這兩年Redis火得可以，Redis也常常被當作Memcached的挑戰者被提到桌面上來。關于Redis與Memcached的比較更是比比皆是。然而，Redis真的在功能、性能以及內存使用效率上都超越了Memcached嗎？

下面內容來自Redis作者在stackoverflow上的一個回答，對應的問題是《Is memcached a dinosaur in comparison to Redis?》（相比Redis，Memcached真的過時了嗎？）

• You should not care too much about performances. Redis is faster per core with small values, but memcached is able to use multiple cores with a single executable and TCP port without help from the client. Also memcached is faster with big values in the order of 100k. Redis recently improved a lot about big values (unstable branch) but still memcached is faster in this use case. The point here is: nor one or the other will likely going to be your bottleneck for the query-per-second they can deliver.
• 沒有必要過多的關心性能，因為二者的性能都已經足夠高了。由于Redis只使用單核，而Memcached可以使用多核，所以在比較上，平均每一個核上Redis在存儲小數據時比Memcached性能更高。而在100k以上的數據中，Memcached性能要高于Redis，雖然Redis最近也在存儲大數據的性能上進行優化，但是比起Memcached，還是稍有遜色。說了這么多，結論是，無論你使用哪一個，每秒處理請求的次數都不會成為瓶頸。（比如瓶頸可能會在網卡）
• You should care about memory usage. For simple key-value pairs memcached is more memory efficient. If you use Redis hashes, Redis is more memory efficient. Depends on the use case.
• 如果要說內存使用效率，使用簡單的key-value存儲的話，Memcached的內存利用率更高，而如果Redis采用hash結構來做key-value存儲，由于其組合式的壓縮，其內存利用率會高于Memcached。當然，這和你的應用場景和數據特性有關。
• You should care about persistence and replication, two features only available in Redis. Even if your goal is to build a cache it helps that after an upgrade or a reboot your data are still there.
• 如果你對數據持久化和數據同步有所要求，那么推薦你選擇Redis，因為這兩個特性Memcached都不具備。即使你只是希望在升級或者重啟系統后緩存數據不會丟失，選擇Redis也是明智的。
• You should care about the kind of operations you need. In Redis there are a lot of complex operations, even just considering the caching use case, you often can do a lot more in a single operation, without requiring data to be processed client side (a lot of I/O is sometimes needed). This operations are often as fast as plain GET and SET. So if you don’t need just GEt/SET but more complex things Redis can help a lot (think at timeline caching).
• 當然，最后還得說到你的具體應用需求。Redis相比Memcached來說，擁有更多的數據結構和并支持更豐富的數據操作，通常在Memcached里，你需要將數據拿到客戶端來進行類似的修改再set回去。這大大增加了網絡IO的次數和數據體積。在Redis中，這些復雜的操作通常和一般的GET/SET一樣高效。所以，如果你需要緩存能夠支持更復雜的結構和操作，那么Redis會是不錯的選擇。

來源：Is memcached a dinosaur in comparison to Redis?（其他人的回答同樣值得一看）