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https://issues.apache.org/jira/browse/HBASE-4218?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Mikhail Bautin updated HBASE-4218:
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    Attachment: Delta-encoding.patch-2012-01-05_16_31_44_copy.patch

Attaching a patch that applies. (A new unit test is coming for HFile v1 to 
encoded HFile v2 upgrade, so the patch is not final yet.)
                
> Data Block Encoding of KeyValues  (aka delta encoding / prefix compression)
> ---------------------------------------------------------------------------
>
>                 Key: HBASE-4218
>                 URL: https://issues.apache.org/jira/browse/HBASE-4218
>             Project: HBase
>          Issue Type: Improvement
>          Components: io
>    Affects Versions: 0.94.0
>            Reporter: Jacek Migdal
>            Assignee: Mikhail Bautin
>              Labels: compression
>             Fix For: 0.94.0
>
>         Attachments: 0001-Delta-encoding-fixed-encoded-scanners.patch, 
> 0001-Delta-encoding.patch, 4218-v16.txt, 4218.txt, D447.1.patch, 
> D447.10.patch, D447.11.patch, D447.12.patch, D447.13.patch, D447.14.patch, 
> D447.15.patch, D447.16.patch, D447.17.patch, D447.18.patch, D447.19.patch, 
> D447.2.patch, D447.3.patch, D447.4.patch, D447.5.patch, D447.6.patch, 
> D447.7.patch, D447.8.patch, D447.9.patch, 
> Data-block-encoding-2011-12-23.patch, 
> Delta-encoding.patch-2011-12-22_11_52_07.patch, 
> Delta-encoding.patch-2012-01-05_15_16_43.patch, 
> Delta-encoding.patch-2012-01-05_16_31_44.patch, 
> Delta-encoding.patch-2012-01-05_16_31_44_copy.patch, 
> Delta_encoding_with_memstore_TS.patch, open-source.diff
>
>
> A compression for keys. Keys are sorted in HFile and they are usually very 
> similar. Because of that, it is possible to design better compression than 
> general purpose algorithms,
> It is an additional step designed to be used in memory. It aims to save 
> memory in cache as well as speeding seeks within HFileBlocks. It should 
> improve performance a lot, if key lengths are larger than value lengths. For 
> example, it makes a lot of sense to use it when value is a counter.
> Initial tests on real data (key length = ~ 90 bytes , value length = 8 bytes) 
> shows that I could achieve decent level of compression:
>  key compression ratio: 92%
>  total compression ratio: 85%
>  LZO on the same data: 85%
>  LZO after delta encoding: 91%
> While having much better performance (20-80% faster decompression ratio than 
> LZO). Moreover, it should allow far more efficient seeking which should 
> improve performance a bit.
> It seems that a simple compression algorithms are good enough. Most of the 
> savings are due to prefix compression, int128 encoding, timestamp diffs and 
> bitfields to avoid duplication. That way, comparisons of compressed data can 
> be much faster than a byte comparator (thanks to prefix compression and 
> bitfields).
> In order to implement it in HBase two important changes in design will be 
> needed:
> -solidify interface to HFileBlock / HFileReader Scanner to provide seeking 
> and iterating; access to uncompressed buffer in HFileBlock will have bad 
> performance
> -extend comparators to support comparison assuming that N first bytes are 
> equal (or some fields are equal)
> Link to a discussion about something similar:
> http://search-hadoop.com/m/5aqGXJEnaD1/hbase+windows&subj=Re+prefix+compression

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