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https://issues.apache.org/jira/browse/HBASE-4218?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13179335#comment-13179335
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Phabricator commented on HBASE-4218:
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mcorgan has commented on the revision "[jira] [HBASE-4218] HFile data block
encoding framework and delta encoding implementation".
Trying to review this with an eye on schema changes and compactions.
INLINE COMMENTS
src/main/java/org/apache/hadoop/hbase/io/hfile/HFileDataBlockEncoderImpl.java:241
What about the situation where regionserver is running for a while with
ENCODING_IN_MEMORY=true and block cache gets filled with encoded blocks, and
then user does schema change to disable encoding altogether. Now the block
cache may return an old encoded block. (Assuming online schema change doesn't
invalidate all blocks for a table?)
If i'm understanding that correctly, then it shouldn't be an
IllegalStateException but should be handled normally. It should probably
invalidate the encoded block from the block cache if possible, otherwise it
will expire normally. Then it should return null so that HfileReaderV2 knows
to go to the filesystem to get the block.
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-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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