[CentOS] 4.4/64-bit Supermicro/ Nvidia RAID [thanks]

Wed Dec 6 04:15:16 UTC 2006
Aleksandar Milivojevic <alex at milivojevic.org>

Sudev Barar wrote:
> On 06/12/06, Karl R. Balsmeier <karl at klxsystems.net> wrote:
>> >The reason why booting fails is most likely due to the fact that your
>> >BIOS is attempting to emulate (in software) RAID-5 volume when loading
>> >boot loader, kernel and initrd image.  While in reality those are
>> >probably stored on your first drive.
>> >
> Copying over bootloader/grub on to all drives would then solve this?

No.  It won't solve it.

Let say you configured fake-RAID, level 5, with stripe size of 64kB in
the BIOS.

The installation was done onto first disk, since that is what Linux saw
(4 individual drives).  The BIOS thinks that what you have is 4 drives
in software RAID-5, because that's how it was configured.

Depending on the location of RAID metadata on the drives, and other
things, BIOS might load bootloader from MBR.  MBR is in the first
stripe, so it's in the same place on the first drive be it RAID-5 volume
or single drive.

Bootloader than attempts to load stuff from your filesystems.  It has to
use BIOS calls.  Now let say it needs to read first few MB from the disk
(to load stage2, to load kernel, or whatever, not important what).  That
stuff is written by Linux on the first drive.  What BIOS will do, it
will read first 64k from disk 1, next 64k from disk2, next 64k from
disk3, next 64k from disk2 and so on.  Why?  Because it thinks it is
RAID-5 volume, and it attempts to emulate RAID-5.  Of course, this is
completely wrong.  There's nothing on disks 2, 3 and 4.

Not to mention couple of other things.  The checksums for RAID-5 will be
completely wrong, if BIOS bothers to check them on reads at all.  The
partition table will be bogus (it was generated by Linux for single
drive, and BIOS thinks it is RAID-5 volume).  And so on, and so on...

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