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title Distributed Replicated Block Device (DRBD)

Distributed Replicated Block Device (DRBD)

Dec 15, 2018

Distributed Replicated Block Device (DRBD) mirrors block devices between multiple hosts. The replication is transparent to other applications on the host systems.

(Tested on ubuntu:16.04 and ubuntu:18.04)

Create a Logical Volume

References:

It is assumed that there's a volume group named ubuntu-vg exists with sufficient free space. You need to create logical volumes on each node.

lvcreate -L 10G --name drbd-lv ubuntu-vg

Our block devices on each node will be /dev/ubuntu-vg/drbd-lv.

You should disable the LVM cache in /etc/lvm/lvm.conf on both nodes:

write_cache_state = 0

and remove any stale cache entries by:

sudo rm -rf /etc/lvm/cache/.cache

Install DRBD9 Packages

Add repository in: https://launchpad.net/~linbit/+archive/ubuntu/linbit-drbd9-stack

sudo add-apt-repository ppa:linbit/linbit-drbd9-stack
sudo apt-get update

Install DRBD packages

sudo apt install -y drbd-utils python-drbdmanage drbd-dkms

Configure DRBD

References:

It is assumed that there are two nodes, and their hostnames are alpha and delta. Our primary node will be alpha for this setup.

On each nodes, edit /etc/hosts file and add respective hostname and ip addresses. Make sure adding 127.0.1.1 as hostname.

127.0.1.1       delta
192.168.7.123   alpha
192.168.7.140   delta

Select the same timezone on each node, UTC is recommended.

sudo dpkg-reconfigure tzdata

Install ntp to synchronize clocks.

sudo apt-get update
sudo apt-get install -y ntp

On each node, edit /etc/drbd.conf. Make sure removing other config file includes.

global {
       usage-count no;
}
common {
       protocol C;
}
resource r0 {
         net {
            fencing resource-only;
         }
         handlers {
            fence-peer "/usr/lib/drbd/crm-fence-peer.9.sh";
            after-resync-target "/usr/lib/drbd/crm-unfence-peer.9.sh";
         }
         on alpha {
            device /dev/drbd0;
            disk /dev/ubuntu-vg/drbd-lv;
            address 192.168.7.123:7788;
            meta-disk internal;
         }
         on delta {
            device /dev/drbd0;
            disk /dev/delta-vg/drbd-lv;
            address 192.168.7.140:7788;
            meta-disk internal;
         }
}

Protocol C is synchronous replication protocol. "Local write operations on the primary node are considered completed only after both the local and the remote disk write(s) have been confirmed.": https://docs.linbit.com/docs/users-guide-9.0/#s-replication-protocols

Fencing peer is crucial to avoid split brain when the connection between nodes is lost. Handlers ensure that when a node disconnects and falls back from other node, it has to re-sync all the contents from the other node before being primary again.

On each node, load the drbd kernel module, create meta data and bring it online.

sudo modprobe drbd
sudo drbdadm create-md r0
sudo drbdadm up r0

You can check the status with:

sudo drbdadm status r0

You should see something similar to the following.

r0 role:Secondary
  disk:Inconsistent
  delta role:Secondary
    disk:Inconsistent

Initial synchronization

Initial synchronization has to be started on only one node. Run the following on the primary node - the node selected as the synchronization source.

sudo drbdadm primary --force r0

The synchronization should take a while depending on the logical volume size ~ around 8% per hour for a 500GB HDD volume (PS: It took nearly 13 hours to complete). You should instead start with a small disk and gradually extend the logical volume beneath it as you need more size. You can watch its status by running:

sudo drbdadm status

You should see something similar to the following.

r0 role:Secondary
  disk:Inconsistent
  alpha role:Primary
    replication:SyncTarget peer-disk:UpToDate done:11.31

Once it is complete you will see a status with disks up to date:

r0 role:Primary
  disk:UpToDate
  delta role:Secondary
    peer-disk:UpToDate

Now, only on the primary node, you can format the disk and mount it.

sudo mkfs.ext4 /dev/drbd0
sudo mount /dev/drbd0 /srv

Currently, the data is only served on the primary node, on path /srv. If we wanted it to be served on the secondary node, we'd have to unmount it from the primary, and make the drbd resource online and mount it on the secondary manually. To make it highly available, we still need to install and configure a Cluster Resource Manager called pacemaker. I will explain it in a different post.

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