXt1d0 cXt2d0
  3. Buy 3+ more identically sized disks. (e.g. 4x2TB)
  4. Create a new filesystem: zfs create yourPool/fake
  5. Create two sparse files: mkfile -n 1TB /yourpool/fake/fakehda
  6. Create a double parity zpool: zpool create yourNewPool raidz2 cXt3d0 cXt4d0 cXt5d0 cXt6d0 /yourpool/fake/fakehda /yourpool/fake/fakehdb
  7. zfs send/recv your filesystems from one pool to the other.
  8. Detach one disk from your mirror zpool detach yourPool cXt1d0
  9. Replace your a fake disk with the real disk zpool replace yourNewPool /yourpool/fake/fakehda cXt1d0
  10. Wait for resilvering to complete. Monitor progress with zpool status yourNewPool.
  11. When it's completed resilvering, murder the mirror zpool destroy yourPool
  12. Re-use the second old disk zpool replace yourNewPool /yourpool/fake/fakehda cXt2d0

During this entire process you would be able to survive any single disk failing without data loss.

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