MDADM Help Urgently Needed!!! Raid 5 Array not starting.
I have 4 WD10EARS drives running in a RAID 5 array using MDADM.
Yesterday my OS Drive failed. I have replaced this and installed a fresh copy of Ubuntu 11.04 on it.
I then installed MDADM, and rebooted the machine, hoping that it would automatically rebuild the array.
It hasnt, when i look at the array using Disk Utility, it says that the array is not running. If i try to start the array it says :

Error assembling array: mdadm exited with exit code 1: mdadm: failed to RUN_ARRAY /dev/md0: Input/output error
mdadm: Not enough devices to start the array.

I have tried MDADM --assemble --scan and it gives this output:

mdadm: /dev/md0 assembled from 2 drives - not enough to start the array.

I know that there are 4 drives present as they are all showing, but it is only using 2 of them.

I also ran MDADM -- detail /dev.md0 which gave:


root@warren-P5K-E:~# mdadm --detail /dev/md0
/dev/md0:
Version : 0.90
Creation Time : Tue Dec 21 20:00:42 2010
Raid Level : raid5
Used Dev Size : 976762496 (931.51 GiB 1000.20 GB)
Raid Devices : 4
Total Devices : 2
Preferred Minor : 0
Persistence : Superblock is persistent

Update Time : Thu Jun 2 08:27:09 2011
State : active, FAILED, Not Started
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0

Layout : left-symmetric
Chunk Size : 64K

UUID : 78bded27:4f36b420:b8c93aaa:68704163
Events : 0.61

Number Major Minor RaidDevice State
0 0 0 0 removed
1 8 32 1 active sync /dev/sdc
2 0 0 2 removed
3 8 48 3 active sync /dev/sdd


By looking at this it looks like only 2 of th 4 drives are activated.

How do I get them all active and start the array again.

There has been nothing written to any of the 4 drives.

Any help would be appreciated.

Are the missing drives physically present?
i.e. is there supposed to be a /dev/sdb etc
Do a sudo fdisk -l to check.
If they are then you could try
mdadm --assemble /dev/md0 --uuid={UUID-of-raid5-array} --force

Hi, Thanks for the reply.

What is the {UUID-of-raid5-array} ?

Do I need to tell the system the order of the 4 drives?

Im not sure if the system is trying to mount the drives in the array in the wrong order.

If I run:

root@warren-P5K-E:/home/warren# mdadm --assemble --scan -v

I get : (I have removed the other OS drive info)

mdadm: looking for devices for /dev/md0
mdadm: /dev/sdd is identified as a member of /dev/md0, slot 3.
mdadm: /dev/sdc is identified as a member of /dev/md0, slot 1.
mdadm: /dev/sdb is identified as a member of /dev/md0, slot 2.
mdadm: /dev/sda is identified as a member of /dev/md0, slot 0.
mdadm: added /dev/sda to /dev/md0 as 0
mdadm: added /dev/sdb to /dev/md0 as 2
mdadm: added /dev/sdd to /dev/md0 as 3
mdadm: added /dev/sdc to /dev/md0 as 1
mdadm: /dev/md0 assembled from 2 drives - not enough to start the array.


I cant get hold of my mdadm.conf file, as it was on the OS drive that died.

Any help would be appreciated. I really don't want to loose the data

as you have already discovered
UUID : 78bded27:4f36b420:b8c93aaa:68704163

Thanks Brak,

WIll it try to rebuild the array in the right order?
If you look at the second post, it looks like 2 of the disks are in the wrong order.
If I force it with them in the wrong order will it ruin my data?
I had a problem with the array before the os drive went, it couldnt find the mdadm.conf file then, and could not start it for some reason. As soon as I fixed the OS drive and it could access the mdadm.conf file it worked fine. Unfortunatley I cant find a backup of the mdadm.conf file (I think its backed up on the array)
Thanks again for your speedy reply, and all your help.
I will let you know how it goes tonight when I return home.

Hi There,

I tried :

root@warren-P5K-E:/home/warren# mdadm --assemble /dev/md0 --uuid={78bded27:4f36b420:b8c93aaa:68704163} --force

and it came back with:

mdadm: Bad uuid: {78bded27:4f36b420:b8c93aaa:68704163}

I still cant get it started, so any help would be appreciated,

What does
mdadm -A /dev/md0 /dev/sd[a-d]
come back with?

Thanks Brak, I really need to get this sorted.

root@warren-P5K-E:/home/warren# mdadm -A /dev/md0 /dev/sd[a-d]
mdadm: /dev/md0 assembled from 2 drives - not enough to start the array.
root@warren-P5K-E:/home/warren#

Whooo..

Got my array working with:

sudo mdadm --assemble --scan -f

It built in seconds, and all the data is still there.

Time to finish the backup now I think.

How do I save a copy of the current config to mdadm.conf ?
Or backup the arrays settings to store on a flash drive?

Thanks to anyone that helped.

It may have already re-created the /etc/mdadm.conf file as the man page says
in the assemble scan section

If udev is active, mdadm does not create any entries in /dev but leaves
that to udev. It does record information in /var/run/mdadm/map which
will allow udev to choose the correct name.

If mdadm detects that udev is not configured, it will create the
devices in /dev itself.