When setting up my SSD zpool – which I basically wanted because I wanted *no* constant disk-access to my disk-based zpool, I was lazy. Laziness almost never pays off.
Before starting with ZFS, I had almost my full SSD as an LVM physical volume (except the bootdisk). Then, I added my external disk cabinet and set up a mirrored ZFS pool on it:
hassio# zpool status pool: nasdisk state: ONLINE scan: scrub repaired 0B in 0 days 03:34:33 with 0 errors on Fri Dec 17 16:02:07 2021 config: NAME STATE READ WRITE CKSUM nasdisk ONLINE 0 0 0 mirror-0 ONLINE 0 0 0 sda ONLINE 0 0 0 sdb ONLINE 0 0 0
Then, after a while, I added a second pool, znvm, that only had one disk: A logical volume from the internal SSD.
But ZFS on top of a logical volume isn’t a particulary good idea. You risk confusing ZFS thoroghly, especially IO-wise, and might end up with less performance and more overhead on your I/O.
Since my NAS-box is headless, I hate booting it and I kept putting off the operation of cleaning up. I wanted to reduce the size of the phyical volume to basically only contain the root volume, and then create a new partitition that could hold my SSD zpool, znvm.
After some research today, I decided it was worth it to try doing it on a live machine. Warning: If doing this, is probably an extremely good idea to take a full backup first!
Here is my steps:
- Replace the logical volume in the zpool with a partition on a spare disk. Luckily, I had one.
- Remove the logical volume that the zpool earlier was residing on
- Reduce the size of the physical volume to the new desired size, as seen from LVM
- Reduce the size of the SSD partitition the physical volume is living on to the size of the physical volume, Here, it is a good idea to add an extra GB or two, for your piece of mind. Too little disk here, and your volume group is corrupt!
- Add the new partitition that the zpool should use on the SSD.
- Replace the temporary partition in the zpool with the new partition in step 5.
- And that’s actually it!
Here are the commands:
zpool status znvm (and find the name of the volume) fdisk /dev/sdc (temporary disk) - create partition sdc1 zpool replace znvm <old_volume_name> /dev/sdc1 zpool status znvm (and wait until it is finished!) lvremove /dev/ubuntu-vg/zpool_nvm (that was used for znvm) pvresize 201G /dev/nvme0n1p3 (201G is the new size, nvme0n1p3 is the partition holding the volume) cfdisk /dev/nvme0n1 (change the size of nvme0n1p3 to 205G and add a new nvme0n1p4 for the zpool that is as large as /dev/sdc1) zpool replace znvm sdc1 /dev/nvme0n1p4 zpool status (until it is finished)
And that is the whole operation!