flushdb

Delete all the keys of the currently selected DB.
This command never fails.
By default, 
FLUSHDB will synchronously flush all keys from the database.
Setting the lazyfree-lazy-user-flush configuration directive to "yes" changes the default flush mode to asynchronous.
It is possible to use one of the following modifiers to dictate the flushing mode explicitly:
ASYNC: flushes the database asynchronously
!SYNC: flushes the database synchronously
Note: an asynchronous 
FLUSHDB command only deletes keys that were present at the time the command was invoked. Keys created during an asynchronous flush will be unaffected.