When memtable receives a tombstone it can happen under some workloads
that it covers data which is still in the memtable. Some workloads may
insert and delete data within a short time frame. We could reduce the
rate of memtable flushes if we eagerly drop tombstoned data.
One workload which benefits is the raft log. It stores a row for each
uncommitted raft entry. When entries are committed they are
deleted. So the live set is expected to be short under normal
conditions.
Fixes#652.
Closes#10807
* github.com:scylladb/scylla:
memtable: Add counters for tombstone compaction
memtable, cache: Eagerly compact data with tombstones
memtable: Subtract from flushed memory when cleaning
mvcc: Introduce apply_resume to hold state for partition version merging
test: mutation: Compare against compacted mutations
compacting_reader: Drop irrelevant tombstones
mutation_partition: Extract deletable_row::compact_and_expire()
mvcc: Apply mutations in memtable with preemption enabled
test: memtable: Make failed_flush_prevents_writes() immune to background merging
4bd4aa2e88