By default OpenKM store the binary information of the documents in the file system.
Usually you will get the best perfomance storing document in the file system. Basically with this configuration, when you upload or download a document it's open a file system stream directly from the server.
The parameter repository.datastore.backend help you to indicate what datastore will be used by OpenKM. You must change the configuration properties to be set at $TOMCAT_HOME/openkm.properties file and restart OpenKM to take effect.
You can not change datastore after you have uploaded documents into OpenKM.
You can only change it only at the first start up of the application or if you still have not uploaded documents.
File system datastore
The file system datastore is the default datastore and what we suggest for almost OpenKM installations.
When database datastore is enabled, the binary information of the documents are stored into the database.
Usually with this configuration you should get less performance. Basically when you upload or download a document, it's created a temporary file in the OpenKM server what is uploaded in the database, that means more steps and extra delay for setting and retrieving binary data from database.