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From "Jacek Migdal (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-4218) Delta Encoding of KeyValues (aka prefix compression)
Date Wed, 17 Aug 2011 19:25:27 GMT
Delta Encoding of KeyValues  (aka prefix compression)
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                 Key: HBASE-4218
                 URL: https://issues.apache.org/jira/browse/HBASE-4218
             Project: HBase
          Issue Type: Improvement
          Components: io
            Reporter: Jacek Migdal


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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