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From "Phabricator (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-4218) Delta Encoding of KeyValues (aka prefix compression)
Date Wed, 14 Dec 2011 01:46:31 GMT

    [ https://issues.apache.org/jira/browse/HBASE-4218?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13168970#comment-13168970

Phabricator commented on HBASE-4218:

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

  Addressing Michael's comments. A new version of the diff will follow. Running unit tests.

  src/main/java/org/apache/hadoop/hbase/HColumnDescriptor.java:99 Renamed to DEFAULT_DELTA_ENCODING_IN_MEMORY_ENABLED.
  src/main/java/org/apache/hadoop/hbase/KeyValue.java:2022 How about SamePrefixComparator?
This means the same thing as the latter but is shorter.
  src/main/java/org/apache/hadoop/hbase/io/deltaencoder/BitsetKeyDeltaEncoder.java:34-42 Done.
  src/main/java/org/apache/hadoop/hbase/io/deltaencoder/BitsetKeyDeltaEncoder.java:56 Done.
  src/main/java/org/apache/hadoop/hbase/io/deltaencoder/BitsetKeyDeltaEncoder.java:69 Done.
  src/main/java/org/apache/hadoop/hbase/io/deltaencoder/DeltaEncoder.java:32-35 Done.
  src/main/java/org/apache/hadoop/hbase/io/hfile/AbstractHFileWriter.java:90 Fixed.
  src/main/java/org/apache/hadoop/hbase/KeyValue.java:2020 This extension to the comparator
interface is used in BufferedDeltaEncoder to improve performance if the supplied comparator
implements this interface. We don't need to compare the first commonPrefix bytes of the two
keys if we already know they are the same.
  src/main/java/org/apache/hadoop/hbase/KeyValue.java:2148 This is the same as the old comparator
code. We are assuming that the two KVs are valid.
  src/main/java/org/apache/hadoop/hbase/KeyValue.java:2156 I've looked into this and indeed
saw some code duplication. I refactored the rest of this function into a common one shared
between the two comparators.
  src/main/java/org/apache/hadoop/hbase/io/deltaencoder/BitsetKeyDeltaEncoder.java:89 I guess
we might need to think about a bigger unified compression framework for HFiles, HLogs, and
RPC at some point.


> 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,
> 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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