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Comment by cm2187

5 days ago

So for instance I have a ZFS pool with 3 HDD data vdevs, and 2 SSD special vdevs. I want to convert the two SSD vdevs into a single one (or possibly remove one of them). From what I read the only way to do that is to destroy the entire pool and recreate it (it's in a server in a datacentre, don't want to reupload that much data).

In windows, you can set a disk for removal, and as long as the other disks have enough space and are compatible with the virtual disks (eg you need at least 5 disks if you have parity with number of columns=5), it will rebalance the blocks onto the other disks until you can safely remove the disk. If you use thin provisioning, you can also change your mind about the settings of a virtual disk, create a new one on the same pool, and move the data from one to the other.

Mdadm/lvm will do the same albeit with more of a pain in the arse as RAID requires to resilver not just the occupied space but also the free space so takes a lot more time and IO than it should.

It's one of my beef with ZFS, there are lots of no return decisions. That and I ran into some race conditions with loading a ZFS array on boot with nvme drives on ubuntu. They seem to not be ready, resulting in randomly degraded arrays. Fixed by loading the pool with a delay.

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cm2187

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formerly_proven  5 days ago

My understanding is that ZFS does virtual <-> physical translation in the vdev layer, i.e. all block references in ZFS contain a (vdev, vblock) tuple, and the vdev knows how to translate that virtual block offset into actual on-disk block offset(s).

This kinda implies that you can't actually remove data vdevs, because in practice you can't rewrite all references. You also can't do offline deduplication without rewriting references (i.e. actually touching the files in the filesystem). And that's why ZFS can't deduplicate snapshots after the fact.

On the other hand, reshaping a vdev is possible, because that "just" requires shuffling the vblock -> physical block associations inside the vdev.

  • ryao  5 days ago

    There is a clever trick that is used to make top level removal work. The code will make the vdev readonly. Then it will copy its contents into free space on other vdevs (essentially, the contents will be stored behind the scenes in a file). Finally, it will redirect reads on that vdev into the stored vdev. This indirection allows you to remove the vdev. It is not implemented for raid-z at present though.

    • formerly_proven  5 days ago

      Though the vdev itself still exists after doing that? It just happens to be backed by, essentially, a "file" in the pool, instead of the original physical block devices, right?

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