Replication Oplog Length

Replication uses an operation log ("oplog") to store write operations. These operations replay asynchronously on other nodes.

The length of the oplog is important if a secondary is down. The larger the log, the longer the secondary can be down and still recover. Once the oplog has exceeded the downtime of the secondary, there is no way for the secondary to apply the operations; it will then have to do a full synchronization of the data from the primary.

By default, on 64 bit builds, oplogs are quite large - perhaps 5% of disk space. Generally this is a reasonable setting.

The oplog is a capped collection, and fixed size once allocated. Once it is created it is not easy to change without losing the existing data. This will be addressed in future versions so that it can be extended.

The mongod --oplogSize command line parameter sets the size of the oplog. Changing this parameter after the oplog is created does not change the size of your oplog.

This collection is named:

• local.oplog.$main for master/slave replication
• local.oplog.rs for replica sets

See also

• The Halted Replication page
• Resyncing a Very Stale Replica Set Member
• Replica Sets - Oplog