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https://issues.apache.org/jira/browse/HBASE-6435?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13419414#comment-13419414
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nkeywal commented on HBASE-6435:
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The patch is not finished. Actually, it contains for code for the hdfs hook and
the related test, but not the code for defining the location order from the
file name. But as it is different from what we initially discussed, I post it
here in case someone sees something I missed.
It does not mean it should not be fixed in hdfs as well, just that this is
likely to be much simpler than patching the 1.0 branch...
> Reading WAL files after a recovery leads to time lost in HDFS timeouts when
> using dead datanodes
> ------------------------------------------------------------------------------------------------
>
> Key: HBASE-6435
> URL: https://issues.apache.org/jira/browse/HBASE-6435
> Project: HBase
> Issue Type: Improvement
> Components: master, regionserver
> Affects Versions: 0.96.0
> Reporter: nkeywal
> Assignee: nkeywal
> Attachments: 6435.unfinished.patch
>
>
> HBase writes a Write-Ahead-Log to revover from hardware failure.
> This log is written with 'append' on hdfs.
> Through ZooKeeper, HBase gets informed usually in 30s that it should start
> the recovery process.
> This means reading the Write-Ahead-Log to replay the edits on the other
> servers.
> In standards deployments, HBase process (regionserver) are deployed on the
> same box as the datanodes.
> It means that when the box stops, we've actually lost one of the edits, as we
> lost both the regionserver and the datanode.
> As HDFS marks a node as dead after ~10 minutes, it appears as available when
> we try to read the blocks to recover. As such, we are delaying the recovery
> process by 60 seconds as the read will usually fail with a socket timeout. If
> the file is still opened for writing, it adds an extra 20s + a risk of losing
> edits if we connect with ipc to the dead DN.
> Possible solutions are:
> - shorter dead datanodes detection by the NN. Requires a NN code change.
> - better dead datanodes management in DFSClient. Requires a DFS code change.
> - NN customisation to write the WAL files on another DN instead of the local
> one.
> - reordering the blocks returned by the NN on the client side to put the
> blocks on the same DN as the dead RS at the end of the priority queue.
> Requires a DFS code change or a kind of workaround.
> The solution retained is the last one. Compared to what was discussed on the
> mailing list, the proposed patch will not modify HDFS source code but adds a
> proxy. This for two reasons:
> - Some HDFS functions managing block orders are static
> (MD5MD5CRC32FileChecksum). Implementing the hook in the DFSClient would
> require to implement partially the fix, change the DFS interface to make this
> function non static, or put the hook static. None of these solution is very
> clean.
> - Adding a proxy allows to put all the code in HBase, simplifying dependency
> management.
> Nevertheless, it would be better to have this in HDFS. But this solution
> allows to target the last version only, and this could allow minimal
> interface changes such as non static methods.
> Moreover, writing the blocks to the non local DN would be an even better
> solution long term.
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