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https://issues.apache.org/jira/browse/HBASE-4608?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13146269#comment-13146269
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jirapos...@reviews.apache.org commented on HBASE-4608:
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bq. On 2011-11-07 23:39:59, Lars Hofhansl wrote:
bq. > Cool stuff.
bq. >
bq. > I am probably just missing something... But when is the dictionary
itself stored? Don't we need to read out the logs again.
bq. >
bq. > Just so I understand: We build up the dictionary as we go along. In the
beginning most things won't be in the dictionary, we write them out and add
them to the dict, and from that time on when we encounter them again we just
write the index.
bq. > On the read we could also build up the dict as we go along, because when
values weren't in the dictionary they where written into the file, so we can
recreate the dictionary as we read. Right?
bq. >
bq. > (As I said, I am probably missing something).
bq. >
bq. > See minor comments inline.
bq.
bq. Li Pi wrote:
bq. You aren't missing anything! Thats exactly how it works.
bq.
bq. Each WAL starts off with a brand new shiny dictionary. We build up the
dictionary as we write, and when we read, we start off with a shiny new
dictionary again. The dictionary is recreated upon read.
bq.
bq. Lars Hofhansl wrote:
bq. Ok... What I cannot find then, is the code that builds the dictionary
during read :)
bq.
bq. Also as a general concern... We write these WAL logs (in part) for
redundancy. Compression is the opposite of redundancy... So say, we garble the
beginning of a WAL file, then the entire file will be useless to us... I don't
think that is a big deal, though. As the WAL entries are variable length this
is mostly true even today.
bq.
bq.
bq. Li Pi wrote:
bq. Oops, somehow I deleted that line. There are comments for it. Added it
back in.
bq.
bq. //if this isn't in the dictionary, we need to add to the dictionary.
bq.
bq. As for the more general concern: HBase won't return a write to the
client until the WALEdit write is completely done. So aborting midway won't be
an issue - and even if we abort midway, we can recover everything thats been
written so far.
bq.
bq. For the beginning of the file getting garbled? - True but we'd lose
some information with or without compression. With compression we lose more
information, but that's the nature of compression. Recovering a partially
garbled WAL fully is impossible no matter what approach we use. Either way, its
not a contingency the WAL is built to handle - a partial recovery after all WAL
replica's have been corrupted.
well, in the non-compressed WAL case, we can re-sync to a SequenceFile "SYNC"
marker and continue reading from there in the face of arbitrary corruption.
Perhaps the compression mechanism should have some kind of "maximum lookback" -
ie when a dictionary is being built, keep the file offset where each dictionary
word was used. Then, when deciding to use a dict reference vs a literal, if the
curOffset - lastUsedOffset > MAX_LOOKBACK_THRESHOLD, we re-write the entry.
This would bound the size of unrecoverable WAL portions while still providing
good compression (similar to what we have today)
- Todd
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On 2011-11-07 23:12:37, Li Pi wrote:
bq.
bq. -----------------------------------------------------------
bq. This is an automatically generated e-mail. To reply, visit:
bq. https://reviews.apache.org/r/2740/
bq. -----------------------------------------------------------
bq.
bq. (Updated 2011-11-07 23:12:37)
bq.
bq.
bq. Review request for hbase, Eli Collins and Todd Lipcon.
bq.
bq.
bq. Summary
bq. -------
bq.
bq. Heres what I have so far. Things are written, and "should work". I need to
rework the test cases to test this, and put something in the config file to
enable/disable. Obviously this isn't ready for commit at the moment, but I can
get those two things done pretty quickly.
bq.
bq. Obviously the dictionary is incredibly simple at the moment, I'll come up
with something cooler sooner. Let me know how this looks.
bq.
bq.
bq. This addresses bug HBase-4608.
bq. https://issues.apache.org/jira/browse/HBase-4608
bq.
bq.
bq. Diffs
bq. -----
bq.
bq. src/main/java/org/apache/hadoop/hbase/KeyValue.java e68e486
bq.
src/main/java/org/apache/hadoop/hbase/regionserver/wal/CompressedKeyValue.java
PRE-CREATION
bq.
src/main/java/org/apache/hadoop/hbase/regionserver/wal/SimpleDictionary.java
PRE-CREATION
bq.
src/main/java/org/apache/hadoop/hbase/regionserver/wal/WALDictionary.java
PRE-CREATION
bq. src/main/java/org/apache/hadoop/hbase/regionserver/wal/WALEdit.java
e1117ef
bq.
bq. Diff: https://reviews.apache.org/r/2740/diff
bq.
bq.
bq. Testing
bq. -------
bq.
bq.
bq. Thanks,
bq.
bq. Li
bq.
bq.
> HLog Compression
> ----------------
>
> Key: HBASE-4608
> URL: https://issues.apache.org/jira/browse/HBASE-4608
> Project: HBase
> Issue Type: New Feature
> Reporter: Li Pi
> Assignee: Li Pi
> Attachments: 4608v1.txt
>
>
> The current bottleneck to HBase write speed is replicating the WAL appends
> across different datanodes. We can speed up this process by compressing the
> HLog. Current plan involves using a dictionary to compress table name, region
> id, cf name, and possibly other bits of repeated data. Also, HLog format may
> be changed in other ways to produce a smaller HLog.
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