Handbook mirroring section

[Andrey V. Elsukov]

On 06.06.2012 15:07, Hiroki Sato wrote:
> ae> 1. When geom_mirror module is not loaded GEOM_PART will complain that the
> ae> backup GPT header is not in the last LBA and partition table will be marked
> ae> as CORRUPT. The recover operation will destroy the GEOM_MIRROR's metadata.
> ae>
> ae> 2. If primary GPT header or table become damaged, then gptboot will not
> ae> detect GPT because the backup GPT header is not in the last LBA. So, the
> ae> system will not boot.
> 
>  Thanks, I see.  Do you think the attached patch is too aggressive for
>  the problem 2?  The value of altlba should be matched with one in the
>  original secondary header when the primary header is corrupted and
>  the secondary header is looked up in this way.

Yes, i also have thought about this and this should work for most GEOM classes,
but i revised again PMBR code and it seems that it will not work anyway :)
Our PMBR doesn't use backup GPT header and table, and it doesn't verify
correctness of primary GPT.

From the other side, there are three situations when we use GPT:
1. FreeBSD is only one system on the disk and we use PMBR and gptboot to boot it.

In case if we will fix PMBR your patch will help.

2. FreeBSD is no one on the disk, but we still use legacy boot method (PMBR+gptboot).

I don't know what behavior have other systems when they detect invalid GPT (i.e.
when backup header is not in the last LBA).

3. We use UEFI (it is not work yet).

Also i don't know what UEFI firmware will do with invalid GPT.

-- 
WBR, Andrey V. Elsukov

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