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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