original submission:

> Currently "zstd" compression is supported for BRTFS, but not for ZFS.  As the version of ZFS shipped with many distributions now supports zstd, it is possible for a user to set the root-on-ZFS filesystem to use zstd and inadvertently cause their system to become un-bootable and virtually irrecoverable.
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Just to clarify the impact a little  ...On any Ubuntu 21.10 or 22.04 system, if the "compression" setting for the "bpool" (/boot) filesystem is changed to "zstd" (and the system actually compresses any data on that filesystem using zstd), the system will be effectively "bricked" on the next reboot.

The UEFI boot will call Grub, the boot menu will appear, but none of the listed kernels will boot and the roll-back snapshots will, likewise, not boot.

Just changing the compression type back to "none" or "lz4" will not work (because the data is already compressed with "zstd").  For most casual users, the only viable (system) recovery method is a complete re-installation.


FOR ANYONE LOOKING TO RECOVER FROM THIS ISSUE:-

  The data on your filesystems is intact (this is a Grub issue, -not- a ZFS issue), therefore you can recover your whole system using an alternative boot method which has "zstd" support, such as ZFSBootMenu.

   Alternatively, if you are comfortable with ZFS basics, you can boot from a live-CD, use ZFS send-receive to back-up your bpool to either an external disk, system or SD-card, recreate the bpool  (hint:- zpool create -o compatibility=grub2 bpool) and then re-import the data  again.