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From Michael McCandless <>
Subject Re: Background merge hit exception
Date Thu, 18 Sep 2008 14:19:55 GMT
Lucene tries to carry forward the root cause exception from the merge,
into that IOException that optimize throws.  But it doesn't always
succeed in doing so; I'll open a Jira issue and try to figure out why
this is the case.

All the exception "means" is that the optimize didn't finish -- you
still have multiple segments in the index.  The index should still be
fine (not corrupt, nothing lost).

What's happening is a BG merge thread is hitting an unhandled
exception.  The JRE will log such unhandled exceptions to System.err
by default, so, you should scour the app server's logs to find it (it
should be there).

BTW, that merge that's being attempted is particularly inefficient --
you are merging an immense segment (the first one) with a bunch of
tiny ones.  A partial optimize could be much better.

Things to try w/o code changes:

   * Use a separate tool, eg Luke, to run optimize and see what
     root exception is thrown.

Things to try with code changes:

   * Switch to SerialMergeScheduler -- it should still throw the
     exception, but you'll see the full root cause.

   * As of 2.4 (coming soon), subclass ConcurrentMergeScheduler and
     override the handleMergeException to do your own logging so you
     that you see the detailed root-cause exception.

   * Alternatively, before Lucene 2.4, if you are running in JRE 1.5+
     environment, you can set the default exception handler for
     threads to do your own logging:


On Sep 17, 2008, at 4:24 PM, vivek sar wrote:

> Hi,
> We have been running Lucene 2.3 for last few months with our
> application and all the sudden we have hit the following exception,
>       java.lang.RuntimeException: background
> merge hit exception: _2uxy:c11345949 _2uxz:c150 _2uy0:c150 _2uy1:c150
> _2uy2:c150 _2uy3:c150 _2uy4:c82 into _2uy5 [optimize]
> I don't see any other error messages (or stacktrace) around this
> exception message. This problem doesn't seem to be recoverable and the
> indexer process is failing even after the reboot of the machine.
> I've gone through the mailing list over this issue and saw few  
> suggestions,
> 1) Make sure you've enough disk space (x2 the index size)  - our
> index size is around 5 GB and we have around 50GB space available so
> this shouldn't be the case
> 2) Is your machine multi-core - yes, this application is running on
> Linux box with 8 CPUs, not sure if this is the problem
> I can't update the code as this is running on the customer site. Here
> are my questions,
>   a) Is there any workaround to this problem without updating the  
> code base?
>   b) Is there a jira opened on this issue?
>   c) Has this been fixed in the subsequent Lucene releases?
> Thanks,
> -vivek
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