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The following page has been changed by EricEvans: http://wiki.apache.org/cassandra/MemtableThresholds The comment on the change is: added a bit, still needs more work ------------------------------------------------------------------------------ + '''''THIS DOCUMENT IS A WORK IN PROGRESS''''' + When performing write operations, Cassandra stores values to column-family - specific, in-memory data structures, called Memtables. Memtables + specific, in-memory data structures called Memtables. These Memtables are - are flused to disk whenever a threshold is exceeded, so it's important to - have these thresholds configured properly to keep Memtable size - in check, and overall memory usage within limits. + flushed to disk whenever one of the configurable thresholds is exceeded. + Proper tuning of these thresholds is important since the more memory that + can be put to use the better, while running out of memory is a sure way to + bring down the node. - == MemtableSizeInMB == - If left unset, defaults to 128MB. + Since Memtables store actual column values, they consume at least as + much memory as the size of data inserted. However, there is also overhead + associated with the data-structures used to index this data. When the + number of columns and rows is high compared to the size of values, this + overhead can become quite significant. + == Threshold Configuration == + Listed below are the thresholds found in `storage-conf.xml`, along with a + description. + + === MemtableSizeInMB === + As the name indicates, this sets the max size in megabytes that the + Memtable will store before triggering a threshold violation and causing + it to be flushed to disk. It corresponds to the size of the values + inserted, (plus the size of the containing column). + - == MemtableObjectCountInMillions == + === MemtableObjectCountInMillions === If left unset, defaults to 1, (or 1,000,000 objects). == Using Jconsole To Optimize Thresholds == Cassandra's column-family mbeans have a number of attributes that can - prove invaluable in determining optimal thresholds. Once way to access + prove invaluable in determining optimal thresholds. Onc way to access this instrumentation is using Jconsole, a graphical monitoring and management application that ships with your JDK.
