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From "Phabricator (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-4218) Delta Encoding of KeyValues (aka prefix compression)
Date Tue, 29 Nov 2011 17:39:41 GMT

    [ https://issues.apache.org/jira/browse/HBASE-4218?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13159391#comment-13159391

Phabricator commented on HBASE-4218:

tedyu has commented on the revision "[jira] [HBASE-4218] Delta encoding for keys in HFile".

  Nice work, Mikhail and Jacek.

  Please add category to the new tests.

  Are there performance numbers for various encoders other than Prefix encoder ?

  src/main/java/org/apache/hadoop/hbase/io/hfile/HFileReaderV1.java:337 As Matt pointed out,
the return value should be stored in hfileBlock so that we don't incur double encoding.
  src/main/java/org/apache/hadoop/hbase/io/hfile/HFileReaderV2.java:305 Similar to the case
in HFileReaderV1, return value should be stored in dataBlock.
  src/main/java/org/apache/hadoop/hbase/io/deltaencoder/DeltaEncoderAlgorithms.java:33 Matt
suggested alternative names for DeltaEncoding:
  KeyValueEncoding, DataBlockEncoding, HCellEncoding, BlockEncoding.

  DataBlockEncoding sounds good.
  src/main/java/org/apache/hadoop/hbase/io/deltaencoder/DiffKeyDeltaEncoder.java:405 Misspelling:
comperator should be comparator.
  src/main/java/org/apache/hadoop/hbase/io/deltaencoder/DeltaEncoderAlgorithms.java:65 Javadoc
doesn't match actual class name.
  src/main/java/org/apache/hadoop/hbase/io/deltaencoder/FastDiffDeltaEncoder.java:53 The tail
should read '128 bit encoding'
  src/main/java/org/apache/hadoop/hbase/io/deltaencoder/FastDiffDeltaEncoder.java:28 This
class is only used locally. It should be an inner class.
  src/main/java/org/apache/hadoop/hbase/io/deltaencoder/DiffKeyDeltaEncoder.java:49 Tail should
read '128 bit encoding'
  src/main/java/org/apache/hadoop/hbase/io/deltaencoder/DiffKeyDeltaEncoder.java:346 Please
remove extra blank line.
  src/main/java/org/apache/hadoop/hbase/io/deltaencoder/DiffKeyDeltaEncoder.java:28 Please
change this class to inner class.
Should read 'which indicates'


> Delta Encoding of KeyValues  (aka 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
>         Attachments: D447.1.patch, D447.2.patch, D447.3.patch, D447.4.patch, D447.5.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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