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.
ASYNC option was added to FLUSHALL and FLUSHDB in order to let the entire dataset or a single database to be freed asynchronously.