Replica Set Oplog

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Oplog Size

Workloads that Might Require a Larger Oplog Size
Oplog Status
Slow Oplog Application
Oplog Collection Behavior

The oplog (operations log) is a special capped collection that keeps a rolling record of all operations that modify the data stored in your databases. If write operations do not modify any data or fail, they do not create oplog entries.

Unlike other capped collections, the oplog can grow past its configured size limit to avoid deleting the majority commit point.

MongoDB applies database operations on the primary and then records the operations on the primary's oplog. The secondary members then copy and apply these operations in an asynchronous process. All replica set members contain a copy of the oplog, in the local.oplog.rs collection, which allows them to maintain the current state of the database.

To facilitate replication, all replica set members send heartbeats (pings) to all other members. Any secondary member can import oplog entries from any other member.

Each operation in the oplog is idempotent. That is, oplog operations produce the same results whether applied once or multiple times to the target dataset.

Oplog Size

When you start a replica set member for the first time, MongoDB creates an oplog of a default size if you do not specify the oplog size.

For Unix and Windows systems

The default oplog size depends on the storage engine:

Storage Engine	Default Oplog Size
WiredTiger Storage Engine	5% of free disk space
In-Memory Storage Engine for Self-Managed Deployments	5% of physical memory

The default oplog size has the following constraints:

The minimum oplog size is 990 MB. If 5% of free disk space or physical memory (whichever is applicable based on your storage engine) is less than 990 MB, the default oplog size is 990 MB.
The maximum default oplog size is 50 GB. If 5% of free disk space or physical memory (whichever is applicable based on your storage engine) is greater than 50 GB, the default oplog size is 50 GB.

For 64-bit macOS systems

The default oplog size is 192 MB of either free disk space or physical memory depending on the storage engine:

Storage Engine	Default Oplog Size
WiredTiger Storage Engine	192 MB of free disk space
In-Memory Storage Engine for Self-Managed Deployments	192 MB of physical memory

In most cases, the default oplog size is sufficient. For example, if an oplog is 5% of free disk space and fills up in 24 hours of operations, then secondaries can stop copying entries from the oplog for up to 24 hours without becoming too stale to continue replicating. However, most replica sets have much lower operation volumes, and their oplogs can hold much higher numbers of operations.

Before mongod creates an oplog, you can specify its size with the oplogSizeMB option. Once you have started a replica set member for the first time, use the replSetResizeOplog administrative command to change the oplog size. replSetResizeOplog enables you to resize the oplog dynamically without restarting the mongod process.

Minimum Oplog Retention Period

You can specify the minimum number of hours to preserve an oplog entry where mongod only removes an oplog entry if both of the following criteria are met:

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

By default MongoDB does not set a minimum oplog retention period and automatically truncates the oplog starting with the oldest entries to maintain the configured maximum oplog size.

To configure the minimum oplog retention period when starting the mongod, either:

Add the storage.oplogMinRetentionHours setting to the mongod configuration file.
-or-
Add the --oplogMinRetentionHours command line option.

To configure the minimum oplog retention period on a running mongod, use replSetResizeOplog. Setting the minimum oplog retention period while the mongod is running overrides any values set on startup. You must update the value of the corresponding configuration file setting or command line option to persist those changes through a server restart.

Oplog Window

oplog entries are time-stamped. The oplog window is the time difference between the newest and the oldest timestamps in the oplog. If a secondary node loses connection with the primary, it can only use replication to sync up again if the connection is restored within the oplog window.

Workloads that Might Require a Larger Oplog Size

If you can predict your replica set's workload to resemble one of the following patterns, then you might want to create an oplog that is larger than the default. Conversely, if your application predominantly performs reads with a minimal amount of write operations, a smaller oplog may be sufficient.

The following workloads might require a larger oplog size.

Updates to Multiple Documents at Once

The oplog must translate multi-updates into individual operations in order to maintain idempotency. This can use a great deal of oplog space without a corresponding increase in data size or disk use.

Deletions Equal the Same Amount of Data as Inserts

If you delete roughly the same amount of data as you insert, the database will not grow significantly in disk use, but the size of the operation log can be quite large.

Significant Number of In-Place Updates

If a significant portion of the workload is updates that do not increase the size of the documents, the database records a large number of operations but does not change the quantity of data on disk.

Oplog Status

To view oplog status, including the size and the time range of operations, issue the rs.printReplicationInfo() method. For more information on oplog status, see Check the Size of the Oplog.

Replication Lag and Flow Control

Under various exceptional situations, updates to a secondary's oplog might lag behind the desired performance time. Use db.getReplicationInfo() from a secondary member and the replication status output to assess the current state of replication and determine if there is any unintended replication delay.

Starting in MongoDB 4.2, administrators can limit the rate at which the primary applies its writes with the goal of keeping the majority committed lag under a configurable maximum value flowControlTargetLagSeconds.

By default, flow control is enabled.

Note

For flow control to engage, the replica set/sharded cluster must have: featureCompatibilityVersion (fCV) of 4.2 and read concern majority enabled. That is, enabled flow control has no effect if fCV is not 4.2 or if read concern majority is disabled.

See Replication Lag for more information.

Slow Oplog Application

Secondary members of a replica set log oplog entries that take longer than the slow operation threshold to apply. These messages are logged for the secondaries under the REPL component with the text applied op: <oplog entry> took <num>ms.

2018-11-16T12:31:35.886-05:00 I REPL   [repl writer worker 13] applied op: command { ... }, took 112ms

The slow oplog application logging on secondaries are:

Not affected by the logLevel/systemLog.verbosity level (or the systemLog.component.replication.verbosity level); i.e. for oplog entries, the secondary logs only the slow oplog entries. Increasing the verbosity level does not log all oplog entries.
Not captured by the profiler and not affected by the profiling level.

For more information on setting the slow operation threshold, see

mongod --slowms
slowOpThresholdMs
The profile command or db.setProfilingLevel() shell helper method.

Oplog Collection Behavior

You cannot drop the local.oplog.rs collection from any replica set member if your MongoDB deployment uses the WiredTiger Storage Engine. You cannot drop the local.oplog.rs collection from a standalone MongoDB instance. mongod requires the oplog for both Replication and recovery of a node if the node goes down.

Starting in MongoDB 5.0, it is no longer possible to perform manual write operations to the oplog on a cluster running as a replica set. Performing write operations to the oplog when running as a standalone instance should only be done with guidance from MongoDB Support.

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