On creating my RAID-Z pool on ZoL I assumed, I could easily just drop-in additional disks later on. Meanwhile I learned that this is yet not possible.

But... I had a similar problem on creating my initial pool. Only 4 free SATA ports, but an old RAID5 with three 2TB disks and a new RAIDZ1 with three 4TB disk. The solution was to a) degrade the RAID5 and b) build the initial RAIDZ with a sparse file as "virtual third drive", which was taken immediately taken offline after pool creation:

  1. Create sparse file: dd if=/dev/zero of=/zfs1 bs=1 count=1 seek=4100G
  2. Create the raidz pool: zpool create zfspool raidz /dev/disk1 /dev/disk2 /zfs1
  3. Immediately take off the sparse file: zpool offline zfspool /zfs1
  4. Migrate data to zfspool. Uninstall old RAID5 disks, add third, new 4TB disk
  5. Replace & resilver the sparse file in the pool with the actual, third drive:
    zpool replace zfspool /zfs1 /dev/disk3

This worked out really great! Now I learned that though ZFS does not directly support adding a single disk to RAIDz, but that it supports one-by-one replacing disks with larger ones.

So here is my plan. Does anybody see a flaw in it?

  • Buy a fourth 4TB disk and take one disk offline from the existing pool
  • Create 2x2TB paritions on these two, free disks.
  • Build a RAIDz out of these four "disks": 3x2TB = 6TB net storage.
  • For performance reason: Take one of the 2nd partition immediately offline
  • Migrate max. 6TB data to the new pool & destroy the old pool
  • Replace offline "2TB disk" with a real 4TB one of the old pool. Wait to resilver.
  • On the drive with 2 active partitions: Take the 2nd 2TB partitions offline and replace it with the second 4TB disk from the old pool. Wait for resilvering.
  • One-by-one: Take a remaining 2TB partitions offline, grow the partition with 4TB and re-add the disk the pool. Wait for resilvering.
  • Rinse & repeat for the very last 2TB disk/partition

Will this work? I know that I'm higher vulnerable to data loss due to the missing redundancy during the process, but I will have backup of the most important data. Just not enough for the whole 6TB payload.

And will ZFS automatically grow the pool to (3+1)x4TB = 12TB after the last step?

2 Answers 2


Ugly, but this would work.

Except when it doesen't;).

  • Be very careful when specifying the partitions and when replacing the disks
  • try it in am VM beforehand, setup the virtual disks like your hardware an dry run it 1 or 2 times.
  • make a scrub before you start and take a look at the S.M.A.R.T info from the disks. You would not try this with an already flakey disk.

Important: You better have a tested backup on another medium or machine before trying it!

Yes, ZFS will grow the pool if the last 2TB disk or partition is replaced with a 4TB one ( if you have autoexpand=on for the pool )

zpool get autoexpand $pool

zpool set autoexpand=on $pool

On a sidenote: you should not use RAID-Z on disks bigger than 2TB. Your chance of getting an error on resilvering when replacing a faulted disk is very high. Please consider RAID-Z2.

  • 2
    Remember that autoexpand has to be enabled before you replace the first disk in the pool.
    – bodgit
    Dec 22, 2014 at 13:49
  • 1
    Correct. This should work. But: 1. It will take a darn long time. If possible, move the data to something else and rebuild from scratch to save a lot of time. 2. If any disk has an error during this process (probably somewhere in the 1:1000 to 1:10 likelihood) you'll be losing data as there wont be any redundant disks. 3. Backups, backups, backups.
    – Chris S
    Dec 22, 2014 at 14:46
  • Thanks for the feedback. I know its ugly, but the only way to avoid useless hardware. I was missing the autoexpandproperty. IMHO RAID-Z2 recommendation rather depends on disk count, not size. Would use starting at 4 data disks. Also: Doing monthly ZFS scrubs mitigates bit rot risk.
    – bentolor
    Dec 23, 2014 at 9:24

This sounds terrible. It's your data, so you can do as you wish... Nobody would endorse the solution, though.

This is really a situation where you should just start over.

Move your data somewhere temporarily and rebuild.

  • 1
    It's ugly - yes. And I do backups of the important data. To start over I'd need at least 2 additional HDDs and a controller just for the migration.
    – bentolor
    Dec 23, 2014 at 9:16

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