Delete all the keys of all the existing databases, not just the currently selected one. This command never fails.
The time-complexity for this operation is O(N), N being the number of keys in all existing databases.
*FLUSHALL ASYNC (Redis 4.0.0 or greater)
Redis is now able to delete keys in the background in a different thread without blocking the server.
An ASYNC option was added to FLUSHALL and FLUSHDB in order to let the entire dataset or a single database to be freed asynchronously.
Asynchronous FLUSHALL and FLUSHDB commands only delete keys that were present at the time the command was invoked. Keys created during an asynchronous flush will be unaffected.