Ahmed Hussein created HDFS-15644:
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Summary: Failed volumes can cause DNs to stop block reporting
Key: HDFS-15644
URL: https://issues.apache.org/jira/browse/HDFS-15644
Project: Hadoop HDFS
Issue Type: Bug
Components: block placement, datanode
Reporter: Ahmed Hussein
Assignee: Ahmed Hussein
[~daryn] found a corner case where remove failed volumes can cause a NPE in
[FsDataSetImpl.getBlockReports()|https://github.com/apache/hadoop/blob/trunk/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/FsDatasetImpl.java#L1939].
+Scenario:+
* Inside {{Datanode#HandleVolumeFailures()}}, removing a failed volume is a
2-step process.
** First it's removed from from the volumes list
** Later in time are the replicas scrubbed from the volume map
* A concurrent thread generating blockReports may access the replicaMap
accessing a non existing VolumeID.
He made a fix for that and we have been using it on our clusters since
Hadoop-2.7.
By analyzing the code, the bug is still applicable to Trunk.
* The path Datanode#removeVolumes() is safe because the two step process in
{{FsDataImpl.removeVolumes()}}
[FsDatasetImpl.java#L577|https://github.com/apache/hadoop/blob/trunk/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/FsDatasetImpl.java#L577]
is protected by {{datasetWriteLock}} .
* The path Datanode#handleVolumeFailures() is not safe because the failed
volume is removed from the list without acquiring
{{datasetWriteLock}}.[FsVolumList#239|https://github.com/apache/hadoop/blob/trunk/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/FsVolumeList.java#L239]
The race condition can cause the caller of getBlockReports() to throw NPE if
the RUR is referring to a volume that has already been removed
[FsDatasetImpl.java#L1976|https://github.com/apache/hadoop/blob/trunk/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/FsDatasetImpl.java#L1976].
{code:java}
case RUR:
ReplicaInfo orig = b.getOriginalReplica();
builders.get(volStorageID).add(orig);
break;
{code}
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