replSetResizeOplog

Minimum Oplog Retention Period

A mongod has the following behavior when configured with a minimum oplog retention period (New in 4.4):

• The oplog can grow without constraint so as to retain oplog entries for the configured number of hours. This may result in reduction or exhaustion of system disk space due to a combination of high write volume and large retention period.

• If the oplog grows beyond its maximum size, the mongod may continue to hold that disk space even if the oplog returns to its maximum size or is configured for a smaller maximum size. See Reducing Oplog Size Does Not Immediately Return Disk Space.

• The mongod compares the system wall clock to an oplog entries creation wall clock time when enforcing oplog entry retention. Clock drift between cluster components may result in unexpected oplog retention behavior. See Clock Synchronization for more information on clock synchronization across cluster members.

replSetResizeOplog Does Not Replicate To Other Members

Changing the oplog size or minimum oplog retention period (new in 4.4) of a given replica set member with replSetResizeOplog does not change the oplog size of any other member in the replica set. You must run replSetResizeOplog on each replica set member in your cluster to change the oplog size or minimum retention period for all members.

Reducing Oplog Size Does Not Immediately Return Disk Space

Reducing the oplog size does not immediately reclaim that disk space. This includes oplog size reduction due to truncation of oplog events older than of the minimum oplog retention period (New in 4.4).

To immediately free unused disk space after reducing the oplog size, run compact against the oplog.rs collection in the local database during a maintenance period. compact blocks all operations on the database it runs against. Running compact against oplog.rs therefore prevents oplog synchronization. For a procedure on resizing the oplog and compacting oplog.rs, see Change the Size of the Oplog.

Resource Locking

• For MongoDB 4.2.2 and later, replSetResizeOplog takes an exclusive (W) lock on the oplog and blocks other operations on the collection until it finishes.

• For MongoDB 4.2.1 and earlier, replSetResizeOplog takes a global exclusive (W) lock and blocks all other operations until it finishes.

For more information on locking in MongoDB, see FAQ: Concurrency.

Change the Maximum Oplog Size

Use the db.collection.stats() mongo shell method to display the current maximum oplog size, maxSize, in megabytes. For example:

db.getSiblingDB("local").oplog.rs.stats(1024*1024).maxSize

The above command returns the oplog size of this member in megabytes:

990

The following command uses replSetResizeOplog to change the oplog size of this member to 16384 megabytes:

db.adminCommand({ "replSetResizeOplog": 1, size: 16384})

To verify the new oplog size, rerun the db.collection.stats() method:

db.getSiblingDB("local").oplog.rs.stats(1024*1024).maxSize

The above command returns:

"maxSize": NumberLong("16834")

Change the Minimum Oplog Retention Period

1. Connect a mongo shell to the mongod replica set member.

2. Optional. Use the db.serverStatus() command to verify the current minimum oplog retention value as oplogTruncation.oplogMinRetentionHours:

   db.getSiblingDB("admin").serverStatus().oplogTruncation.oplogMinRetentionHours

   The command returns the currently configured minimum oplog retention period for the mongod. For example:

   1.5

   If the mongod has no minimum oplog retention period, the operation returns an empty result.

3. Use the replSetResizeOplog command to modify the configured minimum oplog retention period. For example, the following sets the minimum oplog retention period to 2 hours:

   db.adminCommand({
     "replSetResizeOplog" : 1,
     "minRetentionHours" : 2
   })