To expand on that a bit: when redis is in AOF mode, all write commands are appended to a journal file. When fsync is set to 'always', this journal file is fsync

To expand on that a bit: when redis is in AOF mode, all write commands are appended to a journal file. When fsync is set to 'always', this journal file is fsynced on every write. Redis has a reputation as an "ephemeral database", because it's usually used as one, but there's nothing inherently ephemeral about it. This isn't magic; it's just file I/O.