管理片式集群平衡器

Check the Balancer Lock¶

To see if the balancer process is active in your cluster, do the following:

1. Connect to any mongos in the cluster using the mongo shell.

2. Issue the following command to switch to the 配置数据库内容:

   use config
   

3. Use the following query to return the balancer lock:

   db.locks.find( { _id : "balancer" } ).pretty()
   

When this command returns, you will see output like the following:

{   "_id" : "balancer",
"process" : "mongos0.example.net:1292810611:1804289383",
  "state" : 2,
     "ts" : ObjectId("4d0f872630c42d1978be8a2e"),
   "when" : "Mon Dec 20 2010 11:41:10 GMT-0500 (EST)",
    "who" : "mongos0.example.net:1292810611:1804289383:Balancer:846930886",
    "why" : "doing balance round" }

This output confirms that:

• The balancer originates from the mongos running on the system with the hostname mongos0.example.net.
• The value in the state field indicates that a mongos has the lock. For version 2.0 and later, the value of an active lock is 2; for earlier versions the value is 1.

sh.getBalancerState()

Schedule the Balancing Window¶

In some situations, particularly when your data set grows slowly and a migration can impact performance, it’s useful to be able to ensure that the balancer is active only at certain times. Use the following procedure to specify a window during which the balancer will be able to migrate chunks:

1. Connect to any mongos in the cluster using the mongo shell.

2. Issue the following command to switch to the 配置数据库内容:

   use config
   

3. Use an operation modeled on the following example update() operation to modify the balancer’s window:

   db.settings.update({ _id : "balancer" }, { $set : { activeWindow : { start : "<start-time>", stop : "<stop-time>" } } }, true )
   

   Replace <start-time> and <end-time> with time values using two digit hour and minute values (e.g HH:MM) that describe the beginning and end boundaries of the balancing window. These times will be evaluated relative to the time zone of each individual mongos instance in the sharded cluster. For instance, running the following will force the balancer to run between 11PM and 6AM local time only:

   db.settings.update({ _id : "balancer" }, { $set : { activeWindow : { start : "23:00", stop : "6:00" } } }, true )
   

Remove a Balancing Window Schedule¶

If you have set the balancing window and wish to remove the schedule so that the balancer is always running, issue the following sequence of operations:

use config
db.settings.update({ _id : "balancer" }, { $unset : { activeWindow : true } })

Disable the Balancer¶

By default the balancer may run at any time and only moves chunks as needed. To disable the balancer for a short period of time and prevent all migration, use the following procedure:

1. Connect to any mongos in the cluster using the mongo shell.

2. Issue one of the following operations to disable the balancer:

   sh.stopBalancer()
   

3. Later, issue one the following operations to enable the balancer:

   sh.startBalancer()
   

use config
while( db.locks.findOne({_id: "balancer"}).state ) {
       print("waiting..."); sleep(1000);
}

The above process and the sh.setBalancerState(), sh.startBalancer(), and sh.stopBalancer() helpers provide wrappers on the following process, which may be useful if you need to run this operation from a driver that does not have helper functions:

1. Connect to any mongos in the cluster using the mongo shell.

2. Issue the following command to switch to the 配置数据库内容:

   use config
   

3. Issue the following update to disable the balancer:

   db.settings.update( { _id: "balancer" }, { $set : { stopped: true } } , true );
   

4. To enable the balancer again, alter the value of “stopped” as follows:

   db.settings.update( { _id: "balancer" }, { $set : { stopped: false } } , true );
   

Disable Balancing During Backups¶

If MongoDB migrates a chunk during a backup, you can end with an inconsistent snapshot of your sharded cluster. Never run a backup while the balancer is active. To ensure that the balancer is inactive during your backup operation:

• Set the balancing window so that the balancer is inactive during the backup. Ensure that the backup can complete while you have the balancer disabled.
• manually disable the balancer for the duration of the backup procedure.

Confirm that the balancer is not active using the sh.getBalancerState() method before starting a backup operation. When the backup procedure is complete you can reactivate the balancer process.