Manual:$wgFileBackends

Details
This is an array of configuration arrays. Each backend configuration has the following parameters:
 * name: A unique name for the backend
 * class: The file backend class to use
 * wikiId: A unique string that identifies the wiki (container prefix)
 * lockManager: The name of a lock manager (see $wgLockManagers)
 * Additional parameters are specific to the class used.