C* 2.1.7 cluster is behaving really bad after 1-2 days. gauges.cassandra.jmx.org.apache.cassandra.metrics.ColumnFamily.AllMemtablesHeapSize.Value jumps to 7 GB (https://www.dropbox.com/s/vraggy292erkzd2/Screenshot%202015-06-29%2019.12.53.png?dl=0) on 3/6 nodes in each data center and then there are many long GC pauses. Cluster is using default heap size values (-Xms8192M -Xmx8192M -Xmn2048M)
Before C* 2.1.5 memtables heap size was basically constant ~500MB (https://www.dropbox.com/s/fjdywik5lojstvn/Screenshot%202015-06-29%2019.30.00.png?dl=0)
After restarting all nodes is behaves stable for 1-2days. Today I've done that and long GC pauses are gone (~18:00 https://www.dropbox.com/s/7vo3ynz505rsfq3/Screenshot%202015-06-29%2019.28.37.png?dl=0). The only pattern we've found so far is that long GC pauses are happening basically at the same time on all nodes in the same data center - even on the ones where memtables heap size is not growing.
Cliffs on the graphs are nodes restarts.
Used memory on boxes where AllMemtabelesHeapSize grows, stays at the same level - https://www.dropbox.com/s/tes9abykixs86rf/Screenshot%202015-06-29%2019.37.52.png?dl=0.
Replication factor is set to 3.