Backing Up DRBD From the Secondary

DRBD® replicates data so you always have more than a single copy. Wouldn’t it be convenient to be able to take a backup from a Secondary node where doing so would have less chance of interference or overloading the Primary node that is currently serving up the data and running the production workload? Well, with LVM snapshots you can do exactly that!

In this environment DRBD is backed by a LVM volume. This means that storage device used for DRBD is a LVM volume. You can confirm this easily by entering an lsblk command. For example:

root@booth-1:~# lsblk
NAME               MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
[...]
vdb                252:16   0    12G  0 disk
├─vg_drbd-files    253:0    0     1G  0 lvm
│ └─drbd0          147:0    0  1024M  0 disk

Above you can see that drbd0 is backed by the vg_drbd-files LVM device.

To take the backup from the secondary you will be using LVM snapshots. The first step here is to identify the path of the LVM device. You can easily verify this by looking at the *.res resource configuration file for your DRBD resource. In this example it’s /dev/vg_drbd/files.

root@booth-1:~# cat /etc/drbd.d/files.res | grep disk
  disk      /dev/vg_drbd/files;
  meta-disk internal;
  disk      /dev/vg_drbd/files;
  meta-disk internal;

Next you create the snapshot. Please note that this does require some free-space remain on the volume group, enough to store the delta of what’s changed between the time the snapshot is created and when you delete it after taking your backup.

root@booth-1:~# lvcreate --size 1G --snapshot --name files_snap /dev/vg_drbd/files
  Logical volume files_snap created.

root@booth-1:~# lvs
[...]
  files      vg_drbd owi-aos--- 1.00g
  files_snap vg_drbd swi-a-s--- 1.00g      files  0.00

At this point you can either backup the /dev/vg_drbd/files_snap at the block layer by using a command like dd, or you can mount it to take a file based backup using a utility like tar, or rsync.

root@booth-1:~# mount /dev/vg_drbd/files_snap /files_snap -t xfs
root@booth-1:~# ls /files_snap/
testfile0  testfile1  testfile2  testfile3  testfile4  testfile5  testfile6  testfile7  testfile8

Please note that LVM snapshots operate on a Copy-On-Write design. This means that every write operation that would alter existing data will now require two writes. One for the original volume, and a copy to the snapshot. It is because of this it is advised to remove the LVM snapshot once the backup is complete.

root@booth-1:~# umount /files_snap
root@booth-1:~# lvremove /dev/vg_drbd/files_snap
Do you really want to remove and DISCARD active logical volume vg_drbd/files_snap? [y/n]: y
  Logical volume files_snap successfully removed

Written by DJV 2022-05-10