hadoop-common-user mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From "Abdul Qadeer" <qadeer.qad...@gmail.com>
Subject Re: Strange behavior with bzip2 input files w/release 0.19.0
Date Thu, 04 Dec 2008 20:48:37 GMT
Andy,

As was mentioned earlier that splitting support is being added for bzip2
files
and actually patch is under review now.  I think, pbzip2 generated files
should
work fine with that because the split algorithm finds the next start of
block
marker and does not use end of stream marker.  We rather use physical
end of file to know when stream ends.


So if you see at https://issues.apache.org/jira/browse/HADOOP-4012
you can download version 4 patch and apply it on Hadoop code and see
if its working for you or you can wait for the review process to complete
so that code becomes a part of standard Hadoop.  You can add yourself
as a watcher there at JIRA 4012, so that you know when its done.  Please
let me know, if pbzip2 generated files does not work even on that code.

Thank you,
Abdul Qadeer


On Thu, Dec 4, 2008 at 11:46 AM, Andy Sautins
<andy.sautins@returnpath.net>wrote:

>
>   Thanks for the response Abdul.
>
>   So, the bzip2 file in question is _kindof_ a concatenation of
> multiple bzip2 files.  It's not concatenated using cat a.bz2 b.bz2 >
> yourFile.bz2, but it is created using pbzip2 ( pbzip2 v1.0.2 running on
> CentOS 5.2 installed from the EPEL repository ).  My understanding is
> that pbzip does roughly what you're saying and concatenates in some
> manner.
>
>   I created a simple test case that reproduces the behavior.  I created
> a file using the following perl script:
>
> for($i=0;$i<20000000;$i++) {
>  print "Line $i\n";
> }
>
>    I then created two different bzip2 files.  One with bzip2 and one
> with pbzip2.  The do have different sizes:
>
> 21994233 simple.bzip2.txt.bz2
> 21999416 simple.pbzip2.txt.bz2
>
>    They do decompress to give the same output file
> bunzip2 -c simple.bzip2.txt.bz2 | md5sum
> 581ad242e6cf22650072edd44d6a2d38  -
>
> bunzip2 -c simple.pbzip2.txt.bz2 | md5sum
> 581ad242e6cf22650072edd44d6a2d38  -
>
>   Running both through the simple line count MapReduce job I get the
> same behavior where bzip2 correctly calculates 20,000,000 records, but
> the pbzip2 generated file only processes the first block ( 82,829
> records ).
>
>   So, it sounds like what you're saying of having multiple end of
> stream markers makes sense.  I will say it would be very beneficial to
> be able to use pbzip2 generated files to compress hadoop input files.
> Using pbzip2 can greatly reduce the amount of time required to bzip2
> compress files and seems to generate a valid bzip2 file ( at least it
> bunzip2 decompresses correctly ).
>
>   Thank you
>
>   Andy
>
> -----Original Message-----
> From: Abdul Qadeer [mailto:qadeer.qadeer@gmail.com]
> Sent: Thursday, December 04, 2008 12:07 PM
> To: core-user@hadoop.apache.org
> Subject: Re: Strange behavior with bzip2 input files w/release 0.19.0
>
> Andy,
>
> As you said, you suspect that only one bzip2 block is being decompressed
> and used; is you bzip2 file the concatenation of multiple bzip2 files
> (i.e.
> are
> you doing something like cat a.bz2 b.bz2 c.bz2 > yourFile.bz2 ?)  In
> such
> a case, there will be many bzip2 end of stream markers in a single file
> and
> bzip2 decomprssor will stop on encountering the first end of block
> marker
> when in fact, the stream has more data in it.
>
> If this is not the case, then bzip2 should work as gzip or plaintext are
> working.
> Currently only one mapper gets the whole file (just like gzip and
> splitting
> support
> for bzip is being added in HADOOP-4012, as Alex mentioned).  The
> LineRecordReader
> get the uncompressed data and does rest of the things same as in the
> case
> of gzip or plaintext.  So can you provide your bzip2 compressed file?
> (May
> be
> uploading it somewhere and sending in the link)  I will look into this
> issue.
>
>
> Abdul Qadeer
>
> On Thu, Dec 4, 2008 at 9:11 AM, Andy Sautins
> <andy.sautins@returnpath.net>wrote:
>
> >
> >
> >    I'm seeing some strange behavior with bzip2 files and release
> > 0.19.0.  I'm wondering if anyone can shed some light on what I'm
> seeing.
> > Basically it _looks_ like the processing of a particular bzip2 input
> > file is stopping after the first bzip2 block.  Below is a comparison
> of
> > tests  between a .gz file which seems to do what I expect, and the
> same
> > file .bz2 which doesn't behave as I expect.
> >
> >
> >
> >    I have the same file stored in hadoop compressed as both bzip2 and
> > gz formats.  The uncompressed file size is 660,841,894 bytes.
> Comparing
> > the files they both seem to be valid archives of the exact same file.
> >
> >
> >
> > /usr/local/hadoop/bin/hadoop dfs -cat
> > bzip2.example/data.bz2/file.txt.bz2 | bunzip2 -c | md5sum
> >
> > 2c82901170f44245fb04d24ad4746e38  -
> >
> >
> >
> > /usr/local/hadoop/bin/hadoop dfs -cat
> bzip2.example/data.gz/file.txt.gz
> > | gunzip -c | md5sum
> >
> > 2c82901170f44245fb04d24ad4746e38  -
> >
> >
> >
> >    Given the md5 sums match it seems like the files are the same and
> > uncompress correctly.
> >
> >
> >
> >    Now when I run a simple Map/Reduce application that just counts
> > lines in the file I get different results.
> >
> >
> >
> >  Expected Results:
> >
> >
> >
> >  /usr/local/hadoop/bin/hadoop dfs -cat
> > bzip2.bug.example/data.gz/file.txt.gz | gunzip -c | wc -l
> >
> > 6884024
> >
> >
> >
> >   Gzip input file Results: 6,884,024
> >
> >   Bzip2 input file Results: 9,420
> >
> >
> >
> >
> >
> >   Looking at the task log files the MAP_INPUT_BYTES of the .gz file
> > looks correct ([(MAP_INPUT_BYTES)(Map input bytes)(660,841,894)] ) and
> > matches the size of the uncompressed file.  However, looking at
> > MAP_INPUT_BYTES for the .bz2 file it's 900,000 ([(MAP_INPUT_BYTES)(Map
> > input bytes)(900000)] ) which matches the block size of the bzip2
> > compressed file.  So that makes me think for some reason that only the
> > first bzip2 block of the bzip2 compressed file is being processed.
> >
> >
> >
> >    So I'm wondering if my analysis is correct and if there could be an
> > issue with the processing of bzip2 input files.
> >
> >
> >
> >   Andy
> >
> >
>

Mime
  • Unnamed multipart/alternative (inline, None, 0 bytes)
View raw message