Re: FreeBSD boot from root pool larger than 1Tb/cannot read MOS/all block copies unavailable

From: Zaphod Beeblebrox <>
Date: Thu, 28 Apr 2022 04:37:51 UTC
The point I want to reiterate, however, is this error is tossed out when my
setup fails and the answer is a full powerdown of the motherboard --- after
which the disks are adequately described.

On Wed, Apr 27, 2022 at 9:26 AM Warner Losh <> wrote:

> On Wed, Apr 27, 2022 at 7:05 AM Larry Rosenman <> wrote:
>> On 04/27/2022 7:45 am, Tomoaki AOKI wrote:
>> [snip]
>> > I've booting with around 1.5T or above (but under 2T) Root-on-ZFS
>> > installation via UEFI for years.
>> >
>> > Basically, UEFI requires GPT partitioning. So many of those would boot
>> > from larger than 1TB boot pool.
>> >
>> > As, IIRC, it is optional, some UEFI firmware would be able to boot from
>> > MBR-patritioned disk but there would be some risk that they have the
>> > same limitations as legacy BIOS in such situations.
>> >
>> > Note that possibly I'm just lucky enough not to be bitten.
>> I've been booting UEFI from 10T pools for literally years, and I have a
>> HP server
>> with a 5T pool that IIRC boots GPT with BIOS, and a different Dell that
>> has a 6T
>> pool also GPT and UEFI.
> Both bsdlabel and mbr have a 32-bit block count, which means for 512-byte
> blocks, the largest they can describe is 2TB. That's likely the root cause
> of this issue. The BIOS can't read beyond that limit on the disks. It's
> hit or
> miss what BIOSes do with 4k drives, though most of the later ones before
> uefi was well established coped well enough.
> This isn't to say that the old code in FreeBSD 12's boot loader didn't have
> other issues with LBAs > 32-bits for ZFS. And for MBR ZFS booting the
> update of the boot blocks is, at best, super weird. If you've not done
> that,
> you'll continue to have the older boot loader, even if the latest just
> works.
> I've never encountered this problem, so I can't offer better words of
> advice,
> though. All my ZFS booting started after I transitioned to UEFI booting.
> Warner