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Old 12-04-2010, 05:49 PM
Robert Heller
 
Default Fiddling with software RAID1 : continue working with one of two disks failing?

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
>
>
>
>
>
> _______________________________________________
> CentOS mailing list
> CentOS@centos.org
> http://lists.centos.org/mailman/listinfo/centos
>
>

--
Robert Heller -- 978-544-6933 / heller@deepsoft.com
Deepwoods Software -- http://www.deepsoft.com/
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