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

[John Robinson <john.robinson@anonymous.org.uk>]

On 01/05/2009 23:33, Goswin von Brederlow wrote:
> Dan Williams <dan.j.williams@intel.com> writes:
> 
>> On Fri, May 1, 2009 at 2:04 PM, Goswin von Brederlow <goswin-v-b@web.de> wrote:
>>> And is horribly ugly when you have to switch hardware. And try doing
>>> it with disks from different controlers? What bios supports that?
>> This is just a simple trade-off between two options.
>>
>> 1/ If you want to reliably boot from multiple disks then make a native
>> md-raid1 array for /boot like hpa suggests.
> 
> Which still has the same problems as the raid5. How to handle the
> bootloader. Say I have a raid1 over sda/b with lvm on it. How do I
> boot that? How do I get the bootloader cloned to a new spare disk when
> it gets added?

`dd if=/dev/originaldrive of=/dev/newdrive bs=512 count=1` to clone the 
boot sector from one of the remaining original drives; this clones your 
partition table too. Once that's done, and you've added the new drive's 
partition(s) back into the RAID set(s) and it's synced, it will boot.

Or just tell your bootloader to install itself on the new drive.

> It realy makes no difference if it is a raid0/1/10/4/5/6.

Yes it does - anything which needs access to more discs than whatever 
the BIOS offers as the boot drive via int13 is asking for trouble.

>> 2/ If you do not want to make /boot into an md-raid1, and you still
>> want to boot from raid5, then use $VENDOR's raid5 option-rom and
>> mdadm's external metadata support.
> 
> There is no such thing as $VENDOR's raid5 option-rom in pretty much
> all cases. And even raid0/1 roms are horribly.

Dan was being modest referring to $VENDOR; any recent board with an 
Intel ICHxR has Intel Matrix RAID with RAID-5 in the BIOS/option ROM. I 
imagine other chipsets have RAID-5 too. I would have used this method on 
my Asus P5Q Pro but the CentOS 5 installer doesn't support it; I think 
recent Fedora and Ubuntu installers do, thanks to excellent work by Dan, 
Neil Brown and others improving md's support for 3rd-party metadata formats.

Cheers,

John.