want to (safely) test raid setup and ability to boot from alternate drives
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install grub with software raid (mdadm) - safetly boot from alternate drive(s)
Hello all,
I have successfully setup software raid1 with hot spare (total 3 drives) on my system and I want to test its ability to boot from one of the alternate drives if the primary dies.
backup. be sure to have the grub bootloader on all drives MBRs (important w/ sw-raid; grub-install hdX). now just remove one drive.. . there's an alternative way by marking a drive faulty - see man mdadm for more info.
I choose to use RAID1 over RAID5 primarily for reliability over speed.
I bought all 3 drives together and if for some reason 1 dies, I would naturally start to wonder about the other 2. Also, in the rare chance that 2 drives have faults I'm still ok.
I've also read about it and it seems that using RAID5 on a 3 drive setup doesn't really offer a huge amount of performance. I would rather have RAID5 with 5 or more drives.
backup. be sure to have the grub bootloader on all drives MBRs (important w/ sw-raid; grub-install hdX). now just remove one drive.. . there's an alternative way by marking a drive faulty - see man mdadm for more info.
sl, ritch
Thank you. I will try that out sometime this week, when I get a chance.
OK - before I attempt to install grub on my alternate hard drives I want to check to see if it is already installed. I also want to know (after I try to install it) whether or not it is installed.
Is there a way to verify that grub is installed on my alternate hard drives?
Well, I didn't find a (good) way to verify if grub was installed on my alternate drives. The only way I could really find out is unplug my primary drive and boot the system. I was left with a blinking cursor. So I took that as a 'NO'.
After a few searches and fiddling around I found out how to install grub on the alternate drives.
Some of this may be debian specific so double check before using yourself.
I ran:
Code:
grub-install /dev/sdb1
Searching for GRUB installation directory ... found: /boot/grub
Installation finished. No error reported.
This is the contents of the device map /boot/grub/device.map.
Check if this is correct or not. If any of the lines is incorrect,
fix it and re-run the script `grub-install'.
(fd0) /dev/fd0
(hd0) /dev/sda
(hd1) /dev/sdb
(hd2) /dev/sdc
grub-install /dev/sdc1
Searching for GRUB installation directory ... found: /boot/grub
Installation finished. No error reported.
This is the contents of the device map /boot/grub/device.map.
Check if this is correct or not. If any of the lines is incorrect,
fix it and re-run the script `grub-install'.
(fd0) /dev/fd0
(hd0) /dev/sda
(hd1) /dev/sdb
(hd2) /dev/sdc
Note: If you get an error like "grub: no corresponding bios drive" try running:
Code:
grub-install --recheck /dev/sdx
Where X is your primary drive (ie: sda1)
It checks for all the available drives to boot from and puts them in the device.map file. I choose to just re-install on the primary drive, because it doesn't work if you don't put a drive location... It should IMO - it just updates the device.map. I suppose you can manually alter this file also... but I would read the man page to find out for sure.
Then I ran:
Code:
#grub
GNU GRUB version 0.97 (640K lower / 3072K upper memory)
[ Minimal BASH-like line editing is supported. For
the first word, TAB lists possible command
completions. Anywhere else TAB lists the possible
completions of a device/filename. ]
grub> root (hd1,0)
Filesystem type is ext2fs, partition type 0xfd
grub> setup (hd1)
Checking if "/boot/grub/stage1" exists... yes
Checking if "/boot/grub/stage2" exists... yes
Checking if "/boot/grub/e2fs_stage1_5" exists... yes
Running "embed /boot/grub/e2fs_stage1_5 (hd1)"... 15 sectors are embedded.
succeeded
Running "install /boot/grub/stage1 (hd1) (hd1)1+15 p (hd1,0)/boot/grub/stage2 /boot/grub/menu.lst"... succeeded
Done.
grub> root (hd2,0)
Filesystem type is ext2fs, partition type 0xfd
grub> setup (hd2)
Checking if "/boot/grub/stage1" exists... yes
Checking if "/boot/grub/stage2" exists... yes
Checking if "/boot/grub/e2fs_stage1_5" exists... yes
Running "embed /boot/grub/e2fs_stage1_5 (hd2)"... 15 sectors are embedded.
succeeded
Running "install /boot/grub/stage1 (hd2) (hd2)1+15 p (hd2,0)/boot/grub/stage2 /boot/grub/menu.lst"... succeeded
Done.
Finally I added a couple boot entries to my menu.lst for fallback and to manually select if I need to.
my menu.lst (this is VERY debian specific):
Code:
== cut to save space ==
## fallback num
# Set the fallback entry. This allows you to boot from an alternate drive
# if the first drive of the array fails.
fallback 5
== cut to save space ==
### END DEBIAN AUTOMAGIC KERNELS LIST
## Fallback Entry (fallback 5)
title Debian GNU/Linux, kernel 2.6.26 (sdb)
root (hd1,0)
kernel /boot/vmlinuz-2.6.26 root=/dev/md0 ro
initrd /boot/initrd.img-2.6.26
## Fallback Entry (fallback 6)
title Debian GNU/Linux, kernel 2.6.26 (sdc)
root (hd2,0)
kernel /boot/vmlinuz-2.6.26 root=/dev/md0 ro
initrd /boot/initrd.img-2.6.26
I unplugged my primary drive (after shutdown of course) and booted it up. Lo and behold it came alive!
I haven't tried to boot off of one drive, but I'm sure it would work. I'm going to make a boot floppy as an additional backup just in case.
Hope this helps someone.
Last edited by checkmate3001; 08-03-2008 at 04:50 AM.
Reason: fix a code section and add a title
Nice post on raid 1 and grub; I'm sure it'll be helpful for a lot of people who ask the same grub questions. Not at all Debian specific though; other than the word "Debian" it ought to work with any distro. Certainly ought to work with minimal renaming of files for slackware, pclinux, ubuntu as far as I know. Still, probably a good idea to put a warning/disclaimer on it.
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. there's an alternative way by marking a drive faulty - see man mdadm for more info.
