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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 08:47:39 GMT

    [ https://issues.apache.org/jira/browse/HBASE-4218?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13179349#comment-13179349
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Mikhail Bautin commented on HBASE-4218:
---------------------------------------

A brief status update. I am in the process of implementing support for column family data
block encoding configuration changes. Those changes are coming in the next version of the
patch that I will post tomorrow. After discussing this with Kannan, our solution is:
* Assign an in-cache data block encoding to every HFile reader. This in-cache encoding is
determined as follows:
** If the HFile is not encoded on disk, the in-cache encoding is set to the column family's
DATA_BLOCK_ENCODING.
** If the HFile is encoded on disk, the in-cache encoding is set to the HFile encoding to
avoid the wasted effort of re-encoding blocks for cache.
* When a non-encoded block is loaded from disk, it is encoded using the in-cache encoding
and put in cache.
* When an encoded block is loaded from disk, its encoding is left as is.
* To reduce the complexity of data block encoding switching, we can include the in-cache encoding
type in the block cache key. For example, if ENCODED_IN_CACHE_ONLY is turned on without encoding
on disk, and then the encoding is turned off altogether, the cache will be populated with
non-encoded blocks (since they will have completely different keys) and encoded blocks will
age out from the cache. While this is suboptimal, the implementation is very simple and the
common case (when the CF encoding options do not change) is not complicated with unnecessary
corner cases.

                
> 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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