When monitoring JMX metrics of cassandra 0.8.7 loaded by write-only test I observe significant read activity on column family where I write to. It seems strange to me, but I expected no read activity on write-only load. The read activity is caused by writes, as when I stop the write test, reads activity disappears. The test performs parallel column writes to a single row, writing the values of fixed column set over and over again. Furthermore, the second problem is that parallel massive reads of such row degrade over time (even without parallel write load) and cassandra starts burning 100% of CPU with read latency degrading x20 times comparing with exactly the same row created from scratch. The test setup is 3 cassandra nodes, read/write consistency = Quorum. Row has 10 and above columns (tested with 10, 100, 1000, 10000 cols), the higher is the number of columns, the worse is observed degradation. Column family has 2 indexed columns that are written with exactly the same values on each and every write. Row key, column name and column value are all Utf8Type. Column family compaction on all the nodes does not help, and the row remains "degraded". Read here means one of: read all the the columns with slice query without bounds/with bounds; executing column count query for a row with bounds/without bounds. I use Hector as cassandra client. I would be thankful if anyone could explain the read activity on write load and give any hints on row read degradation after massive write load on that row.
Regards, Oleg
