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.
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.
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
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.
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.
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.
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.
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.
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
systemLog.verbositylevel (or the
systemLog.component.replication.verbositylevel); 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