DRBD(without clustering)

Do you need transparent, real-time replication of block devices without the need for specialty hardware without paying anything ?

If your answer is YES. DRBD is your solution. DRBD is a distributed replicated storage system for the Linux platform. It is implemented as a kernel driver, several userspace management applications, and some shell scripts. DRBD is traditionally used in high availability (HA)

In this post, I am going create HA cluster block storage. Switching-over will be handled manually. But in the next post I will add cluster software. I have two Debian systems for this lab. It is depicted in the Figure-1 sample architecture.

Figure-1 Sample HA Block Storage

Reference:https://www.ibm.com/developerworks/jp/linux/library/l-drbd/index.html

Installing DRDB packages:

Install drbd8-utils on each of the node.

root@debian1:~# apt-get install drbd8-utils 

Add hostnames into the /etc/hosts file on each of the node.

192.168.122.70 debian1
192.168.122.71 debian2

Creating a file system:

Instead of adding a disk storage we create a file and use it as a storage on each of the node.

root@debian1:~# mkdir /replicated
root@debian1:~# dd if=/dev/zero of=drbd.img bs=1024K count=512
root@debian2:~# mkdir /replicated
root@debian2:~# dd if=/dev/zero of=drbd.img bs=1024K count=512
root@debian1:~# losetup /dev/loop0 /root/drbd.img
root@debian2:~# losetup /dev/loop0 /root/drbd.img

Configuring DRBD:

Add the configuration below on each of the node.

root@debian1:~# cat /etc/drbd.d/replicated.res
resource replicated {
protocol C;          
on debian1 {
                device /dev/drbd0;
                disk /root/drbd.img;
                address 192.168.122.70:7788;
                meta-disk internal;
                }
on debian2 {
                device /dev/drbd0;
                disk /root/drbd.img;
                address 192.168.122.71:7788;
                meta-disk internal;
                }
               
} 


Initializing metadata storage(on each node)

root@debian1:~# drbdadm create-md replicated
initializing activity log
NOT initializing bitmap
Writing meta data...
New drbd meta data block successfully created.
root@debian1:~# 

Make sure that drbd service is running on both nodes.

● drbd.service - LSB: Control DRBD resources.
   Loaded: loaded (/etc/init.d/drbd; generated; vendor preset: enabled)
   Active: active (exited) since Fri 2019-02-01 15:32:34 +04; 6min ago
     Docs: man:systemd-sysv-generator(8)
  Process: 1399 ExecStop=/etc/init.d/drbd stop (code=exited, status=0/SUCCESS)
  Process: 1420 ExecStart=/etc/init.d/drbd start (code=exited, status=0/SUCCESS)
    Tasks: 0 (limit: 4915)
   CGroup: /system.slice/drbd.service

root@debian2:~# lsblk 
NAME              MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
loop0               7:0    0  512M  0 loop 
└─drbd0           147:0    0  512M  1 disk 
sr0                11:0    1 1024M  0 rom  
vda               254:0    0   10G  0 disk 
└─vda1            254:1    0   10G  0 part 
  ├─vgroot-lvroot 253:0    0  7.1G  0 lvm  /
  └─vgroot-lvswap 253:1    0  976M  0 lvm  [SWAP]

DRDB uses only one node at a time as a primary node where read and write can be preformed. We will at first specify node 1 as primary node.

root@debian1:~# drbdadm primary replicated --force

root@debian1:~# cat /proc/drbd 
version: 8.4.7 (api:1/proto:86-101)
srcversion: AC50E9301653907249B740E
0: cs:SyncSource ro:Primary/Secondary ds:UpToDate/Inconsistent C r-----
ns:516616 nr:0 dw:0 dr:516616 al:8 bm:0 lo:0 pe:0 ua:0 ap:0 ep:1 wo:f oos:7620
[==================>.] sync'ed: 99.3% (7620/524236)K
finish: 0:00:00 speed: 20,968 (13,960) K/sec
root@debian1:~# cat /proc/drbd
version: 8.4.7 (api:1/proto:86-101)
srcversion: AC50E9301653907249B740E
0: cs:SyncSource ro:Primary/Secondary ds:UpToDate/Inconsistent C r-----
ns:524236 nr:0 dw:0 dr:524236 al:8 bm:0 lo:0 pe:0 ua:0 ap:0 ep:1 wo:f oos:0
[===================>] sync'ed:100.0% (0/524236)K
finish: 0:00:00 speed: 20,300 (13,792) K/sec

Initializing the Filesystem:

root@debian1:~# mkfs.ext4 /dev/drbd0 
root@debian1:~# mount /dev/drbd0 /replicated/

Do not forget to format the disk partition(in this case /dev/drbd0) first node only. Do not issue the command(mkfs.ext4) on the second node again.

Switching-over the Second node

#On first node:
root@debian1:~# umount /replicated
root@debian1:~# drbdadm secondary replicated
#On second node:
root@debian2:~# drbdadm primary replicated
root@debian2:~# mount /dev/drbd0

Switching-back the First Node:

#On second node:
root@debian2:~# umount /replicated
root@debian2:~# drbdadm secondary replicated
#On first node:
root@debian1:~# drbdadm primary replicated
root@debian1:~# mount /dev/drbd0