kvserver: refine est impact for lease transfers

Previously, when estimating the impact a lease transfer would have, we
would not differentiate between rebalance/transfer. This commit adds a
utility method `TransferImpact` to `RangeUsageInfo` which is now used
when making estimations about lease transers.

Currently, this is identical to `Load`, which was previously used
instead for transfers.

Release note: None

pkg/kv/kvserver/allocator/allocatorimpl/allocator.go

@@ -2186,7 +2186,7 @@ func (a *Allocator) TransferLeaseTarget(
 		return candidates[a.randGen.Intn(len(candidates))]
 
 	case allocator.LoadConvergence:
-		leaseReplLoad := usageInfo.Load()
+		leaseReplLoad := usageInfo.TransferImpact()
 		candidates := make([]roachpb.StoreID, 0, len(existing)-1)
 		for _, repl := range existing {
 			if repl.StoreID != leaseRepl.StoreID() {

pkg/kv/kvserver/allocator/range_usage_info.go

@@ -41,3 +41,14 @@ func (r RangeUsageInfo) Load() load.Load {
 	dims[load.Queries] = r.QueriesPerSecond
 	return dims
 }
+
+// TransferImpact returns the impact of transferring the lease for the range,
+// given the usage information. The impact is assumed to be symmetric, e.g. the
+// receiving store of the transfer will have load = prev_load(recv) + impact
+// after the transfer, whilst the sending side will have load =
+// prev_load(sender) - impact after the transfer.
+func (r RangeUsageInfo) TransferImpact() load.Load {
+	dims := load.Vector{}
+	dims[load.Queries] = r.QueriesPerSecond
+	return dims
+}

pkg/kv/kvserver/store_rebalancer.go

@@ -715,17 +715,17 @@ func (sr *StoreRebalancer) chooseLeaseToTransfer(
 		// store's load. It's just unnecessary churn with no benefit to move leases
 		// responsible for, for example, 1 load unit on a store with 5000 load units.
 		const minLoadFraction = .001
-		if candidateReplica.RangeUsageInfo().Load().Dim(rctx.loadDimension) <
+		if candidateReplica.RangeUsageInfo().TransferImpact().Dim(rctx.loadDimension) <
 			rctx.LocalDesc.Capacity.Load().Dim(rctx.loadDimension)*minLoadFraction {
 			log.KvDistribution.VEventf(ctx, 3, "r%d's %s load is too little to matter relative to s%d's %s total load",
-				candidateReplica.GetRangeID(), candidateReplica.RangeUsageInfo().Load(),
+				candidateReplica.GetRangeID(), candidateReplica.RangeUsageInfo().TransferImpact(),
 				rctx.LocalDesc.StoreID, rctx.LocalDesc.Capacity.Load())
 			continue
 		}
 
 		desc, conf := candidateReplica.DescAndSpanConfig()
 		log.KvDistribution.VEventf(ctx, 3, "considering lease transfer for r%d with %s load",
-			desc.RangeID, candidateReplica.RangeUsageInfo().Load())
+			desc.RangeID, candidateReplica.RangeUsageInfo().TransferImpact())
 
 		// Check all the other voting replicas in order of increasing load.
 		// Learners or non-voters aren't allowed to become leaseholders or raft
@@ -787,7 +787,7 @@ func (sr *StoreRebalancer) chooseLeaseToTransfer(
 				1,
 				"transferring lease for r%d load=%s to store s%d load=%s from local store s%d load=%s",
 				desc.RangeID,
-				candidateReplica.RangeUsageInfo().Load(),
+				candidateReplica.RangeUsageInfo().TransferImpact(),
 				targetStore.StoreID,
 				targetStore.Capacity.Load(),
 				rctx.LocalDesc.StoreID,