In the following scenario, in releases without
HDFS-8211, the DN may regenerate its UUIDs unintentionally.
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, with an error similar to:
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.