Mailing-List: contact issues-help@hbase.apache.org; run by ezmlm
Precedence: bulk
Date: Sat, 17 Dec 2011 19:33:31 +0000 (UTC)
From: "Phabricator (Commented) (JIRA)" <jira@apache.org>
To: issues@hbase.apache.org
Message-ID: 
 <1271219412.22899.1324150411459.JavaMail.tomcat@hel.zones.apache.org>
In-Reply-To: 
 <657671452.46177.1313609127186.JavaMail.tomcat@hel.zones.apache.org>
Subject: [jira] [Commented] (HBASE-4218) Delta Encoding of KeyValues  (aka
 prefix compression)
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit


    [ 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

--
This message is automatically generated by JIRA.
If you think it was sent incorrectly, please contact your JIRA administrators: https://issues.apache.org/jira/secure/ContactAdministrators!default.jspa
For more information on JIRA, see: http://www.atlassian.com/software/jira