Thanks for your answer.

I read that doc. But i misunderstand what is 50Gb of Database??? How much records in it?
lazyboy use ONE consistency level, and that is good.

I don't use multi_gets because they don't need for me and not usefull in my case.
I don't think that cassandra has very fast writes. Because when i insert every 10.000 inserts was done about 3.3 seconds, and mysql do the same task about 1,3 seconds (autocommit is on). And here is all right. I have "cluster" in 4 datanodes so in writes cassandra will by faster than mysql, but on reads it steel slow.