Database Reference
In-Depth Information
Understanding oplog
Oplog is used to record all database changes in a collection of data.
An oplog (operations log) is a special capped collection that keeps a rolling record of all
the operations that modify the data stored in your databases ( http://docs.mongodb.org/
manual/core/replica-set-oplog/ ).
It is a fixed-size collection, and it will automatically overwrite old records when it reaches
the maximum size of the collection. Oplog is used in the replication procedure to facilitate
the process of replication between primary and secondary nodes.
The secondary nodes copy the primary's oplog and apply it, so all members will have
oplog that saves the current state and recent history of the database.
The first time you add a replica set to the database, the server creates oplog with its de-
fault size. The size of oplog varies, and it depends on the operating system and machine
architecture. However, you can define and change the default size of oplog using the
oplogSize property.
The following list shows you the default values for the oplog size based on different oper-
ating systems:
• For 64-bit Linux, Solaris, FreeBSD, and Windows systems, MongoDB allocates 5
percent of the available free disk space to oplog . If this amount is smaller than a
gigabyte, then MongoDB allocates 1 gigabyte of space.
• For 64-bit OS X systems, MongoDB allocates 183 megabytes of space to oplog .
• For 32-bit systems, MongoDB allocates about 48 megabytes of space to oplog
( http://docs.mongodb.org/manual/core/replica-set-oplog/ ).
To check the status of oplog and monitor the size of the log and collection, you can use
the following command and run it in the MongoDB shell:
db.printReplicationInfo()
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