Run the tablet rebalancing tool

The kudu CLI contains a rebalancing tool that can be used to rebalance tablet replicas among tablet servers. For each table, the tool attempts to balance the number of replicas per tablet server. It will also, without unbalancing any table, attempt to even out the number of replicas per tablet server across the cluster as a whole.

The rebalancing tool should be run as the Kudu admin user, specifying all master addresses:

sudo -u kudu kudu cluster rebalance,,

When run, the rebalancer will report on the initial tablet replica distribution in the cluster, log the replicas it moves, and print a final summary of the distribution when it terminates:

Per-server replica distribution summary:
       Statistic       |   Value
 Minimum Replica Count | 0
 Maximum Replica Count | 24
 Average Replica Count | 14.400000

Per-table replica distribution summary:
 Replica Skew |  Value
 Minimum      | 8
 Maximum      | 8
 Average      | 8.000000

I0613 14:18:49.905897 3002065792] tablet e7ee9ade95b342a7a94649b7862b345d: 206a51de1486402bbb214b5ce97a633c -> 3b4d9266ac8c45ff9a5d4d7c3e1cb326 move scheduled
I0613 14:18:49.917578 3002065792] tablet 5f03944529f44626a0d6ec8b1edc566e: 6e64c4165b864cbab0e67ccd82091d60 -> ba8c22ab030346b4baa289d6d11d0809 move scheduled
I0613 14:18:49.928683 3002065792] tablet 9373fee3bfe74cec9054737371a3b15d: fab382adf72c480984c6cc868fdd5f0e -> 3b4d9266ac8c45ff9a5d4d7c3e1cb326 move scheduled

... (full output elided)

I0613 14:19:01.162802 3002065792] tablet f4c046f18b174cc2974c65ac0bf52767: 206a51de1486402bbb214b5ce97a633c -> 3b4d9266ac8c45ff9a5d4d7c3e1cb326 move completed: OK

rebalancing is complete: cluster is balanced (moved 28 replicas)
Per-server replica distribution summary:
       Statistic       |   Value
 Minimum Replica Count | 14
 Maximum Replica Count | 15
 Average Replica Count | 14.400000

Per-table replica distribution summary:
 Replica Skew |  Value
 Minimum      | 1
 Maximum      | 1
 Average      | 1.000000

If more details are needed in addition to the replica distribution summary, use the --output_replica_distribution_details flag. If added, the flag makes the tool print per-table and per-tablet server replica distribution statistics as well.

Use the --report_only flag to get a report on table-wide and cluster-wide replica distribution statistics without starting any rebalancing activity.

The rebalancer can also be restricted to run on a subset of the tables by supplying the --tables flag. Note that, when running on a subset of tables, the tool will not attempt to balance the cluster as a whole.

The length of time rebalancing is run for can be controlled with the flag --max_run_time_sec. By default, the rebalancer will run until the cluster is balanced. To control the amount of resources devoted to rebalancing, modify the flag --max_moves_per_server. See kudu cluster rebalance --help for more.

It's safe to stop the rebalancer tool at any time. When restarted, the rebalancer will continue rebalancing the cluster.

The rebalancer tool requires all registered tablet servers to be up and running to proceed with the rebalancing process in order to avoid possible conflicts and races with the automatic re-replication and to keep replica placement optimal for current configuration of the cluster. If a tablet server becomes unavailable during the rebalancing session, the rebalancer will exit. As noted above, it's safe to restart the rebalancer after resolving the issue with unavailable tablet servers.

The rebalancing tool can rebalance Kudu clusters running older versions as well, with some restrictions. Consult the following table for more information. In the table, "RF" stands for "replication factor".

Version Range Rebalances RF = 1 Tables? Rebalances RF > 1 Tables?
v < 1.4.0 No No
1.4.0 <= v < 1.7.1 No Yes
v >= 1.7.1 Yes Yes

If the rebalancer is running against a cluster where rebalancing replication factor one tables is not supported, it will rebalance all the other tables and the cluster as if those singly-replicated tables did not exist.