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https://issues.apache.org/jira/browse/HBASE-6435?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13423280#comment-13423280
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nkeywal commented on HBASE-6435:
--------------------------------
v2. May need some clean up on logs + a check to unactivate it for hadoop 2 for
example.
> 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, 6435.v2.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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