The hints and batchlog flush requests are issued to all nodes for each
repair request when tombstone_gc repair mode is used.
The amount of such flush requests is high when all nodes in the cluster
run repair. It is observed it takes a long time, up to 15s, for a repair
request to finish such a flush request.
To reduce overhead of the flush, each node caches the flush and only
executes the real flush when the cahce time has passed. It is safe to do
so because the real flush_time is returned. Repair uses the smallest
flush_time returned from peers as the repair time.
The nice thing about the cache on the receiver side is that all senders
can hit the cache. It is better than cache on the sender side.
A slightly smaller flush_time compared to the real flush time will be
used with the benefits of significantly dropped hints and batchlog
flush. The trade-off looks reasonable.
Tests: 2 nodes, with 1s batchlog delay:
Before:
Repair nr_repairs=20 cache_time_in_ms=0 total_repair_duration=40.04245328903198
After:
Repair nr_repairs=20 cache_time_in_ms=5000 total_repair_duration=1.252073049545288
Fixes#20259
b3b3e880d3