It is not possible to design something that is both optimized to be an in memory DB or on disk DB, there are tradeoffs. So Redis stresses the right tradeoffs to be good in memory. The VM evolution, called diskstore, will provide us a way to work on bigger datasets as well. But probably Redis will continue to be much better in memory compared to an on disk DB with dataset that happens to fit in memory, and on disk DBs will try to be better than Redis+diskstore.
A few operations that Redis supports are specifically designed to work well in memory and can't be made efficient on disk. See for instance sorted sets and all the commands that this data type supports.
A few operations that Redis supports are specifically designed to work well in memory and can't be made efficient on disk. See for instance sorted sets and all the commands that this data type supports.