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Albert P Tobey commented on CASSANDRA-9274: ------------------------------------------- I've been messing with these values lately and have observed some poor behavior around them. The comment in cassandra.yaml is misleading. I agree we shouldn't set the threshold by default, but I would like to see a comment added to memtable_flush_writers indicating that if you add a large number of flush writers, the default memtable_cleanup_threshold is going to end up small, which leads to small flushes and more frequent compaction. It makes some sense to set the memtable_cleanup_threshold based on the expected number of tables rather than cores or disks. I might be wrong, but that seems more relevant than the hardware or even the number of flush writer threads. > Changing memtable_flush_writes per recommendations in cassandra.yaml causes > memtable_cleanup_threshold to be too small > ----------------------------------------------------------------------------------------------------------------------- > > Key: CASSANDRA-9274 > URL: https://issues.apache.org/jira/browse/CASSANDRA-9274 > Project: Cassandra > Issue Type: Improvement > Reporter: Donald Smith > Priority: Minor > > It says in cassandra.yaml: > {noformat} > # If your data directories are backed by SSD, you should increase this > # to the number of cores. > #memtable_flush_writers: 8 > {noformat} > so we raised it to 24. > Much later we noticed a warning in the logs: > {noformat} > WARN [main] 2015-04-22 15:32:58,619 DatabaseDescriptor.java:539 - > memtable_cleanup_threshold is set very low, which may cause performance > degradation > {noformat} > Looking at cassandra.yaml again I see: > {noformat} > # memtable_cleanup_threshold defaults to 1 / (memtable_flush_writers + 1) > # memtable_cleanup_threshold: 0.11 > #memtable_cleanup_threshold: 0.11 > {noformat} > So, I uncommented that last line (figuring that 0.11 is a reasonable value). > Cassandra.yaml should give better guidance or the code should *prevent* the > value from going outside a reasonable range. -- This message was sent by Atlassian JIRA (v6.3.4#6332)