[PVE-User] zfs root after last update grub unknown filesystem

Stephan Leemburg sleemburg at it-functions.nl
Sun Sep 13 23:11:41 CEST 2020

Hi All,

I have a proxmox system at netcup that was clean installed about two 
weeks ago.

It is full zfs, so root on zfs. I am migrating from one netcup system 
with less storage to this system.

This system is using the pve-no-subscription repository, but after 
migration I will move the subscription from the 'old' system to this system.

The rpool/data dataset is zfs encrypted.

Today I did zfs send/recv from the 'old' system to this system to the 
rpool/data dataset.

After that I did an apt update and noticed there where updates available.

After the upgrade and the mandatory reboot, the system does not come up 
anymore. It is stuck in grub rescue.

Grub mentions that it has a 'unknown filesystem'.

Has anyone else experienced this same situation?

If so and you could recover, what was the reason and fix?

I am still researching what is causing this. If I boot a .iso then I can 
import the pool and see all datasets and subvolumes.

So, it seems that the zpool itself and the datasets are ok, it is just 
that grub is unable to recognize them for some reason.

I have read about other situations like this where large_dnode seemed to 
be the cause. I noticed that on the zpool large_dnode is enabled. And it 
is a creation only setting. It cannot be changed to disabled afterwards.

This must have been done by the Proxmox installer. As booting is about 
the root filesystem, I guess the zfs send / recv to the rpool/data 
dataset would have nothing to do with it, but I could be wrong.

I will continue my research tomorrow evening after some other 
obligations, but if anyone has an idea, please share it. If it is just 
how to get more debugging info out of the zfs module in grub.

Because 'unknown filesystem', with the zfs module loaded is kind of not 
helping enough..

Best regards,


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