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From Oscar Gothberg <Oscar.Gothb...@platform-a.com>
Subject Problem with Hadoop and concatenated gzip files
Date Sat, 10 Jan 2009 00:26:36 GMT
Hi,

I'm having trouble with Hadoop (tested with 0.17 and 0.19) not fully processing certain gzipped
input files. Basically it only actually reads and processes a first part of the gzipped file,
and just ignores the rest without any kind of warning.

It affects at least (but is maybe not limited to?) any gzip files that are a result of concatenation
(which should be legal to do with gzip format):
http://www.gnu.org/software/gzip/manual/gzip.html#Advanced-usage

Repro case, using the "WordCount" example from the hadoop distribution:
$ echo 'one two three' > f1
$ echo 'four five six' > f2
$ gzip -c f1 > combined_file.gz
$ gzip -c f2 >> combined_file.gz

Now, if I run "WordCount" with combined_file.gz as input, it will only find the words 'one',
'two', 'three', but not 'four', 'five', 'six'.

It seems Java's GZIPInputStream may have a similar issue:
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4691425

Now, if I unzip and re-gzip this 'combined_file.gz' manually, the problem goes away.

It's especially dangerous since Hadoop doesn't show any errors or complains in the least.
It just ignores this extra input. The only way of noticing is to run one's app with gzipped-
and unzipped data side by side and notice the record counts being different.

Is anyone else familiar with this problem? Any solutions, workarounds, short of re-gzipping
very large amounts of data?

Thanks!
/ Oscar

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