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replSetResizeOplog

Definition

replSetResizeOplog

New in version 3.6.

Use the replSetResizeOplog administrative command to change the size of a replica set member’s oplog. [1]

New in version 4.4: replSetResizeOplog also supports specifying the minimum number of hours to preserve an oplog entry.

replSetResizeOplog enables you to resize the oplog or its minimum retention period dynamically without restarting the mongod process.

You must run this command against the admin database.

The command has the following form:

db.adminCommand(
  {
    replSetResizeOplog: <int>,
    size: <double>,
    minRetentionHours: <double>
  }
)

replSetResizeOplog takes the following fields:

Field Type Description
replSetResizeOplog int Set to 1.
size double

The maximum size of the oplog in megabytes.

  • The minimum size you can specify is 990 megabytes.
  • The maximum size you can specify is 1 petabytes.
minRetentionHours double

Optional. The minimum number of hours to preserve an oplog entry, where decimal values represent the fractions of an hour. For example, a value of 1.5 represents one hour and thirty minutes.

The value must be greater than or equal to 0. A value of 0 indicates that the mongod should truncate the oplog starting with the oldest entries to maintain the configured maximum oplog size.

A mongod configured with minRetentionHours only removes an oplog entry if:

  • The oplog has reached the maximum configured size, and
  • The oplog entry is older that the configured number of hours based on the host system clock.

To check the currently configured minimum oplog retention period, see the oplogTruncation.minRetentionHours in the output of the serverStatus command.

New in version 4.4.

Behavior

You can only use replSetResizeOplog on mongod instances running with the Wired Tiger storage engine.

See the Change the Size of the Oplog tutorial for a procedure on using replSetResizeOplog command to resize the oplog.

Starting in MongoDB 4.0, MongoDB forbids dropping the local.oplog.rs collection. For more information on this restriction, see Oplog Collection Behavior.

replSetResizeOplog overrides the maximum oplog size or minimum oplog retention period set at startup by replication.oplogSizeMB / --oplogSizeMB and storage.oplogMinRetentionHours / --oplogMinRetentionHours respectively. To persist the changes made by replSetResizeOplog through a server restart, update the referenced configuration file settings or command line options passed to the mongod.

Important

Reducing the maximum oplog size results in truncation of the oldest oplog entries until the oplog reaches the new configured size.

Similarly, reducing the minimum oplog retention period (new in 4.4) results in truncation of oplog entries older that the specified period if the oplog has exceeded the maximum configured size.

Oplog truncation due to reduced oplog size or retention period can result in unexpected behavior from clients still reading those oplog entries, including:

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

Changed in version 4.2.2.

  • 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.

Examples

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 stats command:

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

The above command returns:

"maxSize": NumberLong("16834")

Warning

Reducing the size of the oplog in a node removes data from it. This may cause replica members syncing with that node to become stale. To resync those members, see Resync a Member of a Replica Set.

[1]Starting in MongoDB 4.0, the oplog can grow past its configured size limit to avoid deleting the majority commit point.

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
    })