Index: src/docbkx/book.xml
===================================================================
--- src/docbkx/book.xml (revision 1092508)
+++ src/docbkx/book.xml (working copy)
@@ -873,40 +873,40 @@
-
-
-
- The WAL
-
- HBase's Write-Ahead
- Log
-
- Each RegionServer adds updates to its Write-ahead Log (WAL)
- first, and then to memory.
-
-
- What is the purpose of the HBase WAL
-
-
- See the Wikipedia
- Write-Ahead
- Log article.
-
-
-
+
+
+ Write Ahead Log (WAL)
-
- WAL splitting
+
+ Purpose
- How edits are recovered from a crashed RegionServer
+ Each RegionServer adds updates (Puts, Deletes) to its write-ahead log (WAL)
+ first, and then to the MemStore for the affected Store.
+ This ensures that HBase has durable writes. Without WAL, there is the possibility of data loss in the case of a RegionServer failure
+ before each MemStore is flushed and new StoreFiles are written. HLog
+ is the HBase WAL implementation, and there is one HLog instance per RegionServer.
+ The WAL is in HDFS in /hbase/.logs/ with subdirectories per region.
+
+ For more general information about the concept of write ahead logs, see the Wikipedia
+ Write-Ahead Log article.
+
+
+
+ WAL Flushing
+ TODO (describe).
+
+
- When a RegionServer crashes, it will lose its ephemeral lease in
- ZooKeeper...TODO
+
+ WAL Splitting
-
- hbase.hlog.split.skip.errors
+ How edits are recovered from a crashed RegionServer
+ When a RegionServer crashes, it will lose its ephemeral lease in
+ ZooKeeper...TODO
+
+
+ hbase.hlog.split.skip.errorsWhen set to true, the default, any error
encountered splitting will be logged, the problematic WAL will be
@@ -939,8 +939,9 @@
+
-
+