HADOOP-17596. ABFS: Change default Readahead Queue Depth from num(processors) to const (#3106)

* HADOOP-17596. ABFS: Change default Readahead Queue Depth from num(processors) to const (#2795)
. Contributed by Sumangala Patki.

(cherry picked from commit 76d92eb2a22c71b5fcde88a9b4d2faec81a1cb9f)

hadoop-tools/hadoop-azure/src/main/java/org/apache/hadoop/fs/azurebfs/constants/FileSystemConfigurations.java

@@ -89,7 +89,7 @@ public final class FileSystemConfigurations {
   public static final int MIN_LEASE_DURATION = 15;
   public static final int MAX_LEASE_DURATION = 60;
 
-  public static final int DEFAULT_READ_AHEAD_QUEUE_DEPTH = -1;
+  public static final int DEFAULT_READ_AHEAD_QUEUE_DEPTH = 2;
 
   public static final boolean DEFAULT_ENABLE_FLUSH = true;
   public static final boolean DEFAULT_DISABLE_OUTPUTSTREAM_FLUSH = true;

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

@@ -803,7 +803,7 @@ pattern is detected.
 `fs.azure.readaheadqueue.depth`: Sets the readahead queue depth in
 AbfsInputStream. In case the set value is negative the read ahead queue depth
 will be set as Runtime.getRuntime().availableProcessors(). By default the value
-will be -1. To disable readaheads, set this value to 0. If your workload is
+will be 2. To disable readaheads, set this value to 0. If your workload is
  doing only random reads (non-sequential) or you are seeing throttling, you
   may try setting this value to 0.
 

hadoop-tools/hadoop-azure/src/test/java/org/apache/hadoop/fs/azurebfs/services/TestAbfsInputStream.java

@@ -28,6 +28,7 @@ import org.assertj.core.api.Assertions;
 
 import org.apache.hadoop.conf.Configuration;
 import org.apache.hadoop.fs.FileSystem;
+import org.apache.hadoop.fs.FSDataInputStream;
 import org.apache.hadoop.fs.FSDataOutputStream;
 import org.apache.hadoop.fs.FileStatus;
 import org.apache.hadoop.fs.Path;
@@ -48,6 +49,7 @@ import static org.mockito.Mockito.when;
 
 import static org.apache.hadoop.test.LambdaTestUtils.intercept;
 import static org.apache.hadoop.fs.azurebfs.constants.AbfsHttpConstants.FORWARD_SLASH;
+import static org.apache.hadoop.fs.azurebfs.constants.ConfigurationKeys.FS_AZURE_READ_AHEAD_QUEUE_DEPTH;
 
 /**
  * Unit test AbfsInputStream.
@@ -569,6 +571,20 @@ public class TestAbfsInputStream extends
     testReadAheads(inputStream, FORTY_EIGHT_KB, SIXTEEN_KB);
   }
 
+  @Test
+  public void testDefaultReadaheadQueueDepth() throws Exception {
+    Configuration config = getRawConfiguration();
+    config.unset(FS_AZURE_READ_AHEAD_QUEUE_DEPTH);
+    AzureBlobFileSystem fs = getFileSystem(config);
+    Path testFile = new Path("/testFile");
+    fs.create(testFile);
+    FSDataInputStream in = fs.open(testFile);
+    Assertions.assertThat(
+        ((AbfsInputStream) in.getWrappedStream()).getReadAheadQueueDepth())
+        .describedAs("readahead queue depth should be set to default value 2")
+        .isEqualTo(2);
+  }
+
 
   private void testReadAheads(AbfsInputStream inputStream,
       int readRequestSize,