Appendix A. The virt group profile

The /var/lib/glusterd/groups/virt file defines a configuration that is used for all volumes in the virt group. Defining a group configuration and adding volumes to a group helps save time and effort when you want to update the settings on all volumes in the group.
The default contents of the /var/lib/glusterd/groups/virt file are as follows: 
performance.quick-read=off
performance.read-ahead=off
performance.io-cache=off
performance.low-prio-threads=32
network.remote-dio=enable
cluster.eager-lock=enable
cluster.quorum-type=auto
cluster.server-quorum-type=server
cluster.data-self-heal-algorithm=full
cluster.locking-scheme=granular
cluster.shd-max-threads=8
cluster.shd-wait-qlength=10000
features.shard=on
user.cifs=off
cluster.choose-local=off
client.event-threads=4
server.event-threads=4
performance.client-io-threads=on
The majority of these settings prevent caching in the client stack.
The cluster.eager-lock option optimizes write performance by using synchronous replication when there is a single writer to a file.
The cluster.data-self-heal option specifies how self-heal operations are performed.
The features.shard and features.shard-block-size options enable sharding behavior in the volume and set a block size appropriate for storing disk images.
The cluster.quorum-type=auto and cluster.server-quorum-type options enable client and server-side quorum to help ensure consistency and to prevent split-brain scenarios.

Note

The default settings of cluster.quorum-type=auto and cluster.server-quorum-type=server provide consistency at the cost of fault tolerance. Using these settings, if server-side quorum is not met, virtual machines pause because volumes are not available. If client-side quorum is not met, virtual machines pause even though a read-only replica pair is available. Manual intervention is then required to make the VMs resume the operations after the quorum is restored.
If fault tolerance is preferred over consistency, disable server-side and client-side quorum with the following commands: 
# gluster volume reset volname server-quorum-type
# gluster volume reset volname quorum-type
See Supported Volume Options in the Red Hat Gluster Storage 3.5 Administration Guide for more information about any of these settings.