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From "Matt Corgan (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 23:36:40 GMT

    [ https://issues.apache.org/jira/browse/HBASE-4218?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13179130#comment-13179130
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Matt Corgan commented on HBASE-4218:
------------------------------------

Yes, i think i used the most recent version.  I don't have the code readily available, but
can check into it tonight.  

My main concern from this morning was that the modified settings hid features of already working
code (like Lars mentioned) while not really simplifying things too much.  I guess the big
problem with having the separate ON_DISK and IN_MEMORY settings is that a user would have
to change both of them simultaneously, which is not obvious to a new user.

One option could be to persist the ENCODING_ON_DISK and ENCODING_IN_MEMORY separately in the
HColumnDescriptor no matter what we put in the settings UI.  That way we have the ability
to change the user facing settings in the future without having to go through the painful
process of versioning the HTableDescriptor (i'm not even sure how that works behind the scenes).
 If we did that, I think the simplest setting we could expose to the user would just be ENCODING,
and that would set both of the persistent variables to the same thing.

i hate to overthink it - just might be hard to change once it's in place
                
> 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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