Cassandra commitlog activities but it should not

I use C* v2.1.2 have the following keyspaces

cqlsh> SELECT * FROM system.schema_keyspaces;
 keyspace_name       | durable_writes        | strategy_class                               | strategy_options
---------------+----------------+---------------------------------------------+----------------------------
        system       |           True        | org.apache.cassandra.locator.LocalStrategy   |                         {}
     system_traces   |          False        | org.apache.cassandra.locator.SimpleStrategy  | {"replication_factor":"2"}
       a1_ks         |          False        | org.apache.cassandra.locator.SimpleStrategy  | {"replication_factor":"1"}

I have two disks. Data directory is on sda. Commitlog is on sdb. I do 100% writes into a1_ks.user_table .

Watching IO activities I noticed that C* write something (mutations) into commitlog. But it looks strange because I disabled durable writes for a1_ks. May be it's system activities and they are flushed into commitlog?