Wed 17 December 2014

Filed under sysadmin

Tags linux raid lvm

This little tutorial will first create a RAID array, and one LVM volume using dummy files. Next it will simulate the case where you would like to recover from just one device, and finally reassemble the complete array again.

Creating the Software RAID Array

Required packages

sudo aptitude install \
    lvm2 \

Creating files for testing

dd if=/dev/zero of=disk1 bs=1M count=30
dd if=/dev/zero of=disk2 bs=1M count=30

Create devices for these files

mdadm only accepts valid block devices for raid members. We use the loopback device for this:

sudo losetup /dev/loop0 disk1
sudo losetup /dev/loop1 disk2

Create the RAID array

sudo mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/loop0 /dev/loop1

Verify integrity

Running cat /proc/mdstat should show you the following now:

Personalities : [raid1]
md0 : active raid1 loop1[1] loop0[0]
      30656 blocks super 1.2 [2/2] [UU]

unused devices: <none>

The [UU] shows us that both devices are up.

Creating Logical Volumes

As the devices have never been used in LVM, we need to create everything first:

sudo pvcreate /dev/md0

This flags the devices as member in a volume group. Now we can craete a volume group with this raid array as member:

sudo vgcreate foobar /dev/md0

To see that it worked, run the following:

sudo vgs

You should see the following:

VG     #PV #LV #SN Attr   VSize  VFree
foobar   1   0   0 wz--n- 28.00m 28.00m

This shows us that we have one volume group (you can imagine this as a virtual hard-disk) with a bit less than 30MB of free space (we created 30M files in the very first step).

Finally, we can create our logical volumes (virtual partitions). Let's create two 10M partitions:

sudo lvcreate -L10M -n part1 foobar
sudo lvcreate -L10M -n part2 foobar

This will create two volums with the names part1 and part2. They should now be available in /dev/foobar or /dev/mapper and can be used like regular disks. To see a quick status, you can run the commands pvs, vgs and lvs for quick reports about physical volumes, volume groups and logical volumes.



This is only necessary if you followed the above steps to simulate real devices using regular files. We need to free up all references before we can remove the loopback devices.

To simulate the physical connection, you can run the following commands to clean up everything from the previous step:

sudo vgchange -a n foobar  # This deactivates the volume group from LVM
sudo mdadm --stop /dev/md0  # This removes the /dev/md0 device and frees up the loopback devices /dev/loop[01]
sudo losetup -d /dev/loop0  # Finally, this removes the loopback devices
sudo losetup -d /dev/loop1


Let's assume that you have only one disk, and you need to recover data from this disk. This section shows how to reassemble everything.

As both the software raid and LVM store metadata in the devices (superblocks), the reassembly is much easier than creation.


If you are using the dummy files as created above, we will set only one of the two files as block device. This is equivalent to physically connecting only one device:

sudo losetup /dev/loop0 disk1


First we scan for RAID devices:

sudo mdadm --assemble --scan

You should see the following:

mdadm: /dev/md/0 has been started with 1 drive (out of 2).

And a quick look at /proc/mdstat shows the following:

Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md0 : active raid1 loop0[0]
      30656 blocks super 1.2 [2/1] [U_]

unused devices: <none>

Seeing [U_] shows us that one device is up, and the other one missing.


Now that the raid device /dev/md0 is up and running, we can scan for LVM metadata:

sudo vgscan

This should show up with the following message:

Reading all physical volumes.  This may take a while...
Found volume group "foobar" using metadata type lvm2

Now, running the command pvs, vgs and/or lvs should show the previously created devices.

Restarting the array with all devices

After recovery, you can reassemble the array with all devices in the exact same way as in the previous "recovery" step. But in this case, you should see both devices as "up" in /proc/mdstat.


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