Re: md extension to support booting from raid whole disks.

[Luca Berra <bluca@comedia.it>]

On Thu, Apr 30, 2009 at 08:59:35AM +0200, Gabor Gombas wrote:
>On Wed, Apr 29, 2009 at 09:45:59AM +0200, Luca Berra wrote:
>
>> Personally I despise disk partitioning schemes, it is a concept that
>> should have died long ago, even GPT, while being more sensible than PC
>> partitions, is of no real use to me.  Ok on ia64 the firmware will read
>> a GPT partition table and load the EFI from a partition, so yes on
>> itaniums this would be the way to go, do we really care?
>
>Just because _you_ do not use it does not mean that it is useless. Sure,
>on server machines you can dedicate the whole disk for Linux and do
>whatever you want. But on desktop machines dual booting is still popular
>so unless you fix Windows to boot from an LVM2 volume, partitioning is
>going to stay for quite some time. Virtualization helps in some cases to
>get rid of the extra partition, but not always.

I do understand that, if you are dual booting you are limited my the
minimum feature set of the system you dual boot. Windows is not even
able to boot from an md raid, the viable choice when dual booting
windows and linux and having disk failure protection is using either
fakeraid, or real hw raid. note that Windows is able to boot from a
raid5 fakeraid, linux, afaik, is not.
I think the whole idea was for systems that do not dual boot.

>> I stumbled upon a lovely failure scenario that shows even your scheme is
>> fragile at best.
>> Due to issue i would not dwell on here the first disk was kicked from the
>> raid1 containing /boot, but it was still very well readable by the bios.
>> result: i took a while realizing why the hell after upgrading the kernel
>> the system insisted on booting with the previous kernel ;)
>
>This failure mode also happens exactly the same way in the "reserve some
>space at the beginning ant turn it into a RAID1 without telling enyone"
>scheme.
My idea is that the space at the beginning has no need of being a raid1.
It just need to contain something smart enough to understand there is a
raid set, assemble it readonly and load a linux kernel from it.
This something does not change over time, you set it up once and forget
about it. At most you update it once in a while.
(note: i never said this something has to be grub2)

>> also in your 'one sensible configuration' boot should not only be
>> raid-1, but it should also be entirely contained in the portion of the
>> disk accessible via int-13. i have seen distribution installers enforce
>> the first constraint. not the second.
>
>If you have such an old BIOS then you will have problems with just a
>single disk as well. This has nothing to do with RAID, so I fail to see
>why you bring it up.
ok, lets forget about that.

L.


-- 
Luca Berra -- bluca@comedia.it
         Communication Media & Services S.r.l.
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