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https://issues.apache.org/jira/browse/HDFS-8486?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14633828#comment-14633828
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Daryn Sharp commented on HDFS-8486:
-----------------------------------
Public service notice:
* _Every restart of a 2.6.x or 2.7.0 DN incurs a risk of unwanted block
deletion_.
* Apply this patch if you are running a pre-2.7.1 release.
I previously attributed this as an ancient bug but it's new to 2.6. HDFS-2560
did start the scanner too early but the race caused a benign log warning. In
2.6, HDFS-6931 made an unrelated change that introduced the faulty (mass)
deletion logic.
> DN startup may cause severe data loss
> -------------------------------------
>
> Key: HDFS-8486
> URL: https://issues.apache.org/jira/browse/HDFS-8486
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: datanode
> Affects Versions: 0.23.1, 2.0.0-alpha
> Reporter: Daryn Sharp
> Assignee: Daryn Sharp
> Priority: Blocker
> Fix For: 2.7.1
>
> Attachments: HDFS-8486.patch, HDFS-8486.patch
>
>
> A race condition between block pool initialization and the directory scanner
> may cause a mass deletion of blocks in multiple storages.
> If block pool initialization finds a block on disk that is already in the
> replica map, it deletes one of the blocks based on size, GS, etc.
> Unfortunately it _always_ deletes one of the blocks even if identical, thus
> the replica map _must_ be empty when the pool is initialized.
> The directory scanner starts at a random time within its periodic interval
> (default 6h). If the scanner starts very early it races to populate the
> replica map, causing the block pool init to erroneously delete blocks.
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