Commits on May 15, 2024

1. db: add SharedLowerUserKeyPrefix and WriteSharedWithStrictObsolete op… …

   …tions
   
   SharedLowerUserKeyPrefix, if specified, is an additional lower bound on
   constraint on key prefixes that should be written to shared files. It
   applies only when CreateOnShared permits some shared file creation. It
   will be used by CockroachDB to exclude keys below TableDataMin from
   shared files, for both correctness (they can contain MERGEs for which
   the obsolete bit does not work) and performance reasons (low latency
   is more important and the data volume is tiny).
   
   WriteSharedWithStrictObsolete, when true, causes shared files to be
   written with WriterOptions.IsStrictObsolete set to true. This adds
   an extra measure of configuration protection to accidentally sharing
   files where the MERGE could become visible (we currently share such
   files, but file virtualization hides these MERGEs).
   
   The enforcement of SharedLowerUserKeyPrefix changes how
   PreferSharedStorage is computed during flushes and compactions. It
   will only be set if the next key to be written by compact.Runner
   permits writing to shared storage. compact.Runner optimizes this
   computation for when a compaction is fully within the shared or
   unshared bounds. Additionally compact.Runner uses the existing
   OutputSplitter to decide when a split should happen when transitioning
   from unshared to shared.
   
   While here, we do a tiny optimization in OutputSplitter to remove
   a key comparison on each iteration key.
   
   Fixes cockroachdb#2756

   

   sumeerbhola committed May 15, 2024
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