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From Adam Kocoloski <kocol...@apache.org>
Subject silent view index file corruption
Date Wed, 07 Apr 2010 01:03:33 GMT
Hi all, we recently had an EC2 node go AWOL for about 12 hours.  When it came back, we noticed
after a few days that a number of the view indexes stored on that node were not updating.
 I did some digging into the error logs and with Paul's help pieced together what was going
on.  I won't bother you with all the gory details unless you ask for them, but the gist of
it is that those files are corrupted.

The troubling thing for me is that we only discovered the corruption when it completely broke
the index updates.  In one case, it did this by rearranging the bits so that couch_file thought
that the btree node it was reading from disk had an associated MD5 checksum. It didn't (no
btree nodes do), and so couch_file threw a file_corruption exception.  But if the corruption
had shown up in another part of the file I might never have known.  In fact, some of the other
indices on that node probably are silently corrupted.

You might wonder how likely it is that a file becomes corrupted but still appears to be functioning.
 I checked the last modified timestamps for three broken files.  One was last modified when
the node went down, but the other two had timestamps in between the node's recovery and now.
 To me, that means that the view indexer was able to update those files for quite a while
(~2 days) before it bumped into a part of the btree that was corrupted.

I wonder what we should do about this.  My first thought is to make it optional to write 
btree nodes (possibly only for view index files?) using append_term_md5 instead of append_term.
 It seems like a simple patch, but I don't know a priori what the performance hit would be.
 Other thoughts?

Best, Adam


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