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From "dhruba borthakur (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HDFS-1260) 0.20: Block lost when multiple DNs trying to recover it to different genstamps
Date Wed, 25 Aug 2010 21:56:18 GMT

    [ https://issues.apache.org/jira/browse/HDFS-1260?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12902652#action_12902652

dhruba borthakur commented on HDFS-1260:

> However, when we then call updateBlockMap() it doesn't use a wildcard generation stamp,

yes, that seems to be a bug to me. Great catch!

> 0.20: Block lost when multiple DNs trying to recover it to different genstamps
> ------------------------------------------------------------------------------
>                 Key: HDFS-1260
>                 URL: https://issues.apache.org/jira/browse/HDFS-1260
>             Project: Hadoop HDFS
>          Issue Type: Bug
>    Affects Versions: 0.20-append
>            Reporter: Todd Lipcon
>            Assignee: Todd Lipcon
>            Priority: Critical
>             Fix For: 0.20-append
>         Attachments: hdfs-1260.txt, hdfs-1260.txt, simultaneous-recoveries.txt
> Saw this issue on a cluster where some ops people were doing network changes without
shutting down DNs first. So, recovery ended up getting started at multiple different DNs at
the same time, and some race condition occurred that caused a block to get permanently stuck
in recovery mode. What seems to have happened is the following:
> - FSDataset.tryUpdateBlock called with old genstamp 7091, new genstamp 7094, while the
block in the volumeMap (and on filesystem) was genstamp 7093
> - we find the block file and meta file based on block ID only, without comparing gen
> - we rename the meta file to the new genstamp _7094
> - in updateBlockMap, we do comparison in the volumeMap by oldblock *without* wildcard
GS, so it does *not* update volumeMap
> - validateBlockMetaData now fails with "blk_7739687463244048122_7094 does not exist in
blocks map"
> After this point, all future recovery attempts to that node fail in getBlockMetaDataInfo,
since it finds the _7094 gen stamp in getStoredBlock (since the meta file got renamed above)
and then fails since _7094 isn't in volumeMap in validateBlockMetadata
> Making a unit test for this is probably going to be difficult, but doable.

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