replSetResizeOplog
replSetResizeOplog also supports specifying the minimum number of hours to preserve an oplog entry.
Changed in version 5.0: To set the replSetOplog size in mongosh, use the Double() constructor.
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.
This command is available in deployments hosted in the following environments:
ImportantThis command is not supported in MongoDB Atlas clusters. For information on Atlas support for all commands, see Unsupported Commands.
MongoDB Enterprise: The subscription-based, self-managed version of MongoDB
MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB
The command has the following form:
db.adminCommand( { replSetResizeOplog: <int>, size: <double>, minRetentionHours: <double> } )The command 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.
Explicitly cast the size as a double in mongosh with Double(). See Change the Maximum Oplog Size.
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.oplogMinRetentionHours in the output of the serverStatus command.
You can use replSetResizeOplog on mongod instances running with the Wired Tiger storage engine or the in-memory storage engine.
See the Change the Oplog Size of Self-Managed Replica Set Members tutorial for a procedure on using replSetResizeOplog command to resize the oplog.
The oplog can grow past its configured size limit to avoid deleting the majority commit point.
You cannot drop 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:
storage.oplogMinRetentionHours / --oplogMinRetentionHours respectively.
The new oplog size persists after a server restart, unless you use:
ImportantReducing 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 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:
Open change streams may become invalidated
Secondaries which have not replicated those oplog entries may require resynchronization.
Backups using mongodump with --oplog against the member may not capture entries prior to truncation.
A mongod has the following behavior when configured with a minimum oplog retention period:
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.
Changing the oplog size or minimum oplog retention period 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 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.
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 Oplog Size of Self-Managed Replica Set Members.
replSetResizeOplog takes an exclusive (W) lock on the oplog and blocks other operations on the collection until it finishes.
For more information on locking in MongoDB, see FAQ: Concurrency.
Use the db.collection.stats() mongosh method to display the current maximum oplog size, maxSize, in megabytes. For example:
db.getSiblingDB("local").oplog.rs.stats(1024*1024).maxSizeThe above command returns the oplog size of this member in megabytes:
The following command uses replSetResizeOplog to change the oplog size of this member to 16384 megabytes:
db.adminCommand({ "replSetResizeOplog": 1, size: Double(16384)})To verify the new oplog size, rerun the db.collection.stats() method:
db.getSiblingDB("local").oplog.rs.stats(1024*1024).maxSizeThe above command returns:
Optional. Use the db.serverStatus() command to verify the current minimum oplog retention value as oplogTruncation.oplogMinRetentionHours:
db.getSiblingDB("admin").serverStatus().oplogTruncation.oplogMinRetentionHoursThe command returns the currently configured minimum oplog retention period for the mongod. For example:
If the mongod has no minimum oplog retention period, the operation returns an empty result.
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})RetroSearch is an open source project built by @garambo | Open a GitHub Issue
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