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From "Matt Corgan (Created) (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-5479) Postpone CompactionSelection to compaction execution time
Date Sun, 26 Feb 2012 00:51:48 GMT
Postpone CompactionSelection to compaction execution time
---------------------------------------------------------

                 Key: HBASE-5479
                 URL: https://issues.apache.org/jira/browse/HBASE-5479
             Project: HBase
          Issue Type: New Feature
          Components: io, performance, regionserver
            Reporter: Matt Corgan


It can be commonplace for regionservers to develop long compaction queues, meaning a CompactionRequest
may execute hours after it was created.  The CompactionRequest holds a CompactionSelection
that was selected at request time but may no longer be the optimal selection.  The CompactionSelection
should be created at compaction execution time rather than compaction request time.

The current mechanism breaks down during high volume insertion.  The inefficiency is clearest
when the inserts are finished.  Inserting for 5 hours may build up 50 storefiles and a 40
element compaction queue.  When finished inserting, you would prefer that the next compaction
merges all 50 files (or some large subset), but the current system will churn through each
of the 40 compaction requests, the first of which may be hours old.  This ends up re-compacting
the same data many times.  

The current system is especially inefficient when dealing with time series data where the
data in the storefiles has minimal overlap.  With time series data, there is even less benefit
to intermediate merges because most storefiles can be eliminated based on their key range
during a read, even without bloomfilters.  The only goal should be to reduce file count, not
to minimize number of files merged for each read.

There are other aspects to the current queuing mechanism that would need to be looked at.
 You would want to avoid having the same Store in the queue multiple times.  And you would
want the completion of one compaction to possibly queue another compaction request for the
store.

A alternative architecture to the current style of queues would be to have each Store (all
open in memory) keep a compactionPriority score up to date after events like flushes, compactions,
schema changes, etc.  Then you create a "CompactionPriorityComparator implements Comparator<Store>"
and stick all the Stores into a PriorityQueue (synchronized remove/add from the queue when
the value changes).  The async compaction threads would keep pulling off the head of that
queue as long as the head has compactionPriority > X.

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