ddanielr opened a new pull request, #4682:
URL: https://github.com/apache/accumulo/pull/4682

   This approach handles rollbacks from 2.1.3 to 2.1.2 gracefully by allowing 
scan server references to exist in multiple sections with different storage 
formats and allows the `ScanServerRefTabletFile` to handle the logic 
internally. 
   
   The upgrade code now removes all scan server refs on upgrade to ensure the 
old scan server section is empty.
   However, I'm not sure if that code will be run in the future since the 
`AccumuloDataVersion` has already been incremented in later versions.
   
   * Moves the scan server refs to a different prefix to avoid compatibility 
issues with possible downgrades
   * Modifies the Scan Server Reference format to put the UUID first in the row 
to reduce metadata tablet contention on writes
   * Adds a new "OldScanServerReferences" section to the metadata schema to 
ensure that future changes keep track of the previously used prefixes. 
   
   
   
   
   


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