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From Michael McCandless <luc...@mikemccandless.com>
Subject Re: Corrupt index
Date Thu, 14 Jun 2012 12:36:30 GMT
On Wed, Jun 13, 2012 at 8:45 PM, Itamar Syn-Hershko <itamar@code972.com> wrote:
> Mike,
>
> On Wed, Jun 13, 2012 at 7:31 PM, 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.
>
>
> Definitely, as Christopher noted we are about to release a 3.0.3 compatible
> version, which is line-by-line port of the Java version.

Hmm OK.  Then we still need to explain the corruption...

>> 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).
>
> Somewhat unrelated to this thread, but what should I expect to see? from
> time to time we do see write.lock present after an app-crash or power
> failure. Also, what are the steps that are expected to be performed in such
> cases?

If you are using NativeFSLockFactory, you will see a write.lock but it
will not actually be locked (according to the OS); so, it's fine.

If you are using SimpleFSLockFactory then the presence of write.lock
means the index is still locked and you'll have to remove it.

>> > 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...
>
> Ok, so we do have fsync since LUCENE-1044 is present, and there were
> segments present from previous commits. Any idea what went wrong?

I don't know!

>> > 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...).
>
> So 2328 broke 1044, and this was fixed only in 3.4, right? so 2328 made it
> to a 3.0.x release while the fix for it (3418) was only released in 3.4. Am
> I right?
>
> If this is the case, 2328 probably made it's way to Lucene.Net since we are
> using the released sources for porting, and we now need to apply 3418 in the
> current version.

OK that makes sense: 2328 broke things as of 3.0.3, and 3418 fixed
things in 3.4.

> Does it make sense to just port FSDirectory from 3.4 to 3.0.3? or were there
> API or other changes that will make our life miserable if we do that?

Hmmm I'm not certain offhand: maybe diff the two sources?  The fix in
3418 was trivial in the end, so maybe just backport that.

>> > 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 suspected. Will try to reproduce reliably - any recommendations? not
> really feeling like reinventing the wheel here...
>
> MockDirectoryWrapper wasn't ported yet as it appears to only appear in 3.4,
> and as you said it won't really help here anyway

Use a spare computer and try pulling the plug on it ... or pull a (hot
swappable/pluggable) hard drive while indexing onto it ...

You can also use a virtual machine and power it off ungracefully /
kill the process.

If any of these events can corrupt the index then there's a bug
somewhere (or: the IO system ignores fsync).

Mike McCandless

http://blog.mikemccandless.com

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