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From Frank Geary <>
Subject Re: Index corruption using Lucene 2.4.1 - thread safety issue?
Date Wed, 03 Feb 2010 18:39:23 GMT

For the record - I haven't proven this yet - but here's my current theory of
what is causing the problem:

1) We start with a new RAMDir IW[0] and do some deletes and adds.
2) We create at least one IndexReader based on that IW. The last of which
we'll call IndexReader[A].
3) Then we switch to using the other RAMDir IW[1].
4) While using IW[1], we clear out IW[0] and create a new IW[0].  However,
the last IndexReader, IndexReader[A], is left around.
5) Eventually, we switch to using the other newly re-created RAMDir IW[0]
6) The first search that comes in against RAMDir IW[0] trys to reopen
IndexReader[A].  But IndexReader[A] has nothing to do with the newly
re-created RAMDir index[0].  

So my theory is that this confuses the IndexReader.reopen() code.  
Therefore, we can perhaps fix the problem by deleting the last IndexReader
used whenever we re-create any RAMDir IW.  Furthermore, this problem may be
so infrequent because:
a) It can only happen the first time after two RAMDir swaps.
b) It may depend on what state IndexReader[A] was in when it was created.
c) It may also depend on what state the new RAMDir IW is in when that first
reopen() call is made using the old IndexReader[A].  
d) If the first reopen for a new IW succeeds, we're fine until the next
e) If it fails, it could fail repeatedly or only once.

I'll let you know when I determine if this theory is correct or not.



Michael McCandless-2 wrote:
> It looks like each IW writes to a private log file -- could you zip up
> all such files and attach as a zip file (CC me directly because the
> list strips attachments)?  It can give me a bigger picture than just
> these fragments...
> Mike
> On Fri, Jan 22, 2010 at 12:02 PM, Frank Geary <>
> wrote:
>> Mike,
>> Below are the portions of the merge log files related to the
>> setInfoStream()
>> calls during the time our exception happens.  The ..._1... log file is
>> from
>> one RAMDir index and the ..._0... log file is from the other RAMDir
>> index.
> OK
>> The time period when our Array index out of bounds exception happens is
>> 2009-12-27 19:59:51,244 (when the IndexReader.reopen() is about to be
>> called) to 2009-12-27 19:59:51,259 (when the exception is thrown from the
>> call).
> I suspect the corruption happened before that, ie, somehow the
> deletions file for a segment got overwritten.  To bound the time I
> guess we have look back to the reopen before the corrupt IndexReader
> (which didn't hit the corruption).
>> As you can see from the log files below,
>> our "_1" RAMDir index is the only one being used during the time the
>> exception is thrown - not new information to me.
> Right but we need to see logs from earlier...
>> Also a merge has finished
>> at 2009-12-27 19:59:50,561 and thus it appears that nothing at all is
>> happening (with respect to merges or commits) in either RAMDir index at
>> the
>> time the problem occurs.
> Need to see all the infoStream log files to really be sure...
>> But one thing that is interesting to me - perhaps
>> you can clarify - is there is a very short commit at the very top of the
>> "_1" log file.  Why is that commit different from the commit right below
>> it?
>> What does "DW: apply 1 buffered deleted terms and 0 deleted docIDs" mean?
> There was one delete-by-Term buffered (and no added docs) for the
> first commit, and the reverse for the 2nd case.
>> What does "commit: pendingCommit == null; skip" mean?  Are there
>> different
>> kinds of commit?
> It looks like the deleted Term did not in fact match a document and so
> the flush & commit were skipped.
> You should also try making new RAMDirs instead of reusing the same one
> -- if that still hits GC problems then something else is wrong: once a
> RAMDir & all readers/writers using it are closed, it should be easy to
> GC.
> Or, another thing you could try is to swap in MockRAMDirectory (from
> Lucene's tests): it will throw an exception if you attempt to close it
> when the readers/writers didn't in fact close all their files.  If you
> hit that exception you know something isn't being closed... it's a
> good way to catch bugs...
> Mike
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