At Sat, 04 Dec 2010 18:34:26 +0100 CentOS mailing list centos@centos.org wrote:
Hi,
I'm currently experimenting with software RAID1 on a spare PC with two 40 GB hard disks. Normally, on a desktop PC with only one hard disk, I have a very simple partitioning scheme like this :
/dev/hda1 80 MB /boot ext2 /dev/hda2 1 GB swap /dev/hda3 39 GB / ext3
Here's what I'd like to do. Partition a second hard disk (say, /dev/hdb) with three partitions. Setup RAID1 like this :
/dev/md0 80 MB /boot ext2 /dev/md1 1 GB swap /dev/md2 39 GB / ext3
I somehow managed to get this far. Here's what I have :
[root@raymonde ~]# fdisk -l /dev/hda
Disk /dev/hda: 41.1 GB, 41110142976 bytes 255 heads, 63 sectors/track, 4998 cylinders Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System/dev/hda1 * 1 11 88326 fd Linux raid autodetect /dev/hda2 12 134 987997+ fd Linux raid autodetect /dev/hda3 135 4998 39070080 fd Linux raid autodetect
[root@raymonde ~]# fdisk -l /dev/hdb
Disk /dev/hdb: 41.1 GB, 41110142976 bytes 16 heads, 63 sectors/track, 79656 cylinders Units = cylinders of 1008 * 512 = 516096 bytes
Device Boot Start End Blocks Id System/dev/hdb1 * 1 156 78592+ fd Linux raid autodetect /dev/hdb2 157 2095 977256 fd Linux raid autodetect /dev/hdb3 2096 79656 39090744 fd Linux raid autodetect
During install, my /dev/md1 and /dev/md2 got somehow mixed up, which doesn't really matter :
[root@raymonde ~]# cat /etc/fstab /dev/md1 / ext3 defaults 1 1 /dev/md0 /boot ext2 defaults 1 2 tmpfs /dev/shm tmpfs defaults 0 0 devpts /dev/pts devpts gid=5,mode=620 0 0 sysfs /sys sysfs defaults 0 0 proc /proc proc defaults 0 0 /dev/md2 swap swap defaults 0 0
I wasn't sure where to install GRUB, so I chose /dev/md0.
No, you install GRUB (or alternitively, lilo) on *both* /dev/hda AND /dev/hdb, with your root /dev/hda1. Neither grub (nor lilo) know about RAID (ditto for the BIOS). This is not a problem, since the *elements* of a RAID1 set look like 'normal' partitions with normal file systems on them. You want grub to be in the MBR of /dev/hda -- duping it in /dev/hdb's MBR allows you to boot (with degraded RAID sets) from /dev/hdb (cabled & jumpered to be /dev/hda) in the event /dev/hda dies.
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, see above.
Minor performance nit: Doing RAID with two IDE disks on the *same* controller is not going to buy you anything in terms of performance. I suspect this is just experimental, mostly to get the feel for how to set things up, so this is not a major issue.
Cheers from the freezing South of France,
Niki
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