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From "Harsh J (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-4246) The exclude node list should be more forgiving, for each output stream
Date Fri, 30 Nov 2012 11:50:00 GMT
Harsh J created HDFS-4246:

             Summary: The exclude node list should be more forgiving, for each output stream
                 Key: HDFS-4246
                 URL: https://issues.apache.org/jira/browse/HDFS-4246
             Project: Hadoop HDFS
          Issue Type: Improvement
          Components: hdfs-client
            Reporter: Harsh J
            Priority: Minor

Originally observed by Inder on the mailing lists:


i was wondering if there is any mechanism/logic to move a node back from the excludedNodeList
to live nodes to be tried for new block creation.

In the current DFSOutputStream code i do not see this. The use-case is if the write timeout
is being reduced and certain nodes get aggressively added to the excludedNodeList and the
client caches DFSOutputStream then the excludedNodes never get tried again in the lifetime
of the application caching DFSOutputStream

What this leads to, is a special scenario, that may impact smaller clusters more than larger

1. File is opened for continuos hflush/sync-based writes, such as a HBase WAL for example.
This file is gonna be kept open for a very very long time, by design.
2. Over time, nodes are excluded for various errors, such as DN crashes, network failures,
3. Eventually, exclude list == live nodes list or close, and the write suffers. At time of
equality, the write also fails with an error of not being able to get a block allocation.

We should perhaps make the excludeNodes list a timed-cache collection, so that even if it
begins filling up, the older excludes are pruned away, giving those nodes a try again for

One place we have to be careful about, though, is rack-failures. Those sometimes never come
back fast enough, and can be problematic to retry code with such an eventually-forgiving list.
Perhaps we can retain forgiven nodes and if they are entered again, we may double or triple
the forgiveness value (in time units), to counter this? Its just one idea.

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