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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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stack updated HBASE-4218:
-------------------------
Release Note: Adds a block compression that stores the diff from the
previous key only. Good for big keys and small value datasets. Makes writing
and scanning slower but because the blocks compressed with this feature stay
compressed when in memory up in the block cache, more data is cached. Off by
default. To enable, on the column descriptor set DATA_BLOCK_ENCODING to NONE,
PREFIX, DIFF or FAST_DIFF. Set ENCODE_ON_DISK to true on column descriptor to
have the encoding in place out in the hfile (on by default).
> 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, 4218-2012-01-14.txt, 4218-v16.txt, 4218.txt,
> D1659.1.patch, D1659.2.patch, D1659.3.patch, 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,
> D447.20.patch, D447.21.patch, D447.22.patch, D447.23.patch, D447.24.patch,
> D447.25.patch, D447.26.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-2012-01-17_11_09_09.patch,
> Delta-encoding-2012-01-25_00_45_29.patch,
> Delta-encoding-2012-01-25_16_32_14.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.patch-2012-01-05_18_50_47.patch,
> Delta-encoding.patch-2012-01-07_14_12_48.patch,
> Delta-encoding.patch-2012-01-13_12_20_07.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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