Comment by cm2187 5 days ago
It is possible with windows storage space (remove drive from a pool) and mdadm/lvm (remove disk from a RAID array, remove volume from lvm), which to me are the two major alternatives. Don't know about unraid.
> It is possible with windows storage space (remove drive from a pool) and mdadm/lvm (remove disk from a RAID array, remove volume from lvm), which to me are the two major alternatives. Don't know about unraid.
Perhaps I am misunderstanding you, but you can offline and remove drives from a ZFS pool.
Do you mean WSS and mdadm/lvm will allow an automatic live rebalance and then reconfigure the drive topology?
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.
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.
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.
The man page says that your example is doable with zpool remove:
https://openzfs.github.io/openzfs-docs/man/master/8/zpool-re...
> Do you mean WSS and mdadm/lvm will allow an automatic live rebalance and then reconfigure of the drive topo?
mdadm can convert RAID-5 to a larger or smaller RAID-5, RAID-6 to a larger or smaller RAID-6, RAID-5 to RAID-6 or the other way around, RAID-0 to a degraded RAID-5, and many other fairly reasonable operations, while the array is online, resistant to power loss and the likes.
I wrote the first version of this md code in 2005 (against kernel 2.6.13), and Neil Brown rewrote and mainlined it at some point in 2006. ZFS is… a bit late to the party.
Doing this with the on disk data in a merkle tree is much harder than doing it on more conventional forms of storage.
By the way, what does MD do when there is corrupt data on disk that makes it impossible to know what the correct reconstruction is during a reshape operation? ZFS will know what file was damaged and proceed with the undamaged parts. ZFS might even be able to repair the damaged data from ditto blocks. I don’t know what the MD behavior is, but its options for handling this are likely far more limited.
Well, then they made a design choice in their RAID implementation that made fairly reasonable things hard.
I don't know what md does if the parity doesn't match up, no. (I've never ever had that happen, in more than 25 years of pretty heavy md use on various disks.)
Storage Spaces doesn't dedicate drive to single purpose. It operates in chunks (256MB i think). So one drive can, at the same time, be part of a mirror and raid-5 and raid-0. This allows fully using drives with various sizes. And choosing to remove drive will cause it to redistribute the chunks to other available drives, without going offline.
IIUC the ask (I have a hard time wrapping my head around zfs vernacular), btrfs allows this at least in some cases.
If you can convince btrfs balance to not use the dev to remove it will simply rebalance data to the other devs and then you can btrfs device remove.