DRBD

Distributed Replicated Block Device, makes it possible to replicate an disk/block device across nodes.
You can only mount the DRBD on one node at a time, unless an cluster filesystem is used, but this is ideal to put 'below' and HA NFS server.

DRBD 8.3.13 is used in this setup.

For real HA, please consider pacemaker,corosync or similar, since this simple setup will require manual interfering, if something goes down.

2 identical disks must be present on each node.
In this example we will be using /dev/sdb1 which ~ 1GB

If the partition on the disk is not created, please create it. My partition on both nodes looks like this.

fdisk -l /dev/sdb

Disk /dev/sdb: 1073 MB, 1073741824 bytes
139 heads, 8 sectors/track, 1885 cylinders, total 2097152 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x958c5f64

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1            2048     2097151     1047552   83  Linux

On both nodes install drbd

apt-get install drbd8-utils

On both nodes modprobe the drbd module, yes this is needed, since it is not loaded when installing.

modprobe drbd

On both nodes create the an unique ressource file to the sdb1 disk.

/etc/drbd.d/r0.res
resource r0 {
  device /dev/drbd0;
  disk /dev/sdb1;
  meta-disk internal;

  on testserver01 {
    address 10.0.2.1:7788;
  }
  on testserver02 {
    address 10.0.2.100:7788;
  }
  syncer {
    rate  7M;
  }
}

Create/edit the global_common.conf file with following content.

/etc/drbd.d/global_common.conf
global {
        usage-count no;
        # minor-count dialog-refresh disable-ip-verification
}

common {
        protocol C;

        handlers {
                # The following 3 handlers were disabled due to #576511.
                # Please check the DRBD manual and enable them, if they make sense in your setup.
                # pri-on-incon-degr "/usr/lib/drbd/notify-pri-on-incon-degr.sh; /usr/lib/drbd/notify-emergency-reboot.sh; echo b > /proc/sysrq-trigger ; reboot -f";
                # pri-lost-after-sb "/usr/lib/drbd/notify-pri-lost-after-sb.sh; /usr/lib/drbd/notify-emergency-reboot.sh; echo b > /proc/sysrq-trigger ; reboot -f";
                # local-io-error "/usr/lib/drbd/notify-io-error.sh; /usr/lib/drbd/notify-emergency-shutdown.sh; echo o > /proc/sysrq-trigger ; halt -f";

                # fence-peer "/usr/lib/drbd/crm-fence-peer.sh";
                # split-brain "/usr/lib/drbd/notify-split-brain.sh root";
                # out-of-sync "/usr/lib/drbd/notify-out-of-sync.sh root";
                # before-resync-target "/usr/lib/drbd/snapshot-resync-target-lvm.sh -p 15 -- -c 16k";
                # after-resync-target /usr/lib/drbd/unsnapshot-resync-target-lvm.sh;
        }

        startup {
                # wfc-timeout degr-wfc-timeout outdated-wfc-timeout wait-after-sb
        }

        disk {
                # on-io-error fencing use-bmbv no-disk-barrier no-disk-flushes
                # no-disk-drain no-md-flushes max-bio-bvecs
        }

        net {
                cram-hmac-alg sha1;
                shared-secret "supermand";
                # sndbuf-size rcvbuf-size timeout connect-int ping-int ping-timeout max-buffers
                # max-epoch-size ko-count allow-two-primaries cram-hmac-alg shared-secret
                # after-sb-0pri after-sb-1pri after-sb-2pri data-integrity-alg no-tcp-cork
        }

        syncer {
                # rate after al-extents use-rle cpu-mask verify-alg csums-alg
        }
}

Configuration definitions

Please refer to man pages for more info.

rate - sets syncronization limits
protocol - defines when an sync is marked as OK. (C - OK when both local and remote write are completed)
cram-hmac-alg [alg] - enables peer authentication and the algorithm used, all cryptos in /proc/crypto can be used
shared-secret - the shared secret used in peer authentication

On both nodes, create the DRBD ressource

drbdadm create-md r0
Writing meta data...
initializing activity log
NOT initialized bitmap
New drbd meta data block successfully created.

On both nodes, start the DRBD

service drbd start

On the primary node only, bring up the ressouce

drbdadm up r0

On both nodes, create a filesystem on the DRBD ressource.

mkfs.ext4 /dev/drbd0

create initial sync, primary node only

drbdadm -- --overwrite-data-of-peer primary r0

Now it should be possible to mount the DRBD ressource, if mount point is not created, create first.

mkdir /mnt/drdb
mount /dev/drbd0 /mnt/drdb/

Check if its mounted correctly

df -h |grep drbd
/dev/drbd0                                             1007M   18M  939M   2% /mnt/drdb

View status and sync progress

drbdadm status
<drbd-status version="8.3.13" api="88">
<resources config_file="/etc/drbd.conf">
<resource minor="0" name="r0" cs="Connected" ro1="Primary" ro2="Secondary" ds1="UpToDate" ds2="UpToDate" />
</resources>
</drbd-status>

Caveats

If both nodes reboots, they will most likely not be able to mount automatically, on one host.
If this should work, corosync, heartbeat or similar should be used, but manually it's done this way.
Secondary node

umount /dev/drbd0
drbdadm secondary /dev/drbd0

Primary node

umount /dev/drbd0
drbdadm primary /dev/drbd0
mount /dev/drbd0 /mnt/drdb/