config, tablets: Allow tablets_initial_scale_factor to be a fraction

We may want fewer than 1 tablets per shard in large clusters.

The per-table option is a fraction, so for consistency, this should be
too.

Author: tgrabiec

db/config.hh

@@ -502,7 +502,7 @@ public:
     named_value<int> maximum_replication_factor_warn_threshold;
     named_value<int> maximum_replication_factor_fail_threshold;
 
-    named_value<int> tablets_initial_scale_factor;
+    named_value<double> tablets_initial_scale_factor;
     named_value<unsigned> tablets_per_shard_goal;
     named_value<uint64_t> target_tablet_size_in_bytes;
 

service/tablet_allocator.cc

@@ -597,7 +597,7 @@ class load_balancer {
     load_balancer_stats_manager& _stats;
     std::unordered_set<host_id> _skiplist;
     bool _use_table_aware_balancing = true;
-    int _initial_scale = 1;
+    double _initial_scale = 1;
 private:
     tablet_replica_set get_replicas_for_tablet_load(const tablet_info& ti, const tablet_transition_info* trinfo) const {
         // We reflect migrations in the load as if they already happened,
@@ -707,7 +707,7 @@ class load_balancer {
         _use_table_aware_balancing = use_table_aware_balancing;
     }
 
-    void set_initial_scale(int initial_scale) {
+    void set_initial_scale(double initial_scale) {
         _initial_scale = initial_scale;
     }
 
@@ -1092,7 +1092,6 @@ class load_balancer {
         size_t tablet_count = 0;
         const sstring* winning_dc = nullptr;
 
-        min_per_shard_tablet_count = std::max(1.0, min_per_shard_tablet_count);
         for (auto&& [dc, shards_in_dc] : shards_per_dc) {
             auto rf_in_dc = rs.get_replication_factor(dc);
             if (!rf_in_dc) {

test/boost/tablets_test.cc

@@ -2678,6 +2678,12 @@ SEASTAR_THREAD_TEST_CASE(test_tablet_option_and_config_changes) {
         cfg.tablets_per_shard_goal(100);
         rebalance_tablets(e, &load_stats);
         BOOST_REQUIRE_EQUAL(get_tablet_count(), 16);
+
+        // initial scale can be smaller than 1.
+        // 0.5 tablet/shard * 3 shards = 1.5 tablets =~ 2 tablets.
+        cfg.tablets_initial_scale_factor(0.5);
+        rebalance_tablets(e, &load_stats);
+        BOOST_REQUIRE_EQUAL(get_tablet_count(), 2);
     }, cfg).get();
 }