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

[Robert Heller]

At Sat, 04 Dec 2010 18:34:26 +0100 CentOS mailing list <centos at 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 at 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 at 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 at 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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>                                                                                   

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