Re: Oplog use Ram?

[David Hows]

Hi Tommy,

MongoDB aims to keep as much of your working set of data in RAM as possible.

The oplog is simply a special capped collection which keeps a log of changes to the dataset within it.

When running a replica set secondary nodes will read from the primary node's oplog to keep themselves in sync with all changes.

This means that mongod will try and keep this data in RAM so that it will not need to perform any disk operations (pagefaults) in order to allow secondaries to stay in sync with the primary.

Please see these documents as they contain some relevant information:

http://docs.mongodb.org/manual/faq/storage

http://www.mongodb.org/display/DOCS/Checking+Server+Memory+Usage

Cheers,

David