cassandra massive write perfomance problem

I have server with 4 GB RAM and 2x 4 cores CPU. When I start perform massive writes in Cass开发者_如何转开发andra all works fine initially, but after a couple hours with 10K inserts per second database grows up to 25+ GB, and performance go down to 500 insert per seconds!

I find out this because compacting operations is very slow but I don't understand why? I set 8 concurrent compacting threads but Cassandra don't use 8 threads; only 2 cores are loaded.

Appreciate any help.

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We've seen similar problems with Cassandra out-the-box, see:

http://www.acunu.com/blogs/richard-low/cassandra-under-heavy-write-load-part-ii/

One solution to these sort of performance degradation issues (but by no means the only) is to consider a different storage engine, like Castle, used in the above blog post - its opensource (GPL v2), has much better performance and degrades much more gracefully. The code is here (I've just pushed up a branch for Cassandra 0.8 support):

https://bitbucket.org/acunu/fs.hg

And instructions on how to get started are here:

http://support.acunu.com/entries/20216797-castle-build-instructions

(Full disclosure: I work for Acunu, so may be a little biased ;-)