Replica Set Oplog

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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. 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.

For Unix and Windows systems

The default oplog size depends on the storage engine:

Storage Engine	Default Oplog Size	Lower Bound	Upper Bound
In-Memory Storage Engine	5% of physical memory	50 MB	50 GB
WiredTiger Storage Engine	5% of free disk space	990 MB	50 GB
MMAPv1 Storage Engine	5% of free disk space	990 MB	50 GB

For 64-bit macOS systems

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

Storage Engine	Default Oplog Size
In-Memory Storage Engine	192 MB of physical memory
WiredTiger Storage Engine	192 MB of free disk space
MMAPv1 Storage Engine	192 MB of free disk space

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.

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.

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.

See Replication Lag for more information.

Slow Oplog Application

For MongoDB 3.6 deployments, starting in version 3.6.11, secondary members of a replica set now 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.

copy

2018-11-16T12:31:35.886-0500 I REPL   [repl writer worker 13] applied op: command { ... }, took 112ms

The slow oplog application logging on secondaries are:

Not affected by the slowOpSampleRate; i.e. all slow oplog entries are logged by the secondary.
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.

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