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From "Phabricator (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-4218) Delta Encoding of KeyValues (aka prefix compression)
Date Sat, 17 Dec 2011 19:33:31 GMT

    [ https://issues.apache.org/jira/browse/HBASE-4218?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13171648#comment-13171648
] 

Phabricator commented on HBASE-4218:
------------------------------------

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

  some more comments...

INLINE COMMENTS
  src/main/java/org/apache/hadoop/hbase/io/deltaencoder/DeltaEncoder.java:65 javadoc fix for
the new param "includesMemstoreTS" is needed on a few of these methods.
  src/main/java/org/apache/hadoop/hbase/io/deltaencoder/DeltaEncoder.java:126 little confused
with the doc. Could you clarify what happens in the inexact match case: where are we left
pointing to for the seekBefore = true and seekBefore=false cases.
  src/main/java/org/apache/hadoop/hbase/io/deltaencoder/PrefixKeyDeltaEncoder.java:34 here
and a bunch of other places... 128 bit encoding should read 7 bit encoding
  src/main/java/org/apache/hadoop/hbase/io/deltaencoder/BufferedDeltaEncoder.java:475 It seems
like we are missing a:

  keyBuffer = newKeyBuffer;

  step here after the arrayCopy step.

  src/main/java/org/apache/hadoop/hbase/io/deltaencoder/BufferedDeltaEncoder.java:470 I think
the logic here has an unintentional bug.

  newKeyBufferLength = keyLength * 2;
  should be:
  newKeyBufferLength = keyBuffer.length * 2;

  Otherwise, the check on the subsequent line will always be FALSE.



REVISION DETAIL
  https://reviews.facebook.net/D447

                
> 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: 0001-Delta-encoding-fixed-encoded-scanners.patch, D447.1.patch,
D447.2.patch, D447.3.patch, D447.4.patch, D447.5.patch, D447.6.patch, D447.7.patch, D447.8.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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