Comment 76 for bug 1906476

@enoriel - I've read carefully the info in this ticket; I'm still contemplating the Hirsute to Impish upgrade and *really* glad I didn't pull the trigger earlier!!

The bad news is that it seems clear that the original Impish kernel (5.13.0-19) corrupted filesystems it ran against; thus running any (older/newer) kernel that doesn't include the bug won't "undo" the corruption. I *believe* only ZFS-encrypted filesystems were affected.

https://bugs.launchpad.net/ubuntu/+source/zfs-linux/+bug/1906476/comments/64 and https://bugs.launchpad.net/ubuntu/+source/zfs-linux/+bug/1906476/comments/47 have suggestions on how to rescue your filesystem. It may be best to boot the system to be rescued by other means and import the corrupted filesystem onto it; a Hirsute ISO should do as its ZFS version is close enough to the one used by Impish (2.0.x) but I haven't tested any of this; merely collating info above.

I'm still unclear as to whether a Hirsute -> Impish upgrade is safe. A clear cut statement by Canonical wouldn't go amiss given the severity of this issue (to put it mildly)...