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From Michael McCandless <luc...@mikemccandless.com>
Subject Re: Corrupt index
Date Fri, 15 Jun 2012 00:21:53 GMT
Hmm, OK: in 2.9.4 / 3.0.x, if you open IW on a new directory, it will
make a zero-segment commit.  This was changed/fixed in 3.1 with
LUCENE-2386.

In 2.9.x (not 3.0.x) there is still an autoCommit parameter,
defaulting to false, but if you set it to true then IndexWriter will
periodically commit.

Seeing segment files created and merge is definitely expected, but
it's not expected to see segments_N files unless you pass
autoCommit=true.

Mike McCandless

http://blog.mikemccandless.com

On Thu, Jun 14, 2012 at 8:14 PM, Itamar Syn-Hershko <itamar@code972.com> wrote:
> Not what I'm seeing. I actually see a lot of segments created and merged
> while it operates. Expected?
>
> Reminding you, this is 2.9.4 / 3.0.3
>
> On Fri, Jun 15, 2012 at 3:10 AM, Michael McCandless
> <lucene@mikemccandless.com> wrote:
>>
>> Right: Lucene never autocommits anymore ...
>>
>> If you create a new index, add a bunch of docs, and things crash
>> before you have a chance to commit, then there is no index (not even a
>> 0 doc one) in that directory.
>>
>> Mike McCandless
>>
>> http://blog.mikemccandless.com
>>
>> On Thu, Jun 14, 2012 at 1:41 PM, Itamar Syn-Hershko <itamar@code972.com>
>> wrote:
>> > I'm quite certain this shouldn't happen also when Commit wasn't called.
>> >
>> > Mike, can you comment on that?
>> >
>> > On Thu, Jun 14, 2012 at 8:03 PM, Christopher Currens
>> > <currens.chris@gmail.com> wrote:
>> >>
>> >> Well, the only thing I see is that there is no place where
>> >> writer.Commit()
>> >> is called in the delegate assigned to corpusReader.OnDocument.  I know
>> >> that
>> >> lucene is very transactional, and at least in 3.x, the writer will
>> >> never
>> >> auto commit to the index.  You can write millions of documents, but if
>> >> commit is never called, those documents aren't actually part of the
>> >> index.
>> >>  Committing isn't a cheap operation, so you definitely don't want to do
>> >> it
>> >> on every document.
>> >>
>> >> You can test it yourself with this (naive) solution.  Right below the
>> >> writer.SetUseCompoundFile(false) line, add "int numDocsAdded = 0;".  At
>> >> the
>> >> end of the corpusReader.OnDocument delegate add:
>> >>
>> >> // Example only.  I wouldn't suggest committing this often
>> >> if(++numDocsAdded % 5 == 0)
>> >> {
>> >>    writer.Commit();
>> >> }
>> >>
>> >> I had the application crash for real on this file:
>> >>
>> >>
>> >> http://dumps.wikimedia.org/gawiktionary/20120613/gawiktionary-20120613-pages-meta-history.xml.bz2,
>> >> about 20% into the operation.  Without the commit, the index is empty.
>> >>  Add
>> >> it in, and I get 755 files in the index after it crashes.
>> >>
>> >>
>> >> Thanks,
>> >> Christopher
>> >>
>> >> On Wed, Jun 13, 2012 at 6:13 PM, Itamar Syn-Hershko
>> >> <itamar@code972.com>wrote:
>> >>
>> >>
>> >> > Yes, reproduced in first try. See attached program - I referenced it
>> >> > to
>> >> > current trunk.
>> >> >
>> >> >
>> >> > On Thu, Jun 14, 2012 at 3:54 AM, Itamar Syn-Hershko
>> >> > <itamar@code972.com>wrote:
>> >> >
>> >> >> 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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