[SOLVED] Use UUID of HDDs in RAID 1 Array

[simon_lefisch]

Hey everyone,

I've been searching the interwebs for an answer to my but for whatever reason I am unable to find a clear solution.

I currently have a RAID 1 array composed of 2 x 4TB HDDs. It is configured using /dev/sdb and /dev/sdc. However I would like to change this config to use the UUIDs of each device instead since the /dev/sd* can change when adding extra storage devices. Is there any way to be able to do this with the current config or will I have to nuke the array and start over?

Btw, the reason for wanting to use the UUID is I'm trying to add another HDD for a VM, but when I boot my server I get an error saying my array is degraded. It is due to the array using /dev/sd* for device identifier and when I added the extra HDD it used /dev/sdc.

[TrevorH]

Sure, it's the default way of setting it up anyway and there should be no problem changing over. If you edit /etc/mdadm.conf to look something like this you should be good to go. Use the UUID from mdadm --detail /dev/md0 | grep -i uuid

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$ cat /etc/mdadm.conf
ARRAY /dev/md/0  metadata=1.2 UUID=12318cc4:45663f68:789c462b:abc6c778 name=trevor9.trevor.local:1
MAILADDR trevor.h@my.email

Edit: if you have data on the array that is required by the boot process then you may need yo alter the kernel command line in /boot/grub2/grub.cfg to add rd.md.uuid=<md raid uuid> but if it works now then it's probably ok.

[simon_lefisch]

Sure, it's the default way of setting it up anyway and there should be no problem changing over. If you edit /etc/mdadm.conf to look something like this you should be good to go. Use the UUID from mdadm --detail /dev/md0 | grep -i uuid

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$ cat /etc/mdadm.conf
ARRAY /dev/md/0  metadata=1.2 UUID=12318cc4:45663f68:789c462b:abc6c778 name=trevor9.trevor.local:1
MAILADDR trevor.h@my.email

Edit: if you have data on the array that is required by the boot process then you may need yo alter the kernel command line in /boot/grub2/grub.cfg to add rd.md.uuid=<md raid uuid> but if it works now then it's probably ok.

Hi Trevor,

Thanks for responding. Not sure if my question made sense or if I'm not understanding your answer, so apologies. Here is the output of mdadm --detail /dev/md0 | grep -i uuid:

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ARRAY /dev/md/0 level=raid1 num-devices=2 metadata=1.2 name=server:0 UUID=a0f4b81c:f32bd676:14105dbd:09292f38
   devices=/dev/sdb,/dev/sdc
MAILADDR my.email@address.com

As you can see, the array consists of devices sdb and sdc. However instead of using those device identifiers for the array, I would like to use the UUIDs of sdb and sdc. Does that clarify what I'm trying to do?

[TrevorH]

What's currently in your /etc/mdadm.conf? It can use device names like /dev/sd[ab] or it can use UUIDs. I showed my example file that's using uuids so you would need to adapt yours to match. The metadata= bit cannot be changed as that would need a complete rebuild to implement, the rest can be.

[simon_lefisch]

What's currently in your /etc/mdadm.conf? It can use device names like /dev/sd[ab] or it can use UUIDs. I showed my example file that's using uuids so you would need to adapt yours to match. The metadata= bit cannot be changed as that would need a complete rebuild to implement, the rest can be.

Apologies Trevor, the output of mdadm --detail /dev/md0 | grep -i uuid is:

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UUID : a0f4b81c:f32bd676:14105dbd:09292f38

My /etc/mdadm.conf is:

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ARRAY /dev/md/0 level=raid1 num-devices=2 metadata=1.2 name=server:0 UUID=a0f4b81c:f32bd676:14105dbd:09292f38
   devices=/dev/sdb,/dev/sdc
MAILADDR my.email@address.com

Sorry for the confusion of output.

[TrevorH]

I am about 99% sure that you can just remove the devices=/dev/sdb,/dev/sdc bit of that. The UUID that is already there should be enough - it matches the one from your mdadm --detail. If you are concerned about it, make a backup copy of the file as it is now and make sure that you have an install DVD handy and you can boot that in rescue mode (an option off the initial Troubleshooting menu) and copy the backup copy back.

[simon_lefisch]

I am about 99% sure that you can just remove the devices=/dev/sdb,/dev/sdc bit of that. The UUID that is already there should be enough - it matches the one from your mdadm --detail. If you are concerned about it, make a backup copy of the file as it is now and make sure that you have an install DVD handy and you can boot that in rescue mode (an option off the initial Troubleshooting menu) and copy the backup copy back.

Thanks Trevor, I will give that a shot. Currently out and about so I will let you know what happens when I get back.

I also just came across something that uses the UUID of the array and the disks by ID in fstab like this:

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UUID=a0f4b81c:f32bd676:14105dbd:09292f38       /mnt/raid1    ext4   auto,noatime,nodiratime,device=/dev/disk/by-id/ata-TOSHIBA_MD04ACA400_26F4KJ1NFSAA,device=/dev/disk/by-id/ata-TOSHIBA_MD04ACA400_26F4KJ1OFSAA    0       0

What do think about that? If that works, would I need to change anything? My current fstab entry for the array is:

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/dev/md0		/mnt/raid1		ext4	defaults	0 0

[TrevorH]

UUIDs also work well in fstab and can help avoid things like the device name of /dev/md0 changing. It won't be the _same_ uuid

[simon_lefisch]

UUIDs also work well in fstab and can help avoid things like the device name of /dev/md0 changing. It won't be the _same_ uuid

Hi Trevor,

Sorry for the late response. I did what you said as far as removing the devices line in my mdadm.conf file and that worked perfectly!. Thanks for your help, it is much appreciated