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 -
- <varname>hbase.hlog.split.skip.errors</varname> +
How edits are recovered from a crashed RegionServer + When a RegionServer crashes, it will lose its ephemeral lease in + ZooKeeper...TODO +
+
+ <varname>hbase.hlog.split.skip.errors</varname> When set to true, the default, any error encountered splitting will be logged, the problematic WAL will be @@ -939,8 +939,9 @@
+
- +