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From: Apache Wiki <wikidiffs@apache.org>
To: cassandra-commits@incubator.apache.org
Date: Fri, 15 May 2009 17:47:02 -0000
Message-ID: <20090515174702.25955.78297@eos.apache.org>
Subject: [Cassandra Wiki] Update of "MemtableThresholds" by EricEvans

Dear Wiki user,

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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.