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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 Wed, 04 Jan 2012 22:29:39 GMT

    [ https://issues.apache.org/jira/browse/HBASE-4218?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13179945#comment-13179945
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Mikhail Bautin commented on HBASE-4218:
---------------------------------------

I think that with an 8K line patch we probably should not try to put more complexity into
the first version of delta encoding. We can always make things more complicated later. I like
the two-parameter setup: DATA_BLOCK_ENCODING sets the encoding type (on-disk and in-cache
by default) and ENCODE_ON_DISK (true by default) allows to use in-cache-only encoding (when
explicitly setting ENCODE_ON_DISK=false) and get the benefit of encoding in cache even before
we are 100% sure that our encoding algorithms and encoded scanners are stable. If everyone
agrees with that, I will finish the patch by (1) adding a unit test for switching data block
encoding column family settings; (2) including encoding type in the cache key; and (3) simplifying
the HFileDataBlockEncoder interface, since we assume that the "in-memory format" (used by
scanners) is always the same as the in-cache format and don't need methods such as afterReadFromDiskAndPuttingInCache
anymore.
                
> 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, 4218.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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