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https://issues.apache.org/jira/browse/HDFS-5590?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13836889#comment-13836889
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Suresh Srinivas commented on HDFS-5590:
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How about always persisting to editlog when a block is allocated. This what is
done in HA mode. While doing the same in non-HA mode can be construed as
affecting performance, clearly here there are correctness and data loss issues.
My recommendation would be always persist blocks and hence change the default
value for block persistence from false to true. We also need to document this
and ensure people who set this to false, do it with the understanding of the
risks.
> Sequential block ID may cause data loss when persistBlocks is set to false
> --------------------------------------------------------------------------
>
> Key: HDFS-5590
> URL: https://issues.apache.org/jira/browse/HDFS-5590
> Project: Hadoop HDFS
> Issue Type: Bug
> Affects Versions: 2.2.0
> Reporter: Jing Zhao
> Assignee: Jing Zhao
>
> In a cluster with non-HA setup and dfs.persist.blocks set to false, the
> current sequential block ID mechanism may cause data loss in the following
> case:
> # client creates file1 and requests a block from NN and get blk_id1_gs1
> # client writes blk_id1_gs1 to DN
> # NN is restarted and because persistBlocks is false, blk_id1_gs1 may not be
> persisted in disk
> # another client creates file2 and NN will allocate a new block using the
> same block id blk_id1_gs1 since block ID and generation stamp are both
> increased sequentially.
> Now we may have two versions (file1 and file2) of the blk_id1_gs1 (same id,
> same gs) in the system. It will case data loss.
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