HDFS-16355. Improve block scanner desc

hadoop-hdfs-project/hadoop-hdfs/src/main/resources/hdfs-default.xml

@@ -1640,7 +1640,7 @@
   <name>dfs.block.scanner.volume.bytes.per.second</name>
   <value>1048576</value>
   <description>
-        If this is 0, the DataNode's block scanner will be disabled.  If this
+        If this is configured less than or equal to zero, the DataNode's block scanner will be disabled.  If this
         is positive, this is the number of bytes per second that the DataNode's
         block scanner will try to scan from each volume.
   </description>

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/server/datanode/TestBlockScanner.java

@@ -282,11 +282,17 @@ public void testVolumeIteratorWithCaching() throws Exception {
   public void testDisableVolumeScanner() throws Exception {
     Configuration conf = new Configuration();
     disableBlockScanner(conf);
-    TestContext ctx = new TestContext(conf, 1);
-    try {
-      Assert.assertFalse(ctx.datanode.getBlockScanner().isEnabled());
-    } finally {
-      ctx.close();
+    try(TestContext ctx = new TestContext(conf, 1)) {
+      assertFalse(ctx.datanode.getBlockScanner().isEnabled());
+    }
+  }
+
+  @Test(timeout=60000)
+  public void testDisableVolumeScanner2() throws Exception {
+    Configuration conf = new Configuration();
+    conf.setLong(DFS_BLOCK_SCANNER_VOLUME_BYTES_PER_SECOND, -1L);
+    try(TestContext ctx = new TestContext(conf, 1)) {
+      assertFalse(ctx.datanode.getBlockScanner().isEnabled());
     }
   }