6.4. Creating Replicated Volumes

Important

Creating replicated volume with replica count greater than 3 is under technology preview. Technology Preview features are not fully supported under Red Hat service-level agreements (SLAs), may not be functionally complete, and are not intended for production use.
Tech Preview features provide early access to upcoming product innovations, enabling customers to test functionality and provide feedback during the development process.
As Red Hat considers making future iterations of Technology Preview features generally available, we will provide commercially reasonable efforts to resolve any reported issues that customers experience when using these features.
Replicated volume creates copies of files across multiple bricks in the volume. Use replicated volumes in environments where high-availability and high-reliability are critical.
Use gluster volume create to create different types of volumes, and gluster volume info to verify successful volume creation.
Prerequisites

A trusted storage pool has been created, as described in Section 5.1, “Adding Servers to the Trusted Storage Pool”. Understand how to start and stop volumes, as described in Section 6.10, “Starting Volumes”.

6.4.1. Creating Two-way Replicated Volumes

Two-way replicated volume creates two copies of files across multiple bricks in the volume. The number of bricks must be equal to the replica count for a replicated volume. To protect against server and disk failures, it is recommended that the bricks of the volume are from different servers.
Illustration of a Two-way Replicated Volume

Figure 6.2. Illustration of a Two-way Replicated Volume

Creating two-way replicated volumes
  1. Run the gluster volume create command to create the replicated volume.
    The syntax is # gluster volume create NEW-VOLNAME [replica COUNT] [transport tcp | rdma | tcp,rdma] NEW-BRICK...
    The default value for transport is tcp. Other options can be passed such as auth.allow or auth.reject. See Section 8.1, “Configuring Volume Options” for a full list of parameters.

    Example 6.3. Replicated Volume with Two Storage Servers

    The order in which bricks are specified determines how bricks are replicated with each other. For example, every 2 bricks, where 2 is the replica count forms a replica set. If more bricks were specified, the next two bricks in sequence would replicate each other. The same is illustrated in Figure 6.2. Illustration of a Replicated Volume.
    # gluster volume create test-volume replica 2 transport tcp server1:/exp1/brick server2:/exp2/brick
    Creation of test-volume has been successful
    Please start the volume to access data.
  2. Run # gluster volume start VOLNAME to start the volume.
    # gluster volume start test-volume
    Starting test-volume has been successful
  3. Run gluster volume info command to optionally display the volume information.

Important

You must set client-side quorum on replicated volumes to prevent split-brain scenarios. For more information on setting client-side quorum, see Section 8.10.1.2, “Configuring Client-Side Quorum”

6.4.2. Creating Three-way Replicated Volumes

Three-way replicated volume creates three copies of files across multiple bricks in the volume. The number of bricks must be equal to the replica count for a replicated volume. To protect against server and disk failures, it is recommended that the bricks of the volume are from different servers.
Synchronous three-way replication is now fully supported in Red Hat Storage. Three-way replication volumes are supported only on JBOD configuration.
Illustration of a Three-way Replicated Volume

Figure 6.3. Illustration of a Three-way Replicated Volume

Recommended configuration for geo-replicated volume

The recommended configuration for three-way replication is to have a minimum of three nodes, as only a single brick out of the replica set is involved in syncing the files to the slave. It is expected that, all the bricks of a replica set are in different nodes. It is recommended not to have a brick along with its replica set from the same volume residing in the same node.

The following is an example configuration:
# gluster volume info master
Volume Name: master
Type: Replicate
Volume ID: 6e2e447d-550d-44e4-85be-33b35933de3c
Status: Started
Snap Volume: no
Number of Bricks: 1 x 3 = 3
Transport-type: tcp
Bricks:
Brick1: fedora1:/bricks/brick0/b0 
Brick2: fedora2:/bricks/brick0/b0 
Brick3: fedora3:/bricks/brick0/b0
Creating three-way replicated volumes
  1. Run the gluster volume create command to create the replicated volume.
    The syntax is # gluster volume create NEW-VOLNAME [replica COUNT] [transport tcp | rdma | tcp,rdma] NEW-BRICK...
    The default value for transport is tcp. Other options can be passed such as auth.allow or auth.reject. See Section 8.1, “Configuring Volume Options” for a full list of parameters.

    Example 6.4. Replicated Volume with Three Storage Servers

    The order in which bricks are specified determines how bricks are replicated with each other. For example, every n bricks, where n is the replica count forms a replica set. If more bricks were specified, the next three bricks in sequence would replicate each other. The same is illustrated in Figure 6.2. Illustration of a Replicated Volume.
    # gluster volume create test-volume replica 3 transport tcp server1:/exp1/brick server2:/exp2/brick server3:/exp3/brick
    Creation of test-volume has been successful
    Please start the volume to access data.
  2. Run # gluster volume start VOLNAME to start the volume.
    # gluster volume start test-volume
    Starting test-volume has been successful
  3. Run gluster volume info command to optionally display the volume information.

Important

You must set client-side quorum on replicated volumes to prevent split-brain scenarios. For more information on setting client-side quorum, see Section 8.10.1.2, “Configuring Client-Side Quorum”