Redis has two persistence mechanisms and the documentation presents them as a choice, which invites picking one. They fail differently, and on anything that matters both are usually on.
# RDB — a point-in-time snapshot, compact, fast to restore
save 900 1
save 300 10
# AOF — every write appended, replayed on start
appendonly yes
appendfsync everysecRDB loses everything since the last snapshot, which can be fifteen minutes, and it restores quickly because it is a single compact file. AOF loses at most a second and replays slowly on a large dataset. Running both means the fast restore is available and the window is a second. If Redis is genuinely only a cache, turning both off is a legitimate and deliberate choice — the mistake is having RDB on by default and believing it is a backup.