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1b2140e416
LCS backlog tracker uses STCS tracker for L0. Turns out LCS tracker is calling STCS tracker's replace_sstables() with empty arguments even when higher levels (> 0) *only* had sstables replaced. This unnecessary call to STCS tracker will cause it to recompute the L0 backlog, yielding the same value as before. As LCS has a fragment size of 0.16G on higher levels, we may be updating the tracker multiple times during incremental compaction, which operates on SSTables on higher levels. Inefficiency is fixed by only updating the STCS tracker if any L0 sstable is being added or removed from the table. This may be fixing a quadratic behavior during boot or refresh, as new sstables are loaded one by one. Higher levels have a substantial higher number of sstables, therefore updating STCS tracker only when level 0 changes, reduces significantly the number of times L0 backlog is recomputed. Refs #12499. Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com> Closes #12676