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https://issues.apache.org/jira/browse/HDFS-9752?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15133429#comment-15133429
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Arpit Agarwal commented on HDFS-9752:
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A couple of alternatives for making the unit test faster:
# The test can just verify that pipelineRecoveryCount is not incremented after
DN restart and pipeline recovery. If it is not incremented after one iteration
the {{++pipelineRecoveryCount > 5}} check will never be triggered.
# Add a test hook to reduce the pipelineRecoveryCount threshold from 5 to 1.
> Permanent write failures may happen to slow writers during datanode rolling
> upgrades
> ------------------------------------------------------------------------------------
>
> Key: HDFS-9752
> URL: https://issues.apache.org/jira/browse/HDFS-9752
> Project: Hadoop HDFS
> Issue Type: Bug
> Reporter: Kihwal Lee
> Assignee: Walter Su
> Priority: Critical
> Attachments: HDFS-9752.01.patch
>
>
> When datanodes are being upgraded, an out-of-band ack is sent upstream and
> the client does a pipeline recovery. The client may hit this multiple times
> as more nodes get upgraded. This normally does not cause any issue, but if
> the client is holding the stream open without writing any data during this
> time, a permanent write failure can occur.
> This is because there is a limit of 5 recovery trials for the same packet,
> which is tracked by "last acked sequence number". Since the empty heartbeat
> packets for an idle output stream does not increment the sequence number, the
> write will fail after it seeing 5 pipeline breakages by datanode upgrades.
> This check/limit was added to avoid spinning until running out of nodes in
> the cluster due to a corruption or any other irrecoverable conditions. The
> datanode upgrade-restart should be excluded from the count.
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