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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:
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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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