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https://issues.apache.org/jira/browse/HDFS-1260?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Todd Lipcon updated HDFS-1260:
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Attachment: simultaneous-recoveries.txt
After months of running this test I ran into this failure attached above. One
of the DNs somehow ends up with multiple meta files for the same block, but at
different generation stamps.
I think the issue is in the implementation of DataNode.updateBlock(). The block
passed in doesn't have a wildcard generation stamp, but we don't care - we go
and find the block on disk without matching generation stamps. I think this is
OK based on the validation logic - we still only move blocks forward in
GS-time, and don't revert length. However, when we then call updateBlockMap()
it doesn't use a wildcard generation stamp, so the block can get left in the
block map with the old generation stamp. This inconsistency I think cascades
into the sort of failure seen in the attached log.
I *think* the solution is:
- Change updateBlock to call updateBlockMap with a wildcard generation stamp
key
- Change the interruption code to use a wildcard GS block when interrupting
concurrent writers
I will make these changes and see if the rest of the unit tests still pass,
then see if I can come up with a regression test.
> 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 stamp
> - 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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