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From Ching-Cheng Chen <cc...@evidentsoftware.com>
Subject memtable_flush_after_mins setting not working
Date Thu, 17 Feb 2011 16:17:57 GMT
We have observed the behavior that memtable_flush_after_mins setting not
working occasionally.   After some testing and code digging, we finally
figured out what going on.
The memtable_flush_after_mins won't work on certain condition with current
implementation in Cassandra.

In org.apache.cassandra.db.Table,  the scheduled flush task is setup by the
following code during construction.


*int minCheckMs = Integer.MAX_VALUE;*
*       *
*for (ColumnFamilyStore cfs : columnFamilyStores.values())  *
*{*
*    minCheckMs = Math.min(minCheckMs, cfs.getMemtableFlushAfterMins() * 60
* 1000);*
*}*
*
*
*Runnable runnable = new Runnable()*
*{*
*   public void run()*
*   {*
*       for (ColumnFamilyStore cfs : columnFamilyStores.values())*
*       {*
*           cfs.forceFlushIfExpired();*
*       }*
*   }*
*};*
*flushTask = StorageService.scheduledTasks.scheduleWithFixedDelay(runnable,
minCheckMs, minCheckMs, TimeUnit.MILLISECONDS);*


Now for our application, we will create a keyspacewithout any columnfamily
first.  And only add needed columnfamily later depends on request.

However, when keyspacegot created (without any columnfamily ), the above
code will actually schedule a fixed delay flush check task with
Integer.MAX_VALUE ms
since there is no columnfamily yet.

Later when you add columnfamily to this empty keyspace, the initCf() method
in Table.java doesn't check whether the scheduled flush check task interval
need
to be updated or not.   To fix this, we'd need to restart the Cassandra
after columnfamily added into the keyspace.

I would suggest that add additional logic in initCf() method to recreate a
scheduled flush check task if needed.

Regards,

Chen

www.evidentsoftware.com

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