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From "Harsh J (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-9949) Testcase for catching DN UUID regeneration regression
Date Sun, 13 Mar 2016 02:09:33 GMT
Harsh J created HDFS-9949:

             Summary: Testcase for catching DN UUID regeneration regression
                 Key: HDFS-9949
                 URL: https://issues.apache.org/jira/browse/HDFS-9949
             Project: Hadoop HDFS
          Issue Type: Test
    Affects Versions: 2.6.0
            Reporter: Harsh J
            Assignee: Harsh J
            Priority: Minor

In the following scenario, in releases without HDFS-8211, the DN may regenerate its UUIDs

0. Consider a DN with two disks {{/data1/dfs/dn,/data2/dfs/dn}}
1. Stop DN
2. Unmount the second disk, {{/data2/dfs/dn}}
3. Create (in the scenario, this was an accident) /data2/dfs/dn on the root path
4. Start DN
5. DN now considers /data2/dfs/dn empty so formats it, but during the format it uses {{datanode.getDatanodeUuid()}}
which is null until register() is called.
6. As a result, after the directory loading, {{datanode.checkDatanodUuid()}} gets called with
successful condition, and it causes a new generation of UUID which is written to all disks
{{/data1/dfs/dn/current/VERSION}} and {{/data2/dfs/dn/current/VERSION}}.
7. Stop DN (in the scenario, this was when the mistake of unmounted disk was realised)
8. Mount the second disk back again {{/data2/dfs/dn}}, causing the {{VERSION}} file to be
the original one again on it (mounting masks the root path that we last generated upon).
9. DN fails to start up cause it finds mismatched UUID between the two disks

The DN should not generate a new UUID if one of the storage disks already have the older one.

HDFS-8211 unintentionally fixes this by changing the {{datanode.getDatanodeUuid()}} function
to rely on the {{DataStorage}} representation of the UUID vs. the {{DatanodeID}} object which
only gets available (non-null) _after_ the registration.

It'd still be good to add a direct test case to the above scenario that passes on trunk and
branch-2, but fails on branch-2.7 and lower, so we can catch a regression around this in future.

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