CFR: backup GPT header support in pmbr and loader(8) (Re: Handbook mirroring section)

[Olivier Smedts]

2012/6/14 Andrey V. Elsukov <ae at freebsd.org>:
> On 14.06.2012 17:08, Olivier Smedts wrote:
>> 2012/6/14 Andrey V. Elsukov <ae at freebsd.org>:
>>> Also, our PMBR image doesn't contain any partition entries and
>>> writing it to the disk with dd(1) makes the system unbootable.
>>> What you think if we will add one partition entry to the PMBR image?
>>> I modified your patch (the last hunk added) and it is attached.
>>
>> Isn't it gpart(8)'s job to write the proper boot code from the pmbr
>> image without damaging the partition table ?
>>
>> ie. "gpart bootcode -b /boot/pmbr ada0" instead of using dd.
>
> gpart(8) works only when GPT's geom exists, when PMBR is damaged
> GEOM_PART_GPT can't detect GPT and thus you can't use it.

IMO, GEOM_PART_GPT should not depend on the MBR do detect GPT. That's
the problem which should be fixed ; adding a partition to the MBR is
only a workaround (and what would happen if the sector 0 was
permanently damaged on disk ?). Why does GEOM_PART_GPT depends on the
sector LBA 0 while the GPT header is located at sector LBA 1 ? Don't
know the feasability of this, just saying... :-)

-- 
Olivier Smedts                                                 _
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