jbewing opened a new pull request, #5373:
URL: https://github.com/apache/hbase/pull/5373

   ### What
   
   This PR attempts to optimize the StoreFileScanner.seekToPreviousRow (& 
related reverse scan methods) to reduce the number of seeks from the beginning 
of the block by introducing a variable "previousRow"
   
   ### Implementation Notes
   I still have some work to do to ensure that `StoreFileScanner` obeys it's 
interface/contract in lieu of the new changes as the introduction of state in 
previousRow greatly alters reverse seek behavior. Given that most of the 
StoreFileScanner reverse scan specific methods can/are only used in the context 
of reverse scans, I've just modified those for now. As this PR/idea matures, 
I'll fix up the rest of the interface. 
   
   ### Testing
   I've opened this PR to run the entire suite of hbase-server tests against 
this changeset. 
   
   ### Benchmarking
   I need to benchmark the meta scanning as 1-row reverse scans are very common 
there. Bryan gave a fantastic suggestion in the JIRA to possibly only start 
using the new path if we're on the second seek rather than the first. 
   
   [HBASE-28043](https://issues.apache.org/jira/browse/HBASE-28043)


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