Resize a zvol used by a VM as root disk

e2fsck resize2fs fdisk volsize truenas

Source

• https://askubuntu.com/a/1224578/1152691
• https://www.truenas.com/community/threads/changing-size-shrink-on-active-zvol.42142/post-444130

• shutdown the VM
• edit the VM to add a live ISO image with the device order higher than the disk device of the VM
• start the VM to boot into the live ISO
• resize the root partition to its new smaller size
  • run e2fsck -f /dev/vda3 (where dev/vda3 is the partition to be resized)
  • then run resize2fs /dev/vda3 150G to resize the partition (where 150G is the new smaller size)
• alter the partition table
  • run fdisk /dev/vda
  • enter p to print the table for reference
  • enter d followed by 3 to delete the partition
  • enter n followed by 3 followed by +150G to create a new partition with the smaller size
  • answer no if asked to remove the ext4 marker
  • enter t followed by the correct partition type ID (in this case 23 for Linux root (x86-64))
  • enter w to write the new partition table
• shutdown the VM again
• create a ZFS snapshot of the zvol
• resize the zvol (only possible via the command line)
  • run zfs set volsize=200G <pool>/<zvol> to set the new size for the zvol
  • this will destroy the partition table and maybe even the whole data, but we can get that back when we…
• restore the snapshot from before (this can be done in the GUI again), confirming that this will delete/replace all data
• the size of the zvol will stay the new smaller size, but all data (since it should fit snugly into the new space) will be restored
• then simply start the VM and enjoy

Edit

It seems, the restoration of the snapshot did reset the volsize as well, however, I still have much more free space than before. Also, the partition table and size of the disk inside the VM is still the new smaller size.