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Change Replica Set to WiredTiger

New in version 3.0: The WiredTiger storage engine is available. Also, replica sets may have members with different storage engines.

Changed in version 3.2: WiredTiger is the new default storage engine for MongoDB.

This tutorial gives an overview of changing the storage engine of a member of a replica set to WiredTiger.

Considerations

Replica sets can have members with different storage engines. As such, you can update members to use the WiredTiger storage engine in a rolling fashion. Before changing all the members to use WiredTiger, you may prefer to run with mixed storage engines for some period. However, performance can vary according to workload.

You must be using MongoDB version 3.0 or greater in order to use the WiredTiger storage engine. If upgrading from an earlier version of MongoDB, see the guides on Upgrading to MongoDB 3.0 or Upgrading to MongoDB 3.2 before proceeding with changing your storage engine.

Before enabling the new WiredTiger storage engine, ensure that all replica set/sharded cluster members are running at least MongoDB version 2.6.8, and preferably version 3.0.0 or newer.

Procedure

This procedure completely removes a secondary replica set member’s data, starts mongod with WiredTiger, and performs an initial sync.

To update all members of the replica set to use WiredTiger, update the secondary members first. Then step down the primary, and update the stepped-down member.

1

Shut down the secondary member.

In the mongo shell, shut down the secondary mongod instance you wish to upgrade.

db.shutdownServer()
2

Prepare a data directory for the new mongod running with WiredTiger.

Prepare a data directory for the new mongod instance that will run with the WiredTiger storage engine. mongod must have read and write permissions for this directory. You can either delete the contents of the stopped secondary member’s current data directory or create a new directory entirely.

mongod with WiredTiger will not start with data files created with a different storage engine.

3

Start mongod with WiredTiger.

Start mongod, specifying wiredTiger as the --storageEngine and the prepared data directory for WiredTiger as the --dbpath. Specify additional options as appropriate for this replica set member.

mongod --storageEngine wiredTiger --dbpath <newWiredTigerDBPath> --replSet <replSetName>

Since no data exists in the --dbpath, the mongod will perform an initial sync. The length of the initial sync process depends on the size of the database and network connection between members of the replica set.

You can also specify the options in a configuration file. To specify the storage engine, use the storage.engine setting.

4

Repeat the procedure for other replica set secondaries you wish to upgrade.

Perform this procedure again for the rest of the secondary members of the replica set you wish to use the WiredTiger storage engine.