[HDFS-13719] Docs around dfs.image.transfer.timeout are misleading

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 3.1.0
• Fix Version/s: 2.10.0, 3.2.0, 3.1.1, 2.9.2, 3.0.4
• Component/s: documentation
• Target Version/s: 3.2.0
• Assignee: Kitti Nanasi
• Reporter: Kitti Nanasi
• Created: 04/Jul/18 12:48
• Updated: 30/Oct/18 01:21
• Resolved: 09/Jul/18 13:18

Description

The Jira https://issues.apache.org/jira/browse/HDFS-1490 added the parameter dfs.image.transfer.timeout to HDFS. From the patch (and checking the current code), we can see this parameter governs a socket timeout on the a java.net.HttpURLConnection object:

+    if (timeout <= 0) {
+      // Set the ping interval as timeout
+      Configuration conf = new HdfsConfiguration();
+      timeout = conf.getInt(DFSConfigKeys.DFS_IMAGE_TRANSFER_TIMEOUT_KEY,
+          DFSConfigKeys.DFS_IMAGE_TRANSFER_TIMEOUT_DEFAULT);
+    }
+
+    if (timeout > 0) {
+      connection.setConnectTimeout(timeout);
+      connection.setReadTimeout(timeout);
+    }
+

In the above 'connection' is a java.net.HttpURLConnection.

There is a general disbelief in the community that dfs.image.transfer.timeout is the time the entire image must transfer within, however that does not appear to be the case. The timeout is actually the max time the client will block on the socket before giving up if it cannot get data to read. I guess the idea here is to protect the client from hanging forever if the server hangs.

The docs in hdfs-site.xml are partly what causes this confusion, as they are very misleading:

<property>
  <name>dfs.image.transfer.timeout</name>
  <value>60000</value>
  <description>
        Socket timeout for image transfer in milliseconds. This timeout and the related
        dfs.image.transfer.bandwidthPerSec parameter should be configured such
        that normal image transfer can complete successfully.
        This timeout prevents client hangs when the sender fails during
        image transfer. This is socket timeout during image tranfer.
  </description>
</property>

The start and end of the statement is accurate, but the part "This timeout and the related dfs.image.transfer.bandwidthPerSec parameter should be configured such that normal image transfer can complete successfully." is misleading. There is almost never a reason to change the above in conjunction with the bandwidth setting.