A cache with persistence turned on to hide a design problem

Turning on persistence so a restart would stop losing data, which is treating the symptom of data that should not have been there.

the proposal:  enable AOF, so a restart preserves the
               cache.
what it fixes: the data survives a planned restart.
what it does
not fix:       eviction under memory pressure, a flush,
               a failover, or the fact that three keys
               have no other home.

and what it costs: fsync on the write path, a file that
grows, and a rewrite process that competes for memory
with the thing it is protecting.

Persistence on a cache is right when the cache is expensive to rebuild and wrong when it is being used to make a cache durable. The question that settled it was whether the data would survive an eviction, which no persistence setting affects — and the answer meant the data had to move to the database regardless.