[lug] SW Raid question

[Lee Woodworth]

dio2002 at indra.com wrote:
>>> I guess I'm confused. It seems that you are saying that the block
>>> addresses
>>> that get written into the mbr have some kind of device specifier. I have
>>> moved
>>> a disk from sdb to sda where sdb was prepped using root (hd1,0) in grub.
>>> Booted the former sdb in the sda position just fine with only the one
>>> disk.
>>> Didn't even appear to look for an sdb device. I did adjust the root
>>> statement
>>> in grub.conf to (hd0,0), but did nothing to any device lines.
>>
>> Currently the vote is 2 against 1. :)  I suspect I'll learn something.
>> I've got a CentOS 4.4 minimal image installed now w/ /boot on /dev/md0
>> (which is /dev/sda1 and /dev/sdb1) and everything else in a volume group.
>>
>> I've copied off the first 512 bytes of sda and sdb after the install.
>> I'm snapshotting now.  After that, I'll turn off the VM, disable sda and
>> check to see if it boots.
>>
>> Next, I'll re-enable sda, boot, do the example with
>> device (hd0) /dev/sdb
>>
>> dd off the first 512 bytes of sda and sdb, shutdown, disable sda and see
>> if it boots
>>
>> Next, I'll revert to the snapshot, do the setup (hd1,0) version of the
>> example, dd off the first 512 byes of sda and sdb, disable sda and see
>> if it boots.
>>
>> Finally, I'll diff the two boot sectors for sdb between the two examples.
>>
>> Sound like a reasonable test?
> 
> yuh lost me there somewhere in the middle.  but i think simply trying both
> methods with a pulled drive drive 0 will suffice.  and i'm not sure if a
> diff of the MBR will be decisive, but i would imagine if your comparing
> the MBR on sdb method 1 to sdb method 2 and not sda to sdb, they probably
> should be identical.  the key is that the boot works identically.
Grub seems to write the block address of the 1.5 stage file into the mbr. So
even on the _same_ disk, a change in the stage file locations borks the
stage 1 (mbr) to stage 2 transition. I've done it by redoing the grub
package-level install (not an mbr update).

In short, the block addresses depend on the locations of the stage files
grub uses in its boot process.

> 
> 
> _______________________________________________
> Web Page:  http://lug.boulder.co.us
> Mailing List: http://lists.lug.boulder.co.us/mailman/listinfo/lug
> Join us on IRC: lug.boulder.co.us port=6667 channel=#colug