HDFS-16355. Improve the description of dfs.block.scanner.volume.bytes.per.second (#3724)

Co-authored-by: gf13871 <gf13871@ly.com>
Signed-off-by: Akira Ajisaka <aajisaka@apache.org>
(cherry picked from commit 046a6204b4a895b98ccd41dde1c9524a6bb0ea31)

Change-Id: I2cae5d1c27a492d896da5338a92c7a86f88a8b43

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

@@ -1591,7 +1591,7 @@
   <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 class TestBlockScanner {
   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());
     }
   }