Introduction

The Distributed Replicated Block Device (DRBD) is a distributed replicated storage system for the Linux platform. It is implemented as several userspace management applications and some shell scripts and is normally used on high availability (HA) computer clusters. DRBD refers as well to the logical block devices provided by the scheme and to software that implements it.

The DRBD software is free software released under the terms of the GNU General Public License version 2. DRBD is part of the Lisog open source stack initiative. Distributed Replicated Block Device is actually a network based RAID 1. If you need to secure data on certain disk and are therefore mirroring your data to another disk via network, you have to configure DRBD on your system.

In this tutorial, let us see how to install and configure DRBD on CentOS 6.5.

Requirements

– Two disks  (preferably same size)
– Networking between machines (node1 & node2)
– Working DNS resolution  (/etc/hosts file)
– NTP synchronized times on both nodes
– Selinux Permissive
– Iptables ports (7788) allowed

Let us start DRBD installation.

Install ELRepo repository on your both system:

rpm -Uvh http://www.elrepo.org/elrepo-release-6-6.el6.elrepo.noarch.rpm

Update both repo:

 yum update -y
 setenforce 0

Install DRBD:

 [root@node1 ~]# yum -y install drbd83-utils kmod-drbd83
 [root@node2 ~]# yum -y install drbd83-utils kmod-drbd83

Insert drbd module manually on both machines or reboot:

 /sbin/modprobe drbd

Partition DRBD on both machines:

 [root@node1 ~]# fdisk -cu /dev/sdb
 [root@node2 ~]# fdisk -cu /dev/sdb

Example:

[root@node1 yum.repos.d]# fdisk -cu /dev/sdb

Device contains neither a valid DOS partition table, nor Sun, SGI or OSF disklabel
 Building a new DOS disklabel with disk identifier 0x2a0f1472.
 Changes will remain in memory only, until you decide to write them.
 After that, of course, the previous content won't be recoverable.
Warning: invalid flag 0x0000 of partition table 4 will be corrected by w(rite)
Command (m for help): p
Disk /dev/sdb: 2147 MB, 2147483648 bytes
 255 heads, 63 sectors/track, 261 cylinders, total 4194304 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: 0x2a0f1472
Device Boot Start End Blocks Id System
Command (m for help): n
 Command action
 e extended
 p primary partition (1-4)
 p
 Partition number (1-4): 1

 First sector (2048-4194303, default 2048):
 Using default value 2048
 Last sector, +sectors or +size{K,M,G} (2048-4194303, default 4194303):
 Using default value 4194303
Command (m for help): w
 The partition table has been altered!

Create the Distributed Replicated Block Device resource file (/etc/drbd.d/clusterdb.res):

[root@node1 ~]# vi /etc/drbd.d/clusterdb.res

resource clusterdb
 {
 startup {
 wfc-timeout 30;
 outdated-wfc-timeout 20;
 degr-wfc-timeout 30;
 }

net {
 cram-hmac-alg sha1;
 shared-secret sync_disk;
 }

syncer {
 rate 10M;
 al-extents 257;
 on-no-data-accessible io-error;
 }
 on node1 {
 device /dev/drbd0;
 disk /dev/sdb1;
 address 192.168.1.110:7788;
 flexible-meta-disk internal;
 }
 on node2 {
 device /dev/drbd0;
 disk /dev/sdb1;
 address 192.168.1.110:7788;
 meta-disk internal;
 }
 }

Make sure that DNS resolution is working:

/etc/hosts
 192.168.1.110 node1 node1.example.com
 192.168.1.111 node2 node2.example.com

Set NTP server and add it to crontab  on both machines:

vi/etc/crontab
 5 * * * * root ntpdate your.ntp.server

Copy DRBD configured and hosts file to node2:

 [root@node1 ~]# scp /etc/drbd.d/clusterdb.res node2:/etc/drbd.d/clusterdb.res
 [root@node1 ~]# scp /etc/hosts node2:/etc/

Initialize the DRBD meta data storage on both machines:

[root@node1 ~]# drbdadm create-md clusterdb
[root@node2 ~]# drbdadm create-md clusterdb

 You want me to create a v08 style flexible-size internal meta data block.
 There appears to be a v08 flexible-size internal meta data block
 already in place on /dev/sdb1 at byte offset 2146430976
 Do you really want to overwrite the existing v08 meta-data?
 [need to type 'yes' to confirm] yes
 Writing meta data...
 initializing activity log
 NOT initialized bitmap
 New drbd meta data block successfully created.

Start the drdb  on both nodes:

[root@node1 ~]# service drbd start
[root@node2 ~]# service drbd start

On the PRIMARY node run drbdadm command:

 [root@node1 ~]# drbdadm — –overwrite-data-of-peer primary all

Check if  Device disk initial synchronization to complete (100%) and check to confirm you are on primary node:

[root@node1 yum.repos.d]# cat /proc/drbd
 version: 8.3.16 (api:88/proto:86-97)
 GIT-hash: a798fa7e274428a357657fb52f0ecf40192c1985 build by phil@Build32R6, 2013-09-27 15:59:12
 0: cs:SyncSource ro:Primary/Secondary ds:UpToDate/Inconsistent C r-----
 ns:78848 nr:0 dw:0 dr:79520 al:0 bm:4 lo:0 pe:0 ua:0 ap:0 ep:1 wo:f oos:2017180
 [>....................] sync'ed: 4.0% (2017180/2096028)K
 finish: 0:02:58 speed: 11,264 (11,264) K/sec
ns:1081628 nr:0 dw:33260 dr:1048752 al:14 bm:64 lo:0 pe:0 ua:0 ap:0 ep:1 wo:f oos:0]

Create filesystem on Distributed Replicated Block Device device:

[root@node1 yum.repos.d]# /sbin/mkfs.ext4 /dev/drbd0
 mke2fs 1.41.12 (17-May-2010)
 Filesystem label=
 OS type: Linux
 Block size=4096 (log=2)
 Fragment size=4096 (log=2)
 Stride=0 blocks, Stripe width=0 blocks
 131072 inodes, 524007 blocks
 26200 blocks (5.00%) reserved for the super user
 First data block=0
 Maximum filesystem blocks=536870912
 16 block groups
 32768 blocks per group, 32768 fragments per group
 8192 inodes per group
 Superblock backups stored on blocks:
 32768, 98304, 163840, 229376, 294912

Writing inode tables: done
 Creating journal (8192 blocks): done
 Writing superblocks and filesystem accounting information: done

This filesystem will be automatically checked every 26 mounts or
 180 days, whichever comes first. Use tune2fs -c or -i to override.

You can now mount DRBD device on your primary node:

[root@node1 ~]# mkdir /data
[root@node1 ~]# mount /dev/drbd0  /data

Check:

[root@node1 ~]# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/vg_unixmencentos65-lv_root 19G 3.6G 15G 20% /
tmpfs 1.2G 44M 1.2G 4% /dev/shm
/dev/sda1 485M 80M 380M 18% /boot
/dev/drbd0 2.0G 36M 1.9G 2% /data

Please note:  You don’t need to mount the disk from secondary machines. All data you write on /data folder will be synced to machine2.

To see that, Unmount the /data folder from the primary node, make the secondary node as primary node,  and mount back the /data on the second machine, then you will see the same contents in /data  folder

TIPS:

1. Switch Primary/Secondary

 [root@node1 ~]# drbdadm secondary clusterdb
 [root@node2 ~]# drbdadm  -- --overwrite-data-of-peer primary all

Enjoy!