3.3.2 Creating Replicated Volumes

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3.3.2 Creating Replicated Volumes

This section discusses creating a pool using replicated volumes. The replica count sets the number of copies of files across bricks in the volume. Generally, two or three copies are used. To protect against server and disk failures, the bricks of the volume should be on different nodes.

Split-brain is a situation where two or more replicated copies of a file become divergent, and there is not enough information to select a copy as being pristine and to self-heal any bad copies. Split-brain situations occur mostly due to network issues with clients connecting to the files in the volume.

If you set replica to be an even number (say, 2), you may encounter split-brain as both bricks think they have the latest and correct version. You can use an odd number for the replica count (say, 3), to prevent split-brain.

Using an arbiter brick also enables you to avoid split-brain, yet doesn't require the extra storage required of a replica 3 volume, which needs to store three copies of the files. An arbiter brick contains metadata about the files (but not the files) on other bricks in the volume, so can be much smaller in size. The last brick in each replica subvolume is used as the arbiter brick, for example, if you use replica 3 arbiter 1, every third brick is used as an arbiter brick.

Note

Volumes using an arbiter brick can only be created using the replica 3 arbiter 1 option.

Example 3.2 Creating a replicated volume

This example creates a replicated volume with one brick on three nodes.

# gluster volume create myvolume replica 3 \
  node{1,2,3}:/data/glusterfs/myvolume/mybrick/brick
volume create: myvolume: success: please start the volume to access data
# gluster volume start myvolume
volume start: myvolume: success
# gluster volume info

Volume Name: myvolume
Type: Replicate
Volume ID: ...
Status: Started
Snapshot Count: 0
Number of Bricks: 1 x 3 = 3
Transport-type: tcp
Bricks:
Brick1: node1:/data/glusterfs/myvolume/mybrick/brick
Brick2: node2:/data/glusterfs/myvolume/mybrick/brick
Brick3: node3:/data/glusterfs/myvolume/mybrick/brick
Options Reconfigured:
transport.address-family: inet
nfs.disable: on
performance.client-io-threads: off

Example 3.3 Creating a replicated volume with an arbiter

This example creates a replicated volume with one brick on three nodes, and sets one arbiter brick.

# gluster volume create myvolume replica 3 arbiter 1 \
  node{1,2,3}:/data/glusterfs/myvolume/mybrick/brick
volume create: myvolume: success: please start the volume to access data
# gluster volume start myvolume
volume start: myvolume: success
# gluster volume info

Volume Name: myvolume
Type: Replicate
Volume ID: ...
Status: Started
Snapshot Count: 0
Number of Bricks: 1 x (2 + 1) = 3
Transport-type: tcp
Bricks:
Brick1: node1:/data/glusterfs/myvolume/mybrick/brick
Brick2: node2:/data/glusterfs/myvolume/mybrick/brick
Brick3: node3:/data/glusterfs/myvolume/mybrick/brick (arbiter)
Options Reconfigured:
transport.address-family: inet
nfs.disable: on
performance.client-io-threads: off