[
https://issues.apache.org/jira/browse/HADOOP-7909?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13167318#comment-13167318
]
Tim Broberg commented on HADOOP-7909:
-------------------------------------
Good points, Niels.
Given that, your approach of a separate codec / file type makes a lot of sense,
and I'm inclined to go that way.
Still, a few questions to make sure this is the right approach:
- Is it easy to switch over to the deflate and discard method of HADOOP-7076
if we find the split
headers aren't supported?
- If we advertise a codec as splittable and then we fail to split it, is there
any way to fallback and just return one great big split that gives the same
performance as if we just didn't say we do splitting?
> Implement Splittable Gzip based on a signature in a gzip header field
> ---------------------------------------------------------------------
>
> 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
> Original Estimate: 672h
> Remaining Estimate: 672h
>
> 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.
--
This message is automatically generated by JIRA.
If you think it was sent incorrectly, please contact your JIRA administrators:
https://issues.apache.org/jira/secure/ContactAdministrators!default.jspa
For more information on JIRA, see: http://www.atlassian.com/software/jira
