CI Redis cache driver

No, it is not a redis library it is a cache library. The whole point here is that it provides a wrapper for common functions that exists in all the supported cache systems. This allows you to switch from redis to memcache etc by only changing your config file. It makes no sense to provide functions for a specific system in an abstraction layer like Narf said.

If you want / need redis specific functions you need to use the official phpredis class. This should give you no troubles unless you need to switch from redis to another cache system, then you will need to do some rewriting in your code.