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https://issues.apache.org/jira/browse/HDFS-8782?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14989160#comment-14989160
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Vinayakumar B commented on HDFS-8782:
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HDFS-8578, is also related to upgrade in parallel.
> Upgrade to block ID-based DN storage layout delays DN registration
> ------------------------------------------------------------------
>
> Key: HDFS-8782
> URL: https://issues.apache.org/jira/browse/HDFS-8782
> Project: Hadoop HDFS
> Issue Type: Bug
> Reporter: Haohui Mai
> Priority: Critical
>
> We have seen multiple incidents at production sites that there are long
> delays for DNs to register to the NN when upgrading to post 2.6 release.
> Further investigation shows that the DN is blocked when upgrading the storage
> layout introduced in HDFS-6482. The new storage layout requires making up to
> 64k directories in the underlying file system. Unfortunately the current
> implementation calls {{mkdirs()}} sequentially and upgrades each volume in
> sequential order.
> As a result, upgrading a DN with a lot of disks or with blocks that have
> random block ID takes a long time (usually in hours), and the DN won't
> register to the NN unless it finishes upgrading all the storage directory.
> The excessive delays confuse operations and break the assumption of rolling
> upgrades.
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