High
Availability Hosting with DRBD The term high availability is the buzz word nowadays. Nobody
tolerates downtime, whether it is the owner of a web site, owner of a
server or owner of a
data center. The challenge is how you can offer
the least down time and the term "high availbility", has it all.
DRBD refers to "Distributed
Replicated Block
Device"
and is used for building high availability (HA) clusters. This can be
attained by mirroring or replicating a block device via network. DRBD
can be considered as equivalent to a RAID 1 setup.
In RAID 1,
a drive has its data duplicated on two different drives using a RAID
controller. Same is the case with DRBD, where the local block holds the
data to be replicated and then it is written to another host's
blocks. The only difference here is DRBD allows data replication for
more than 2
nodes.
In simple words DRBD is a Linux Kernel module that
supports a distributed storage system, by which you can share 2 or more
block devices(data or file system).
DRBD works with two or more servers and each of these
are denoted as nodes and the
Node which has the read/write access to
data (also called DRBD data) is known as the primary node. The other
node to which the data is replicated is referred to as the secondary node.
If there are numerous secondary nodes in the high availability cluster,
it is referred as a DRBD cluster.
In a nut shell, DRBD takes the data, writes it to
the local disk, and sends it to the other nodes. This local disk can
either be a physical disk partition, a partition from a volume group,
RAID device or any other block device. This block holds the data to be
replicated.
DRBD can also be used along with LVM2 Logical Volume Manager.
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