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https://issues.apache.org/jira/browse/HDFS-3703?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13452439#comment-13452439
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nkeywal commented on HDFS-3703:
-------------------------------
@Jing
Sure. The scenario is:
- open a file for writing, write some data, keep the file open
- kill one of the datanodes storing one of the replica of the last blocks (ex:
unplug the computer from the network).
- open the file for reading on another machine
During the "open", he DFSClient will try to get the file length, this requires
to connect (via ipc.Client) to one of the datanodes owning the last block. If
you're going to the dead datanodes, the ipc call will fail. It will retry ten
times (by default), and will log the error (in my case, it was a noRouteToHost,
but it could be socketTimeout or alike).
In HBase, we have this when we try to read the Write-Ahead-Log after a
regionserver crash.
> Decrease the datanode failure detection time
> --------------------------------------------
>
> Key: HDFS-3703
> URL: https://issues.apache.org/jira/browse/HDFS-3703
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: data-node, name-node
> Affects Versions: 1.0.3, 2.0.0-alpha
> Reporter: nkeywal
> Assignee: Suresh Srinivas
> Attachments: HDFS-3703-branch2.patch, HDFS-3703.patch,
> HDFS-3703-trunk-read-only.patch, HDFS-3703-trunk-read-only.patch,
> HDFS-3703-trunk-with-write.patch
>
>
> By default, if a box dies, the datanode will be marked as dead by the
> namenode after 10:30 minutes. In the meantime, this datanode will still be
> proposed by the nanenode to write blocks or to read replicas. It happens as
> well if the datanode crashes: there is no shutdown hooks to tell the nanemode
> we're not there anymore.
> It especially an issue with HBase. HBase regionserver timeout for production
> is often 30s. So with these configs, when a box dies HBase starts to recover
> after 30s and, while 10 minutes, the namenode will consider the blocks on the
> same box as available. Beyond the write errors, this will trigger a lot of
> missed reads:
> - during the recovery, HBase needs to read the blocks used on the dead box
> (the ones in the 'HBase Write-Ahead-Log')
> - after the recovery, reading these data blocks (the 'HBase region') will
> fail 33% of the time with the default number of replica, slowering the data
> access, especially when the errors are socket timeout (i.e. around 60s most
> of the time).
> Globally, it would be ideal if HDFS settings could be under HBase settings.
> As a side note, HBase relies on ZooKeeper to detect regionservers issues.
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