Re: gptzfsboot cannot boot from ZFS (mirror) with "can't read MOS of pool znewroot" (FreeBSD 13.5-STABLE)

On 03/04/2025 17:21, Lev Serebryakov wrote:
> Hello!
> 
>    I have ZFS mirror built from two GPT partitions on two SATA drives. It is only ZFS pool on the system, and it looks completely healthy:

[...]

> ZFS: i/o error - all block copies unavailable
> ZFS: can't read MOS of pool znewroot
> 
>    As a hack I'm using `gtpboot` & small `freebsd-ufs` partiton with only `/boot` directory.
> 
>    But I hate this hack, as system update becomes cumbersome, and I cannot convert this system to UEFI boot (because `loader.efi` "Failed to find bootable partition").
> 
>    What is wrong with my ZFS pool and how could I fix it?

Is it possible that you have disk drives larger than 2TB and the boot 
data is written somewhere after that 2TB barrier?

The strangest thing is that it can work for a while when data is written 
in the beginning of the drives, then you make some system update / 
upgrade and then it cannot boot anymore when boot data is written too 
far away.

I experienced this problem on my old HP MicroServer Gen8 with 4TB 
drives. 2TB drives worked fine, also those 4TB drives booted fine in 
another machine (X9 Supermicro) so this was coincidence with "bad" BIOS.
I solved it by creating small partitions on the beginning of the drives 
- about 20GB to contain boot and all the FreeBSD system data. ZFS 
mirrored and booting without an issues. The rest of the drives are ZFS 
mirrored in another pool for storage (the machine serves as NAS)

Kind regards
Miroslav Lachman