any way to REALLY turn off commitlog?

[Keith Freeman]

We're running an insert-heavy use-case and have set "durable_writes = 
false" for all of our keyspaces.  While inserts are coming in (about 
2000 1k-records per second), we are still seeing 50Mb written to files 
in the commitlog directory every 6-10 seconds (using iostat).

Anybody know why so much data would written in this situation? We're 
guessing that these writes are related to the system and system_traces 
keyspaces, right?  Is it considered ok to turn off durable_writes on 
those keyspaces?  (I couldn't find anything on this searching the 
message board or google).

(Note we have double- and triple- checked that all of our keyspaces 
definitely have durable_writes=false, with multiple clean-and-restarts 
of cassandra)