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HBASE-18504 Add documentation for WAL compression 

Signed-off-by: Michael Stack <stack@apache.org>
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Peter Somogyi 2017-08-02 17:00:52 +02:00 committed by Michael Stack
parent 70c4f78ce0
commit 63e313b5c0
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src/main/asciidoc/_chapters/architecture.adoc
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@ -1216,6 +1216,22 @@ This will be the default for HBase 0.99 (link:https://issues.apache.org/jira/bro
You must also enable HFile version 3 (which is the default HFile format starting in HBase 0.99.
See link:https://issues.apache.org/jira/browse/HBASE-10855[HBASE-10855]). Distributed log replay is unsafe for rolling upgrades.
[[wal.compression]]
==== WAL Compression ====
The content of the WAL can be compressed using LRU Dictionary compression.
This can be used to speed up WAL replication to different datanodes.
The dictionary can store up to 2^15^ elements; eviction starts after this number is exceeded.
To enable WAL compression, set the `hbase.regionserver.wal.enablecompression` property to `true`.
The default value for this property is `false`.
By default, WAL tag compression is turned on when WAL compression is enabled.
You can turn off WAL tag compression by setting the `hbase.regionserver.wal.tags.enablecompression` property to 'false'.
A possible downside to WAL compression is that we lose more data from the last block in the WAL if it ill-terminated
mid-write. If entries in this last block were added with new dictionary entries but we failed persist the amended
dictionary because of an abrupt termination, a read of this last block may not be able to resolve last-written entries.
[[wal.disable]]
==== Disabling the WAL