Not exactly. Cassandra writes commit log, and if JVM will crash, cassandra will repair data from that log.

how often it fsyncs that log?

It can be configured: http://wiki.apache.org/cassandra/Durability One option is to fsync changes before telling client that operation is succeeded.

And get what benchmarks?)

Benchmark will be not so good, but do you have better choice?

The idea behind RAM-based databases is that the data is inherently redundant. If it crashes under load the rest of the cluster still has the data. I don't know HBase well at all, but clearly no one sane would argue for a "database" which held data in DRAM on a single host.