HADOOP-17770 WASB : Support disabling buffered reads in positional reads

Author: anoopsjohn

hadoop-tools/hadoop-azure/src/site/markdown/index.md

@@ -545,6 +545,17 @@ The maximum number of entries that that cache can hold can be customized using t
     </property>
 ```
 
+### Performance optimization configurations
+
+`fs.azure.block.blob.buffered.pread.disable`: By default the positional read API will do a
+seek and read on input stream. This read will fill the buffer cache in
+BlockBlobInputStream. If this configuration is true it will skip usage of buffer and do a
+lock free call for reading from blob. This optimization is very much helpful for HBase kind
+of short random read over a shared InputStream instance.
+Note: This is not a config which can be set at cluster level. It can be used as
+an option on FutureDataInputStreamBuilder.
+See FileSystem#openFile(Path path)
+
 ## Further Reading
 
 * [Testing the Azure WASB client](testing_azure.html).