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8a408ac5a8
The learning stage of PAXOS protocol leaves behind an entry in system.paxos table with the last learned value (which can be large). In case not all participants learned it successfully next round on the same key may complete the learning using this info. But if all nodes learned the value the entry does not serve useful purpose any longer. The patch adds another round, "prune", which is executed in background (limited to 1000 simultaneous instances) and removes the entry in case all nodes replied successfully to the "learn" round. It uses the ballot's timestamp to do the deletion, so not to interfere with the next round. Since deletion happens very close to previous writes it will likely happen in memtable and will never reach sstable, so that reduces memtable flush and compaction overhead. Fixes #5779 Message-Id: <20200330154853.GA31074@scylladb.com>