https://www.v2ex.com/t/142644
http://stackoverflow.com/questions/10558465/memcached-vs-redis
简单来说：
redis :支持的数据类型多，读写速度快，支持的单个文件大小要大，可以数据永久化
redis支持：
- Binary-safe strings.
- Lists: collections of string elements sorted according to the order of insertion. They are basically linked lists.
- Sets: collections of unique, unsorted string elements.
- Sorted sets, similar to Sets but where every string element is associated to a floating number value, called score. The elements are always taken sorted by their score, so unlike Sets it is possible to retrieve a range of elements (for example you may ask: give me the top 10, or the bottom 10).
- Hashes, which are maps composed of fields associated with values. Both the field and the value are strings. This is very similar to Ruby or Python hashes.
- Bit arrays (or simply bitmaps): it is possible, using special commands, to handle String values like an array of bits: you can set and clear individual bits, count all the bits set to 1, find the first set or unset bit, and so forth.
- HyperLogLogs: this is a probabilistic data structure which is used in order to estimate the cardinality of a set. Don't be scared, it is simpler than it seems... See later in the HyperLogLog section of this tutorial.