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From "Mikhail Bautin (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-4218) Data Block Encoding of KeyValues (aka delta encoding / prefix compression)
Date Tue, 03 Jan 2012 21:34:40 GMT

    [ https://issues.apache.org/jira/browse/HBASE-4218?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13179036#comment-13179036
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Mikhail Bautin commented on HBASE-4218:
---------------------------------------

@Matt, Ted: the problem with the previous settings was that they were too flexible, and allowed
for different encodings in cache and in memory. We definitely don't want to re-encode a block
using a different encoding algorithm after loading it from disk. After a discussion with Kannan
we decided that the whole benefit of delta encoding is in encoded scanners and allowing to
put more data into cache. If we want to use a compression algorithm on disk but not in cache,
it is possible to implement that using the existing compression framework. Furthermore, Jacek
found in his experiments that encoded scanners were actually faster than scanners on decoded
blocks. Please let me know what use case you have in mind that would require storing decoded
blocks in cache and would not allow for efficient scanning over encoded blocks.

                
> Data Block Encoding of KeyValues  (aka delta encoding / 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
>             Fix For: 0.94.0
>
>         Attachments: 0001-Delta-encoding-fixed-encoded-scanners.patch, 0001-Delta-encoding.patch,
4218-v16.txt, D447.1.patch, D447.10.patch, D447.11.patch, D447.12.patch, D447.13.patch, D447.14.patch,
D447.15.patch, D447.16.patch, D447.17.patch, D447.2.patch, D447.3.patch, D447.4.patch, D447.5.patch,
D447.6.patch, D447.7.patch, D447.8.patch, D447.9.patch, Data-block-encoding-2011-12-23.patch,
Delta-encoding.patch-2011-12-22_11_52_07.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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