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From st...@apache.org
Subject [33/50] [abbrv] hbase git commit: HBASE-18504 Add documentation for WAL compression
Date Thu, 17 Aug 2017 19:15:31 GMT
HBASE-18504 Add documentation for WAL compression

Signed-off-by: Michael Stack <stack@apache.org>


Project: http://git-wip-us.apache.org/repos/asf/hbase/repo
Commit: http://git-wip-us.apache.org/repos/asf/hbase/commit/63e313b5
Tree: http://git-wip-us.apache.org/repos/asf/hbase/tree/63e313b5
Diff: http://git-wip-us.apache.org/repos/asf/hbase/diff/63e313b5

Branch: refs/heads/HBASE-14070.HLC
Commit: 63e313b5c0d7c56d9cf9602e3c204214331189d5
Parents: 70c4f78
Author: Peter Somogyi <psomogyi@cloudera.com>
Authored: Wed Aug 2 17:00:52 2017 +0200
Committer: Michael Stack <stack@apache.org>
Committed: Tue Aug 15 12:54:08 2017 -0700

----------------------------------------------------------------------
 src/main/asciidoc/_chapters/architecture.adoc | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)
----------------------------------------------------------------------


http://git-wip-us.apache.org/repos/asf/hbase/blob/63e313b5/src/main/asciidoc/_chapters/architecture.adoc
----------------------------------------------------------------------
diff --git a/src/main/asciidoc/_chapters/architecture.adoc b/src/main/asciidoc/_chapters/architecture.adoc
index ebb0677..2ded813 100644
--- a/src/main/asciidoc/_chapters/architecture.adoc
+++ b/src/main/asciidoc/_chapters/architecture.adoc
@@ -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
 


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