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https://issues.apache.org/jira/browse/HBASE-4218?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13193130#comment-13193130
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Phabricator commented on HBASE-4218:
------------------------------------
gqchen has commented on the revision "[jira] [HBASE-4218] HFile data block
encoding framework and delta encoding implementation".
Looks really good to me!
I haven't finished reviewing DiffKeyDeltaEncoding (another day or so) and
might probably have a few minor comments about cosmetic things. But definitely
no need to wait for that.
INLINE COMMENTS
src/main/java/org/apache/hadoop/hbase/io/encoding/FastDiffDeltaEncoder.java:198
I think the logic is the following:
1. if the value not the same, copy the whole value.
2. however, if type is also not the same, take advantage of the fact that
"type" field is right ahead of "value", and copy both type and value in one
shot.
So the code would be like:
if ((flag & FLAG_SAME_VALUE) == 0) {
if ((flag & FALG_SAME_TYPE) == 0) {
valueOffset -= ...
valueLength += ...
}
ByteBufferUtils.copy...
}
The headache is if we decide to add one more field between "type" and "value"
in the future, this code will be silently broken.
REVISION DETAIL
https://reviews.facebook.net/D447
> 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, 4218-2012-01-14.txt, 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, D447.18.patch,
> D447.19.patch, D447.2.patch, D447.20.patch, D447.21.patch, D447.22.patch,
> D447.23.patch, D447.24.patch, D447.25.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-2012-01-17_11_09_09.patch,
> Delta-encoding-2012-01-25_00_45_29.patch,
> Delta-encoding.patch-2011-12-22_11_52_07.patch,
> Delta-encoding.patch-2012-01-05_15_16_43.patch,
> Delta-encoding.patch-2012-01-05_16_31_44.patch,
> Delta-encoding.patch-2012-01-05_16_31_44_copy.patch,
> Delta-encoding.patch-2012-01-05_18_50_47.patch,
> Delta-encoding.patch-2012-01-07_14_12_48.patch,
> Delta-encoding.patch-2012-01-13_12_20_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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