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
option when you start
mongod. Alternatively, you can set the
storage.engine variable in the configuration file (by default,
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
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 to before you switch to the new storage
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
dbPathdata 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
Wait for the node to rejoin with the other replica set members 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 --out <dumpDir> $ 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.wiredTiger.engineConfig.cacheSizeGB or $ # storage.inMemory.engineConfig.inMemorySizeGB $ service mongod start $ mongorestore <dumpDir>
Data at Rest Encryption¶
Using Data at Rest Encryption means using the same
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.