[syslinux] Linux software RAID 1.2 superblocks

[Ferenc Wagner]

"H. Peter Anvin" <hpa at zytor.com> writes:

> On 06/11/2010 11:14 AM, H. Peter Anvin wrote:
>
>> On 06/11/2010 10:21 AM, Ferenc Wagner wrote:
>>
>>> Just to bring one more Debian concern to the Syslinux table: the default
>>> metadata format in upstream mdadm changed to 1.2, which means MD
>>> superblocks at the beginning of the partition, after a 4 KB hole.
>>>
>>> Is our favorite bootloader prepared to handle such situations?
>> 
>> Ah, good, they did un-break the universe -- for a while there the
>> default was 1.1, which can't support partition-centric bootloading at all.
>> 
>> Syslinux itself can handle it, but it will need a special boot sector
>> (or special MBR, in case the entire disk in an mdraid -- I think the
>> same code might work for both but I haven't tested it all yet) installed
>> onto the mdraid.  I haven't written that code, but it shouldn't take
>> very long.
>
> Thought some more about it today, and the main trick is to make it
> user-friendly which means adding support to the installer.

Agreed.

> I'm touching some of that code already, and I might have time to
> implement it over the weekend.

That sounds great.  As part of the problem is finding the real partition
offset, some of this code might be reused to support booting from an LVM
logical volume (at least if it's a single linear map).  I even *think*
that the LVM metadata is not placed at the very beginning of the PV,
which would make booting from an entire disk PV theoretically possible.
And yes, I'd use this, especially in virtual setups.

Now, of course I don't say you should try implementing all this over the
weekend, just consider them as possible further directions and leave the
necessary "hooks" around to make future advances easier.  :)
-- 
Thanks,
Feri.