11.7. Rebalancing Volumes
If a volume has been expanded or shrunk using the
add-brick or remove-brick commands, the data on the volume needs to be rebalanced among the servers.
Note
In a non-replicated volume, all bricks should be online to perform the
rebalance operation using the start option. In a replicated volume, at least one of the bricks in the replica should be online.
To rebalance a volume, use the following command on any of the servers:
# gluster volume rebalance VOLNAME start
For example:
# gluster volume rebalance test-volume start Starting rebalancing on volume test-volume has been successful
A
rebalance operation, without force option, will attempt to balance the space utilized across nodes, thereby skipping files to rebalance in case this would cause the target node of migration to have lesser available space than the source of migration. This leads to link files that are still left behind in the system and hence may cause performance issues in access when a large number of such link files are present.
Enhancements made to the file rename and rebalance operations in Red Hat Gluster Storage 2.1 update 5 requires that all the clients connected to a cluster operate with the same or later versions. If the clients operate on older versions, and a rebalance operation is performed, the following warning message is displayed and the rebalance operation will not be executed.
volume rebalance: VOLNAME: failed: Volume VOLNAME has one or more connected clients of a version lower than Red Hat Gluster Storage-2.1 update 5. Starting rebalance in this state could lead to data loss. Please disconnect those clients before attempting this command again.
Red Hat strongly recommends you to disconnect all the older clients before executing the rebalance command to avoid a potential data loss scenario.
Warning
The
Rebalance command can be executed with the force option even when the older clients are connected to the cluster. However, this could lead to a data loss situation.
A
rebalance operation with force, balances the data based on the layout, and hence optimizes or does away with the link files, but may lead to an imbalanced storage space used across bricks. This option is to be used only when there are a large number of link files in the system.
To rebalance a volume forcefully, use the following command on any of the servers:
# gluster volume rebalance VOLNAME start force
For example:
# gluster volume rebalance test-volume start force Starting rebalancing on volume test-volume has been successful
11.7.1. Rebalance Throttling
Rebalance process is made multithreaded to handle multiple files migration for enhancing the performance. During multiple file migration, there can be a severe impact on storage system performance and a throttling mechanism is provided to manage it.
By default, the rebalance throttling is started in the
normal mode. Configure the throttling modes to adjust the rate at which the files must be migrated
# gluster volume set VOLNAME rebal-throttle lazy|normal|aggressive
For example:
# gluster volume set test-volume rebal-throttle lazy
11.7.2. Displaying Status of a Rebalance Operation
To display the status of a volume rebalance operation, use the following command:
# gluster volume rebalance VOLNAME status
For example:
# gluster volume rebalance test-volume status
Node Rebalanced-files size scanned failures status
--------- ----------- ----------- ----------- ----------- ------------
localhost 112 14567 150 0 in progress
10.16.156.72 140 2134 201 2 in progress
The time taken to complete the rebalance operation depends on the number of files on the volume and their size. Continue to check the rebalancing status, and verify that the number of rebalanced or scanned files keeps increasing.
For example, running the status command again might display a result similar to the following:
# gluster volume rebalance test-volume status
Node Rebalanced-files size scanned failures status
--------- ----------- ----------- ----------- ----------- ------------
localhost 112 14567 150 0 in progress
10.16.156.72 140 2134 201 2 in progress
The rebalance status will be shown as
completed the following when the rebalance is complete:
# gluster volume rebalance test-volume status
Node Rebalanced-files size scanned failures status
--------- ----------- ----------- ----------- ----------- ------------
localhost 112 15674 170 0 completed
10.16.156.72 140 3423 321 2 completed11.7.3. Stopping a Rebalance Operation
To stop a rebalance operation, use the following command:
# gluster volume rebalance VOLNAME stop
For example:
# gluster volume rebalance test-volume stop
Node Rebalanced-files size scanned failures status
--------- ----------- ----------- ----------- ----------- ------------
localhost 102 12134 130 0 stopped
10.16.156.72 110 2123 121 2 stopped
Stopped rebalance process on volume test-volume