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From "Wei-Chiu Chuang (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-9923) Datanode disk failure handling should be improved (consistently?)
Date Tue, 08 Mar 2016 20:15:41 GMT
Wei-Chiu Chuang created HDFS-9923:

             Summary: Datanode disk failure handling should be improved (consistently?)
                 Key: HDFS-9923
                 URL: https://issues.apache.org/jira/browse/HDFS-9923
             Project: Hadoop HDFS
          Issue Type: Improvement
          Components: datanode
            Reporter: Wei-Chiu Chuang

Disk failures are hard to handle. This JIRA is created to discuss/improve disk failure handling
in a better/consistent manner.

For one thing, disks can fail in multiple different ways: the hardware might be failing, disk
space is full, checksum error ... For others, hardware abstracts out the details, so it's
hard for software to handle them.

There are currently three disk check mechanisms in HDFS, as far as I know: {{BlockScanner}},
{{BlockPoolSlice#checkDirs}} and {{DU}}. Disk errors are handled differently. 

This JIRA is more focused on {{DU}} error handling. {{DU}} may emit errors like this:
2016-02-18 02:23:36,224 INFO org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.FsDatasetImpl:
Caught exception while scanning /data/8/dfs/dn/current.
Will throw later.
ExitCodeException exitCode=1: du: cannot access `/data/8/dfs/dn/current/BP-1018136951-
1088686909': Input/output error
du: cannot access `/data/8/dfs/dn/current/BP-1018136951-':
ut/output error
I found {{DU}} errors are not handled consistently while working on HDFS-9908 (Datanode should
tolerate disk scan failure during NN handshake), and it all depends on who catches the exception.

For example, 
* if DU returns error during NN handshake, DN will not be able to join the cluster at all(HDFS-9908);
* however, if the same exception is caught in {{BlockPoolSlice#saveDfsUsed}}, data node will
only log a warning and do nothing (HDFS-5498).
* in some cases, the exception handler invokes {{BlockPoolSlice#checkDirs}}, but since it
only checks three directories, it is very unlikely to find the files that have the error.

So my ask is: should the error be handled in a consistent manner? Should data node report
to the name nodes about the disk failures (this is the BlockScanner approach), and should
data node takes this volume offline automatically if DU returns an error? (this is the {{checkDirs}}

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