Skip to content

Switching Storage Engines

By default, Percona Server for MongoDB runs with WiredTiger. There is also the original MMAPv1 storage engine, as well as an optional Percona Memory Engine storage engine to choose from. Each storage engine is designed for specific purposes and workloads.

You can select a storage engine using the --storageEngine command-line option when you start mongod. Alternatively, you can set the storage.engine variable in the configuration file (by default, /etc/mongod.conf).

See also

MongoDB Documentation: Configuration File Options

Data files created by one storage engine are not compatible with the other storage engines, because each one has its own data model.

When changing the storage engine, the mongod node requires an empty dbPath data directory when it is restarted even when using Percona Memory Engine. Though in-memory storage engine stores all data in memory, some metadata files, diagnostics logs and statistics metrics are still written to disk.

Creating a new dbPath data directory for a different storage engine is the simplest solution. Yet when you switch between disk-using storage engines (e.g. from WiredTiger to Percona Memory Engine), you may have to delete the old data if there is not enough disk space for both. Double-check that your backups are solid and/or the replica set nodes are healthy before you switch to the new storage engine.

Procedure

To change to a new storage engine, you have the following options:

  • If you simply want to temporarily test a new storage engine (e.g. Percona Memory Engine), set a different data directory for the dbPath variable in the configuration file. Make sure that the user running mongod has read and write permissions for the new data directory.

    $ service mongod stop
$ # In the configuration file, set the inmemory
$ # value for the storage.engine variable
$ # Set the <newDataDir> for the dbPath variable
$ service mongod start

  • If you want to permanently switch to a new storage engine and do not have any valuable data in your database, clean out the dbPath data directory (by default, /var/lib/mongodb) and edit the configuration file:

    $ service mongod stop
$ rm -rf <dbpathDataDir>
$ # Update the configuration file by setting the new
$ # value for the storage.engine variable
$ # set the engine-specific settings such as
$ # storage.inMemory.engineConfig.inMemorySizeGB
$ service mongod start

  • If there is data that you want to migrate and make compatible with the new storage engine, use the following methods:

    • for replica sets, use the “rolling restart” process. Switch to the new storage engine on the secondary node. Clean out the dbPath data directory and edit the configuration file:

      $ service mongod stop
$ rm -rf <dbpathDataDir>
$ # Update the configuration file by setting the new
$ # value for the storage.engine variable
$ # set the engine-specific settings such as
$ # storage.inMemory.engineConfig.inMemorySizeGB
$ service mongod start

      Wait for the node to rejoin with the other nodes and report the SECONDARY status.

      Repeat the procedure on the remaining nodes.

    • for a standalone instance or a single-node replica set, use the mongodump and mongorestore utilities:

      $ mongodump --out <dumpDir>
$ service mongod stop
$ rm -rf <dbpathDataDir>
$ # Update the configuration file by setting the new
$ # value for the storage.engine variable
$ # set the engine-specific settings such as
$ # storage.inMemory.engineConfig.inMemorySizeGB
$ service mongod start
$ mongorestore <dumpDir>

Data at Rest Encryption

Using Data at Rest Encryption means using the same storage.* configuration options as for WiredTiger. To change from normal to Data at Rest Encryption mode or backward, you must clean up the dbPath data directory, just as if you change the storage engine. This is because mongod cannot convert the data files to an encrypted format ‘in place’. It must get the document data again either via the initial sync from another replica set member, or from imported backup dump.