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https://issues.apache.org/jira/browse/HDFS-4246?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13509089#comment-13509089
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Colin Patrick McCabe commented on HDFS-4246:
--------------------------------------------
This change makes a lot of sense to me. It should help long-running DFSClients
like HBase.
{code}
+ // Forgive nodes in under 2s for this test case.
+ conf.setLong(
+ DFSConfigKeys.DFS_CLIENT_WRITE_EXCLUDE_NODES_CACHE_EXPIRY_INTERVAL,
+ 2000);
...
+ try {
+ Thread.sleep(2000);
+ } catch (InterruptedException e) {
+ fail("Sleep in test (to expire/forgive excluded datanodes)" +
+ "was interrupted for some reason.");
+ }
{code}
In this test, should we set the cache expiry to something like 1s, if we're
sleeping exactly 2 seconds? Otherwise we're cutting it a little bit close,
perhaps? Of course, restarting the DNs will take some time, but it might be
better to err on the side of safety.
Also, you are assuming that the default checksum chunk size is 512 bytes.
While this is currently true, it would be better to manually specify this in
the test, so that if this changes in the future, the test behavior doesn't
change.
> 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
> Affects Versions: 2.0.0-alpha
> Reporter: Harsh J
> Assignee: Harsh J
> Priority: Minor
> Attachments: HDFS-4246.patch
>
>
> Originally observed by Inder on the mailing lists:
> {quote}
> Folks,
> 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
> {quote}
> What this leads to, is a special scenario, that may impact smaller clusters
> more than larger ones:
> 1. File is opened for continuous 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, etc.
> 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 later.
> 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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