[CentOS] Fiddling with software RAID1 : continue working with one of two disks failing?

Sat Dec 4 18:01:20 UTC 2010
Les Mikesell <lesmikesell at gmail.com>

On 12/4/10 11:34 AM, Niki Kovacs wrote:
> I wasn't sure where to install GRUB, so I chose /dev/md0.

Grub doesn't know anything about raid.  It only works because each component of 
a RAID1 looks just like a non-raid filesystem.  You should install grub on the 
master boot partition of both member disks.

> I was wondering if this setup theoretically enabled me to continue
> working with one disk failure. So I tried unplugging the power cord of
> one of my hard disks... which resulted in a "GRUB Disk Error" on boot.
> Question : is there a way to still run the system with either of the two
> disks "damaged" (in this case : unplugged)? And if so, how would I have
> to go about it in my setup?

Yes, raid1 isn't bothered at all by missing members.  You just have to install 
grub on the underlying disks as though you did not have raid. There can be some 
differences in the way ide/sata/scsi controllers handle a disk failure - and 
IDE's often hang the controller if one of two disks on the cable fails. 
Normally the first disk will always work if the 2nd fails, but if the first disk 
fails both the bios and linux have to see the same shift in positions - so even 
though you are installing grub on what it calls hd1, if it boots from there it 
will see its root at hd0,1.   Worst case - just be prepared to boot a rescue 
disk and reinstall grub if your first disk fails.

When you replace the drive and want to rebuild the mirror, just:
mdadm --add /dev/md? /dev/sd??

   Les Mikesell
    lesmikesell at gmail.com