Storage replication with DRBD

Objective: Storage replication with DRBD

Environment : CentOS 6.5 (32-bit)

DRBD Version : 8.3.16

Introduction;

In this article, I am using DRBD(Distributed Replicated Block Device) replicated storage solution mirroring the content of block devices (hard disks) between serves. Not everyone can afford network-attached storage but somehow the data needs to be kept in sync, DRBD which can be thought of as network based RAID-1.

DRBD's position within the Linux I/O stack

Below are some of the basic requirements.

- Two disks  (preferably same size /dev/sdb )
- Networking between machines (drbd-node1 & drbd-node2)
- Working DNS resolution.
- NTP synchronized times on both nodes

Install DRBD packages :

drbd-node1# yum install -y  drbd83-utils kmod-drbd83

drbd-node2# yum install -y  drbd83-utils kmod-drbd83

Load the DRBD modules.

Reboot or /sbin/modprobe drbd

Partition the disk:

drbd-node1# fdisk /dev/sdb

drbd-node2# fdisk /dev/sdb

Create the Distributed Replicated Block Device resource file

Readers are required to change according to your specifications on the servers, which are marked in RED

drbd-node1# cat /etc/drbd.d/drbdcluster.res 

resource drbdcluster

 {

 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 outdated-wfc-timeout 20;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 drbd-node1 {

 device /dev/drbd0;

 disk /dev/sdb1;

 address 192.168.1.XXX:7788;

 flexible-meta-disk internal;

 }

 on drbd-node2 {

 device /dev/drbd0;

 disk /dev/sdb1;

 address 192.168.1.YYY:7788;

 meta-disk internal;

 }

 }

drbd-node1#

Copy DRBD configured to the secondary node(drbd-node2)

drbd-node1# scp /etc/drbd.d/drbdcluster.res root@192.168.1.YYY:/etc/drbd.d/drbdcluster.res

drbd-node1#

Initialize DRBD on both the nodes and start their services(drbd-node1 & drbd-node2)

ALL]# drbdadm create-md drbdcluster

Writing meta data...

initializing activity log

NOT initialized bitmap

New drbd meta data block successfully created.

success

ALL# service drbd start

Starting DRBD resources: [ d(drbdcluster) s(drbdcluster) n(drbdcluster) ]........

- Since both the disks contain garbage values, we are required to tell to the DRBD which set of data would be used as primarily.

drbd-node1# drbdadm -- --overwrite-data-of-peer primary drbdcluster

drbd-node1#

- Device would start for an initial sync, we are required to wait until it completes.

drbd-node1# 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:123904 nr:0 dw:0 dr:124568 al:0 bm:7 lo:0 pe:0 ua:0 ap:0 ep:1 wo:f oos:923580

[=>..................] sync'ed: 12.2% (923580/1047484)K

finish: 0:01:29 speed: 10,324 (10,324) K/sec

Create a file system and populate the data  on the device.

drbd-node1# 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

65536 inodes, 261871 blocks

13093 blocks (5.00%) reserved for the super user

First data block=0

Maximum filesystem blocks=268435456

8 block groups

32768 blocks per group, 32768 fragments per group

8192 inodes per group

Superblock backups stored on blocks: 

32768, 98304, 163840, 229376

Writing inode tables: done                            

Creating journal (4096 blocks): done

Writing superblocks and filesystem accounting information: done

This filesystem will be automatically checked every 30 mounts or

180 days, whichever comes first.  Use tune2fs -c or -i to override.

drbd-node1#

drbd-node1# mount /dev/drbd0 /data

drbd-node1# touch /data/file1

drbd-node1#

You don't need to mount the disk from secondary machines. All data you write on /data folder will be synced to secondary server.

In order to view it, umount /data from the drbd-node1 and maker secondary node as primary node and mount back /data on the drbd-node2. you could see the same contents of /data.

drbd-node1# drbdadm secondary drbdcluster

drbd-node2# drbdadm -- --overwrite-data-of-peer primary drbdcluster

We were successful on storage replication with DRBD.

On further improvement, since DRBD is functioning I would configure cluster and file system as a resource to use it. further in-addition to the Filesystem definition, we also need to tell the cluster where it can be located (only on the DRBD Primary) and when it is allowed to start(after

the Primary was promoted). 

I would publish an article in near future for the same.