cassandra massive write perfomance problem

[lebron james]

Please help me solve one problem I have server with 4 GB RAM and 2x 4 cores
CPU When i start do massive writes in cassandra all works fine. but after
couple hours with 10K inserts per second database grows up to 25+ GB
performance go down to 500 insert per seconds I find out this because
compacting operations is very slow and i dont understand why, i set 8
concurrent compacting threads but cassandra dont use 8 threads only 2 cores
are loaded.