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From "Tim Broberg (Created) (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HADOOP-7909) Implement Splittable Gzip based on a signature in a gzip header field
Date Sat, 10 Dec 2011 18:59:39 GMT
Implement Splittable Gzip based on a signature in a gzip header field
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                 Key: HADOOP-7909
                 URL: https://issues.apache.org/jira/browse/HADOOP-7909
             Project: Hadoop Common
          Issue Type: New Feature
          Components: io
            Reporter: Tim Broberg
            Priority: Minor


I propose to take the suggestion of PIG-42 extend it to
 - add a more robust header such that false matches are vanishingly unlikely
 - repeat initial bytes of the header for very fast split searching
 - break down the stream into modest size chunks (~64k?) for rapid parallel encode and decode
 - provide length information on the blocks in advance to make block decode possible in hardware

An optional extra header would be added to the gzip header, adding 36 bytes.

<sh> := <version><signature><uncompressedDataLength><compressedRecordLength>
<version> := 1 byte version field allowing us to later adjust the deader definition
<signature> := 23 byte signature of the form aaaaaaabcdefghijklmnopr where each letter
represents a randomly generated byte
<uncompressedDataLength> := 32-bit length of the data compressed into this record
<compressedRecordLength> := 32-bit length of this record as compressed, including all
headers, trailers

If multiple extra headers are present and the split header is not the first header, the initial
implementation will not recognize the split.

Input streams would be broken down into blocks which are appended, much as BlockCompressorStream
does. Non-split-aware decoders will ignore this header and decode the appended blocks without
ever noticing the difference.

The signature has >= 132 bits of entropy which is sufficient for 80+ years of Moore's law
before collisions become a significant concern.

The first 7 bytes are repeated for speed. When splitting, the signature search will look for
the 32-bit value aaaa every 4 bytes until a hit is found, then the next 4 bytes identify the
alignment of the header mod 4 to identify a potential header match, then the whole header
is validated at that offset. So, there is a load, compare, branch, and increment per 4 bytes
searched.

The existing gzip implementations do not provide access to the optional header fields (nor
comment nor filename), so the entire gzip header will have to be reimplemented and compression
will need to be done using the raw deflate options of the native library / built in deflater.

There will be some degradation when using splittable gzip:
 - The gzip headers will each be 36 bytes larger. (4 byte extra header header, 32 byte extra
header)
 - There will be one gzip header per block.
 - History will have to be reset with each block to allow starting from scratch at that offset
resulting in some uncompressed bytes that would otherwise have been strings.

Issues to consider:
 - Is the searching fast enough without the repeating 7 bytes in the signature?
 - Should this be a patch to the existing gzip classes to add a switch, or should this be
a whole new class?
 - Which level does this belong at? CompressionStream? Compressor?
 - Is it more advantageous to encode the signature into the less dense comment field?
 - Optimum block size? Smaller splits faster and may conserve memory, larger provides slightly
better compression ratio.



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