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https://issues.apache.org/jira/browse/HDFS-6137?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13948153#comment-13948153
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Tsz Wo Nicholas Sze commented on HDFS-6137:
-------------------------------------------

I think it probably first called doRollback(..) and then readProperties(..) as 
shown below.
{code}
// BlockPoolSliceStorage.doTransition(..)
    if (startOpt == StartupOption.ROLLBACK) {
      doRollback(sd, nsInfo); // rollback if applicable
    } else {
      // Restore all the files in the trash. The restored files are retained
      // during rolling upgrade rollback. They are deleted during rolling
      // upgrade downgrade.
      int restored = restoreBlockFilesFromTrash(getTrashRootDir(sd));
      LOG.info("Restored " + restored + " block files from trash.");
    }
    readProperties(sd);
{code}


> Datanode cannot rollback because LayoutVersion incorrect
> --------------------------------------------------------
>
>                 Key: HDFS-6137
>                 URL: https://issues.apache.org/jira/browse/HDFS-6137
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: datanode
>    Affects Versions: 2.4.0
>            Reporter: Fengdong Yu
>
> upgrade from hadoop-2.0.5-alpha(QJM HA enabled) to the lastest trunk(HA 
> disabled), which is successful. then stop the cluster, and rollback,  then it 
> throw exception:
> {code}
> 2014-03-21 18:33:19,384 FATAL 
> org.apache.hadoop.hdfs.server.datanode.DataNode: Initialization failed for 
> block pool Block pool BP-1123524590-10.204.8.135-1395397158134 (storage id 
> DS-1123524590-10.204.8.135-50010-1395397185148) service to 
> 10-204-8-135/10.204.8.135:9000
> org.apache.hadoop.hdfs.server.common.IncorrectVersionException: Unexpected 
> version of storage directory 
> /data/hdfs/data/current/BP-1123524590-10.204.8.135-1395397158134. Reported: 
> -55. Expecting = -40.
>         at 
> org.apache.hadoop.hdfs.server.common.Storage.setLayoutVersion(Storage.java:1083)
>         at 
> org.apache.hadoop.hdfs.server.datanode.BlockPoolSliceStorage.setFieldsFromProperties(BlockPoolSliceStorage.java:217)
>         at 
> org.apache.hadoop.hdfs.server.common.Storage.readProperties(Storage.java:922)
>         at 
> org.apache.hadoop.hdfs.server.datanode.BlockPoolSliceStorage.doTransition(BlockPoolSliceStorage.java:244)
>         at 
> org.apache.hadoop.hdfs.server.datanode.BlockPoolSliceStorage.recoverTransitionRead(BlockPoolSliceStorage.java:145)
>         at 
> org.apache.hadoop.hdfs.server.datanode.DataStorage.recoverTransitionRead(DataStorage.java:234)
>         at 
> org.apache.hadoop.hdfs.server.datanode.DataNode.initStorage(DataNode.java:913)
>         at 
> org.apache.hadoop.hdfs.server.datanode.DataNode.initBlockPool(DataNode.java:884)
>         at 
> org.apache.hadoop.hdfs.server.datanode.BPOfferService.verifyAndSetNamespaceInfo(BPOfferService.java:280)
>         at 
> org.apache.hadoop.hdfs.server.datanode.BPServiceActor.connectToNNAndHandshake(BPServiceActor.java:222)
>         at 
> org.apache.hadoop.hdfs.server.datanode.BPServiceActor.run(BPServiceActor.java:664)
>         at java.lang.Thread.run(Thread.java:744)
> {code}
>   
> I looked at the datanode dir,  $datanode.dir/VERSION is always new, when we 
> upgrade, this file was overwrited, so it MUST fail during rollback.



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