AndrewsBlog

I recently had a hard drive crash on my home Linux NAS box. The important files were on a software raid 5 array, but the root filesystem (with the majority of the OS) were on a separate IDE hard drive.

Only the IDE drive crashed so my important files were safe, but I needed to remount the raid array. It took me quite a while before I got that right, so I’ll share my knowledge here for you. Here are the commands needed to reconstruct / remount a raid array with a Gentoo live cd:

load the raid module and lvm driver if you need to:

# modprobe raid5
# modprobe dm-mod

Find the UUID’s of the devices

( you can skip this step if you have a backup of your /etc/mdadm.conf file )

# mdadm --examine --scan /dev/sda1

Where /dev/sda1 points to one of the partitions used for the raid array. The output looks like this on my box:

ARRAY /dev/md1 level=raid5 num-devices=4 UUID=94366f38:ccdadda0:06f85cb1:f1263d52

copy it to /etc/mdadm.conf

# mdadm --examine --scan /dev/sda1 >> /etc/mdadm.conf

Now we need to add a list of devices to mdadm.conf of where to look for member disks:

(manually add this to mdadm.conf, the devices i had to add where /dev/sda1 to /dev/sdd1)

DEVICE /dev/sd[abcd]1

At the end that leaves us with this in our mdadm.conf file:

DEVICE /dev/sd[abcd]1
ARRAY /dev/md1 level=raid5 num-devices=4 UUID=94366f38:ccdadda0:06f85cb1:f1263d52

Assemble the raid array

We make the mirror device node:

# mknod /dev/md1 b 9 1

Assemble the array:

# mdadm --assemble --scan

You can check /proc/mdstat to see if the array is online:

# cat /proc/mdstat
Personalities : [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md1 : active raid5 sda1[0] sdd1[3] sdc1[2] sdb1[1]
937705728 blocks level 5, 64k chunk, algorithm 2 [4/4] [UUUU]

Activate the logical volumes

# vgscan

Scans your disk and should show you the Volume Group that was on the raid array and add it to the lvm database

# vgchange -ay

This should activate the volumes and auto create the nodes in /dev

Mount the filesystems

I just needed to mount the filesystems just like I would mount a regular one:

mount --type ext3 /dev/vg2/var /mnt/gentoo/var

Hope this helps someone!