How Do I Manage the Balancer?

Scenario

Constraints

• The balancer applies only to cluster instances.
• DDS enables the balancer by default.

Enabling the Balancer

1. .
2. After connecting to a dds mongos node, run the following command to switch to the config database:

   use config

3. Run the following command to enable the balancer:

   sh.setBalancerState(true)

Disabling the Balancer

1. .
2. After connecting to a dds mongos node, run the following command to switch to the config database:

   use config

3. Run the following command to check the status of the balancer:

   while( sh.isBalancerRunning() ) {
             print("waiting...");
             sleep(500);
   } 

   • If the command output is empty, the balancer is not executing any task. In this case, you can proceed to disable the balancer.
   • If the command output is waiting, the balancer is migrating chunks. In this case, you cannot disable the balancer. Otherwise, data may become inconsistent.

4. If the command output in 3 is empty, run the following command to disable the balancer:

   sh.stopBalancer()

Scheduling a Balancing Window

To prevent chunk migration from affecting your businesses, you can perform the following operations to schedule a balancing window.

Before scheduling a balancing window, ensure that the balancer is enabled by referring to Enabling the Balancer.

1. .
2. After connecting to a dds mongos node, run the following command to switch to the config database:

   use config

3. Run the following commands to schedule a balancing window:

   db.adminCommand({"setBalancerActiveWindow": 1, start : "<start-time>", stop : "<stop-time>"})

   

   • <start-time>: indicates the beginning of the balancing window. Specify the time in the HH:MM format (UTC time). For HH values, use hour values ranging from 00 to 23. For MM value, use minute values ranging from 00 to 59.
   • <stop-time>: indicates the end of the balancing window. Specify the time in the HH:MM format (UTC time). For HH values, use hour values ranging from 00 to 23. For MM value, use minute values ranging from 00 to 59.
   • If the error message "CommandNotFound" is displayed when you run the setBalancerActiveWindow command as user rwuser to schedule a balancing window, the minor version is too early. In this case, upgrade the minor version of the instance and try again. Alternatively, .

   You can run the sh.status() command to check a balancing window schedule. The following command output in Figure 1 shows that the balancing window is 22:00 to 24:00.

   Figure 1 Checking a balancing window schedule
   

Removing a Balancing Window Schedule

If you have set the balancing window and wish to remove the schedule so that the balancer is always running, run the following command:

db.adminCommand({ 'unsetBalancerActiveWindow' : 1 })