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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, 13 Dec 2011 06:35:34 GMT

    [ https://issues.apache.org/jira/browse/HBASE-4218?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13168165#comment-13168165
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Phabricator commented on HBASE-4218:
------------------------------------

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



INLINE COMMENTS
  src/main/java/org/apache/hadoop/hbase/KeyValue.java:2148 Are this calculations dangerous?
 Could they be beyond commonPrefix into unallocated space?


  src/main/java/org/apache/hadoop/hbase/KeyValue.java:2020 I'm not sure I understand what
this is for.   Any chance of an example showing when this would be used?
  src/main/java/org/apache/hadoop/hbase/KeyValue.java:2156 This code looks like the old comparator
code.  We are not duplicating it here are we?  (Thats some ugly code... would be a tradegy
having it show up twice)  We should at miminum tie the two together with comments warning
no change of one w/o changing other.
  src/main/java/org/apache/hadoop/hbase/io/deltaencoder/BitsetKeyDeltaEncoder.java:53 I love
this.
  src/main/java/org/apache/hadoop/hbase/io/deltaencoder/BitsetKeyDeltaEncoder.java:89 I wonder
if we could use this stuff writing over rpc; it might be too costly compressing but maybe
for big KVs..... Anyways.
  src/main/java/org/apache/hadoop/hbase/io/deltaencoder/BitsetKeyDeltaEncoder.java:158 I love
it.

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, 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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