[jira] [Created] (HBASE-26726) Allow disable of region warmup before graceful move

["Andrew Kyle Purtell (Jira)" <ji...@apache.org>]

Andrew Kyle Purtell created HBASE-26726:
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             Summary: Allow disable of region warmup before graceful move
                 Key: HBASE-26726
                 URL: https://issues.apache.org/jira/browse/HBASE-26726
             Project: HBase
          Issue Type: Improvement
          Components: master, Region Assignment
            Reporter: Andrew Kyle Purtell


We have encountered two issues with region warmup before the assignment manager gracefully moves a region from one regionserver to another. 

The first instance is HBASE-26722. Part of the failure chain is temporary double assignment like conditions where both the source and destination regionservers think they have exclusive rights to storefile management (as they should) but warmup opens a region before it is closed. While this can be remediated with more care to this case, it is unclear if warmup affords significant advantage. The motivation of the original commit in 2015 introducing this feature was avoidance of blockcache misses once region ownership transfer was advertised to clients. Depending on use case and additional default-false schema options (like preload) this could still be valuable. Or not.

The second instance is in place upgrade from HBase 1 to HBase 2. In a scenario where regionservers have been replaced by HBase 2 versions, but the master is still HBase 1, region warmup fails. This is not particular harmful but indicates it will not be useful during the transition period. 

We think it would be good to allow for warmup before move to be optionally disabled by a site configuration setting. In particular there have been many unrelated changes committed since 2015 and expectations of invariants in the contribution of the warmup-on-move feature have been invalidated. HBASE-26722 may be the only case, or it might not. 



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