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From Itamar Syn-Hershko <ita...@code972.com>
Subject Re: Corrupt index
Date Thu, 14 Jun 2012 00:54:22 GMT
Christopher,

I used the IndexBuilder app from here
https://github.com/synhershko/Talks/tree/master/LuceneNeatThings with a
8.5GB wikipedia dump.

After running for 2.5 days I had to forcefully close it (infinite loop in
the wiki-markdown parser at 92%, go figure), and the 40-something GB index
I had by then was unusable. I then was able to reproduce this

Please note I now added a few safe-guards you might want to remove to make
sure the app really crashes on process kill.

I'll try to come up with a better way to reproduce this - hopefully Mike
will be able to suggest better ways than manual process kill...

On Thu, Jun 14, 2012 at 1:41 AM, Christopher Currens <
currens.chris@gmail.com> wrote:

> Mike, The codebase for lucene.net should be almost identical to java's
> 3.0.3 release, and LUCENE-1044 is included in that.
>
> Itamar, are you committing the index regularly?  I only ask because I can't
> reproduce it myself by forcibly terminating the process while it's
> indexing.  I've tried both 3.0.3 and 2.9.4.  If I don't commit at all and
> terminate the process (even with a 10,000 4K documents created), there will
> be no documents in the index when I open it in luke, which I expect.  If I
> commit at 10,000 documents, and terminate it a few thousand after that, the
> index has the first ten thousand that were committed.  I've even terminated
> it *while* a second commit was taking place, and it still had all of the
> documents I expected.
>
> It may be that I'm not trying to reproducing it correctly.  Do you have a
> minimal amount of code that can reproduce it?
>
>
> Thanks,
> Christopher
>
> On Wed, Jun 13, 2012 at 9:31 AM, Michael McCandless <
> lucene@mikemccandless.com> wrote:
>
> > Hi Itamar,
> >
> > One quick question: does Lucene.Net include the fixes done for
> > LUCENE-1044 (to fsync files on commit)?  Those are very important for
> > an index to be intact after OS/JVM crash or power loss.
> >
> > More responses below:
> >
> > On Tue, Jun 12, 2012 at 8:20 PM, Itamar Syn-Hershko <itamar@code972.com>
> > wrote:
> >
> > > I'm a Lucene.Net committer, and there is a chance we have a bug in our
> > > FSDirectory implementation that causes indexes to get corrupted when
> > > indexing is cut while the IW is still open. As it roots from some
> > > retroactive fixes you made, I'd appreciate your feedback.
> > >
> > > Correct me if I'm wrong, but by design Lucene should be able to recover
> > > rather quickly from power failures or app crashes. Since existing
> segment
> > > files are read only, only new segments that are still being written can
> > get
> > > corrupted. Hence, recovering from worst-case scenarios is done by
> simply
> > > removing the write.lock file. The worst that could happen then is
> having
> > the
> > > last segment damaged, and that can be fixed by removing those files,
> > > possibly by running CheckIndex on the index.
> >
> > You shouldn't even have to run CheckIndex ... because (as of
> > LUCENE-1044) we now fsync all segment files before writing the new
> > segments_N file, and then removing old segments_N files (and any
> > segments that are no longer referenced).
> >
> > You do have to remove the write.lock if you aren't using
> > NativeFSLockFactory (but this has been the default lock impl for a
> > while now).
> >
> > > Last week I have been playing with rather large indexes and crashed my
> > app
> > > while it was indexing. I wasn't able to open the index, and Luke was
> even
> > > kind enough to wipe the index folder clean even though I opened it in
> > > read-only mode. I re-ran this, and after another crash running
> CheckIndex
> > > revealed nothing - the index was detected to be an empty one. I am not
> > > entirely sure what could be the cause for this, but I suspect it has
> > > been corrupted by the crash.
> >
> > Had no commit completed (no segments file written)?
> >
> > If you don't fsync then all sorts of crazy things are possible...
> >
> > > I've been looking at these:
> > >
> > >
> >
> https://issues.apache.org/jira/browse/LUCENE-3418?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
> > >
> >
> https://issues.apache.org/jira/browse/LUCENE-2328?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
> >
> > (And LUCENE-1044 before that ... it was LUCENE-1044 that LUCENE-2328
> > broke...).
> >
> > > And it seems like this is what I was experiencing. Mike and Mark will
> > > probably be able to tell if this is what they saw or not, but as far
> as I
> > > can tell this is not an expected behavior of a Lucene index.
> >
> > Definitely not expected behavior: assuming nothing is flipping bits,
> > then on OS/JVM crash or power loss your index should be fine, just
> > reverted to the last successful commit.
> >
> > > What I'm looking for at the moment is some advice on what FSDirectory
> > > implementation to use to make sure no corruption can happen. The 3.4
> > version
> > > (which is where LUCENE-3418 was committed to) seems to handle a lot of
> > > things the 3.0 doesn't, but on the other hand LUCENE-3418 was
> introduced
> > by
> > > changes made to the 3.0 codebase.
> >
> > Hopefully it's just that you are missing fsync!
> >
> > > Also, is there any test in the suite checking for those scenarios?
> >
> > Our test framework has a sneaky MockDirectoryWrapper that, after a
> > test finishes, goes and corrupts any unsync'd files and then verifies
> > the index is still OK... it's good because it'll catch any times we
> > are missing calls t sync, but, it's not low level enough such that if
> > FSDir is failing to actually call fsync (that wsa the bug in
> > LUCENE-3418) then it won't catch that...
> >
> > Mike McCandless
> >
> > http://blog.mikemccandless.com
> >
>

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