Kitti Nanasi created HDFS-13719:
-----------------------------------
Summary: Docs around dfs.image.transfer.timeout are misleading
Key: HDFS-13719
URL: https://issues.apache.org/jira/browse/HDFS-13719
Project: Hadoop HDFS
Issue Type: Improvement
Affects Versions: 3.1.0
Reporter: Kitti Nanasi
Assignee: Kitti Nanasi
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:
{code:java}
+ 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);
+ }
+
{code}
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:
{code:xml}
<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>
{code}
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.
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