mjhjmtu

I am confused about how ZFS snapshots and rollback are supposed to work.
I have a zpool containing a couple zvols (each made from partitions of a mirrorvdev). I make a snapshot like this:

~$ sudo zfs snapshot nvme-tank@roll_test

Then I boot a VM located on one of the zvols in the zpool and create a test file

~$ echo "This is a test of the rollback system" > rbtest.txt

Then I shut down the VM and do the rollback. As I understand the concept, the rollback should revert everything in nvme-tank to the state it was when I took the snapshot, before making rbtest.txt.

~$ sudo zfs rollback nvme-tank@roll_test

I then reboot the vm, check for rbtest.txt, and there it is, still there!

The snapshot is there:

~$ zfs list -t all -r nvme-tank
NAME USED AVAIL REFER MOUNTPOINT
nvme-tank 887G 12.1G 24K /media/nvme-tank
nvme-tank@nov82018 0B - 24K -
nvme-tank@roll_test 0B - 24K -
nvme-tank/ext4-zvol 474G 449G 37.8G -
nvme-tank/ntfs-zvol 413G 23.9G 401G -

So, am I misunderstanding what snapshots are supposed to do, using them incorrectly, or is something broken with my zfs?

You only snapshotted nvme-tank, but you didn't snapshot either of nvme-tank/ext4-zvol or nvme-tank/ntfs-zvol, one of which contains the file you created. So nothing happened because you were snapshotting a different dataset than the one you created the file in.

If you want snapshots to be recursive to descendent datasets/zvols, you have to explicitly ask for recursive snapshots, with zfs snapshot -r. However, you can only rollback one specific dataset/zvol at a time with zfs rollback.