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From "Matt Corgan (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-5479) Postpone CompactionSelection to compaction execution time
Date Mon, 27 Feb 2012 20:57:49 GMT

    [ https://issues.apache.org/jira/browse/HBASE-5479?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13217550#comment-13217550
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Matt Corgan commented on HBASE-5479:
------------------------------------

re, outdated requests: i now see that in Store.requestCompaction, you are eliminating already
queued files from consideration, so requested files will never have disappeared between when
a compaction is requested vs executed.

Let me take another stab at explaining the problem.  Say you have hbase.hstore.compactionThreshold=3,
hbase.hstore.compaction.max=20.  You are flushing a particular memstore every minute and compactions
are backed up by an hour for whatever reason.  After 3 minutes of inserting, the CompactSplitThread
will create a CompactionRequest for the first 3 StoreFiles.  During the next hour, while that
first CompactionRequest is sitting in the queue, 60 new StoreFiles were added, and 20 additional
CompactionRequests were queued.

Finally, the first CompactionRequest makes it to the head of the queue and is ready to be
executed.  At this point, there are 63 small StoreFiles in the Store.  While this original
CompactionRequest was correct at the time it was created, I would now prefer that it compacted
the first 20 files, not just the first 3.

Maybe it could abort a CompactionRequest if there are already items in Store.filesCompacting.
                
> 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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