HDFS Disk Failures

Intresting discussion on hortonworks community on disk failures

how-to-alert-for-hdfs-disk-failures.html

> When a single drive fails on a worker node in HDFS, can this adversely affect performance of jobs running on this node?

The answer to this question is it depends. If this node is running a job that is accessing blocks on the failed volume, then yes. it is also possible that the job would be treated as failed if the dfs.datanode.failed.volumes.tolerated is not greater than 0. If it is not a value greater than zero, then HDFS treats a loss of a volume as catastrophic and marks the datanode as failed. If this is set to a value greater than zero, then node will work well until we lose more volumes.

> If this could cause a performance impact, how can our customers monitor for these drive failures in order to take corrective action?

Now this is a hard question to answer without further details. I am tempted to answer that the performance benefit you are going to get by monitoring and relying on a human being to take a corrective action is very doubtful. YARN / MR or whatever execution engine you are using is probably going to be much more efficient at re-scheduling your jobs.

>Or does the DataNode process quickly mark the drive and its HDFS blocks as “unusable”.

Datanode does mark the volume as failed , and namenode will learn that all the blocks on that failed volume are not available on that datanode any more. This happens via something called “block reports”. Once namenode learns that data node has lost the replica of a block then namenode will initiate appropriate replication. Since namenode knows about the loss of blocks, further jobs that need access to those block would most probably not be scheduled on that node. This again depends on the scheduler and its policies.

 

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Author: rajukv

Hadoop(BigData) Architect and Hadoop Security Architect can design and build hadoop system to meet various data science projects.

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