This change makes tablet load balancing more efficient by performing
migrations independently for different tablets, and making new load
balancing plans concurrently with active migrations.
The migration track is interrupted by pending topology change operations.
The coordinator executes the load balancer on edges of tablet state
machine transitions. This allows new migrations to be started as soon
as tablets finish streaming.
The load balancer is also continuously invoked as long as it produces
a non-empty plan. This is in order to saturate the cluster with
streaming. A single make_plan() call is still not saturating, due
to the way algorithm is implemented.
Overload of shards is limited by the fact that load balancer algorithm tracks
streaming concurrency on both source and target shards of active
migrations and takes concurrency limit into account when producing new
migrations.
Closes#14851
* github.com:scylladb/scylladb:
tablets: load_balancer: Remove double logging
tests: tablets: Check that load balancing is interrupted by topology change
tests: tablets: Add test for load balancing with active migrations
tablets: Balance tablets concurrently with active migrations
storage_service, tablets: Extract generate_migration_updates()
storage_service, tablets: Move get_leaving_replica() to tablets.cc
locator: tablets: Move std::hash definition earlier
storage_service: Advance tablets independently
topology_coordinator: Fix missed notification on abort
tablets: Add formatter for tablet_migration_info
dac93b2096