Previously I had a software raid set up using (mdadm) of drives sda and sdb. sdb failed and the only way to reboot the system was by unplugging the second hard drive.
Now I've added fresh sdb and sdc to my RAID setup. sda is the oldest (so most likely to fail) and it is the drive from which we boot (I think, how can I check?).
How can I ensure and test (through GRUB configuration, etc.) that if sda fails, I will still be able to boot my machine.
fdisk -l:
Disk /dev/sda: 250.0 GB, 250000000000 bytes
255 heads, 63 sectors/track, 30394 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000080
Device Boot Start End Blocks Id System
/dev/sda1 * 1 30064 241489048+ fd Linux raid autodetect
/dev/sda2 30065 30394 2650725 5 Extended
/dev/sda5 30065 30394 2650693+ fd Linux raid autodetect
Disk /dev/sdb: 500.1 GB, 500107862016 bytes
255 heads, 63 sectors/track, 60801 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0x00000000
Device Boot Start End Blocks Id System
/dev/sdb1 * 1 30064 241489048+ fd Linux raid autodetect
Partition 1 does not start on physical sector boundary.
/dev/sdb2 30065 30394 2650725 5 Extended
/dev/sdb5 30065 30394 2650693+ fd Linux raid autodetect
Partition 5 does not start on physical sector boundary.
Disk /dev/sdc: 500.1 GB, 500107862016 bytes
255 heads, 63 sectors/track, 60801 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disk identifier: 0x00000000
Device Boot Start End Blocks Id System
/dev/sdc1 * 1 30064 241489048+ fd Linux raid autodetect
Partition 1 does not start on physical sector boundary.
/dev/sdc2 30065 30394 2650725 5 Extended
/dev/sdc5 30065 30394 2650693+ fd Linux raid autodetect
Partition 5 does not start on physical sector boundary.
Disk /dev/md0: 247.3 GB, 247284695040 bytes
2 heads, 4 sectors/track, 60372240 cylinders
Units = cylinders of 8 * 512 = 4096 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Alignment offset: 512 bytes
Disk identifier: 0x00000000
Disk /dev/md0 doesn't contain a valid partition table
Disk /dev/md1: 2714 MB, 2714238976 bytes
2 heads, 4 sectors/track, 662656 cylinders
Units = cylinders of 8 * 512 = 4096 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Alignment offset: 512 bytes
Disk identifier: 0x00000000
Disk /dev/md1 doesn't contain a valid partition table
Best Answer
This is an old chestnut. The short answer is that "grub-install" is often the wrong answer for Software RAID. Here is an example where I have a 3-way RAID-1 array. The /boot partition is stored at /dev/md0. This installs GRUB to each disk, so that if one disk fails, you can boot off one of the other disks.
In the future versions of GRUB, it's much smarter, but CentOS 6 / RHEL 6 still ship with the older GRUB.
To test: Change the "timeout=5" value in your grub.conf file (under /boot) to something like timeout=30. Then swap the location of the two drives before powering the system back on. Or change the the boot order of the hard drives in the BIOS.
(Naturally... make sure you have good backups and know how to put it back to the correct configuration. Testing this out on a throwaway system is always a good idea.)