feat(repair): add MaxRingParallel

MaxRingParallel is a cleaner and more comprehensible way of calculating max repair parallelism based on Ring replication strategy.

pkg/service/repair/plan.go

@@ -320,6 +320,39 @@ func maxRepairRangesInParallel(shards uint, totalMemory int64) Intensity {
 	return NewIntensity(max)
 }
 
+// MaxRingParallel calculates max amount of repair jobs on ring limited to dcs.
+func MaxRingParallel(ring scyllaclient.Ring, dcs []string) int {
+	repairedDCs := strset.New(dcs...)
+	dcNodeCnt := make(map[string]int)
+	for _, dc := range ring.HostDC {
+		dcNodeCnt[dc]++
+	}
+
+	switch ring.Replication {
+	case scyllaclient.SimpleStrategy:
+		repaired := 0
+		for dc, cnt := range dcNodeCnt {
+			if repairedDCs.Has(dc) {
+				repaired += cnt
+			}
+		}
+		return repaired / ring.RF
+	case scyllaclient.NetworkTopologyStrategy:
+		minDC := math.MaxInt / 2
+		for dc, rf := range ring.DCrf {
+			if repairedDCs.Has(dc) {
+				minDC = min(minDC, dcNodeCnt[dc]/rf)
+			}
+		}
+		if minDC == math.MaxInt/2 {
+			minDC = 1
+		}
+		return minDC
+	default:
+		return 1
+	}
+}
+
 // keyspacePlan describes repair schedule and state for keyspace.
 type keyspacePlan struct {
 	Keyspace string

pkg/service/repair/repair_test.go

@@ -0,0 +1,108 @@
+// Copyright (C) 2024 ScyllaDB
+
+package repair_test
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/scylladb/scylla-manager/v3/pkg/scyllaclient"
+	"github.com/scylladb/scylla-manager/v3/pkg/service/repair"
+)
+
+func TestMaxRingParallel(t *testing.T) {
+	hostDC := map[string]string{
+		// dc1 -> 3
+		"h1": "dc1",
+		"h2": "dc1",
+		"h3": "dc1",
+		// dc2 -> 4
+		"h4": "dc2",
+		"h5": "dc2",
+		"h6": "dc2",
+		"h7": "dc2",
+		// dc3 -> 5
+		"h8":  "dc3",
+		"h9":  "dc3",
+		"h10": "dc3",
+		"h11": "dc3",
+		"h12": "dc3",
+	}
+
+	testCases := []struct {
+		Ring     scyllaclient.Ring
+		DCs      []string
+		Expected int
+	}{
+		{
+			Ring: scyllaclient.Ring{
+				HostDC:      hostDC,
+				Replication: scyllaclient.SimpleStrategy,
+				RF:          4,
+			},
+			DCs:      []string{"dc1", "dc2", "dc3"},
+			Expected: 3,
+		},
+		{
+			Ring: scyllaclient.Ring{
+				HostDC:      hostDC,
+				Replication: scyllaclient.SimpleStrategy,
+				RF:          3,
+			},
+			DCs:      []string{"dc1", "dc2"},
+			Expected: 2,
+		},
+		{
+			Ring: scyllaclient.Ring{
+				HostDC:      hostDC,
+				Replication: scyllaclient.NetworkTopologyStrategy,
+				RF:          5,
+				DCrf: map[string]int{
+					"dc1": 1,
+					"dc2": 2,
+					"dc3": 2,
+				},
+			},
+			DCs:      []string{"dc1", "dc2", "dc3"},
+			Expected: 2,
+		},
+		{
+			Ring: scyllaclient.Ring{
+				HostDC:      hostDC,
+				Replication: scyllaclient.NetworkTopologyStrategy,
+				RF:          8,
+				DCrf: map[string]int{
+					"dc1": 1,
+					"dc2": 2,
+					"dc3": 5,
+				},
+			},
+			DCs:      []string{"dc1", "dc2"},
+			Expected: 2,
+		},
+		{
+			Ring: scyllaclient.Ring{
+				HostDC:      hostDC,
+				Replication: scyllaclient.NetworkTopologyStrategy,
+				RF:          4,
+				DCrf: map[string]int{
+					"dc1": 2,
+					"dc2": 1,
+					"dc3": 1,
+				},
+			},
+			DCs:      []string{"dc1", "dc3"},
+			Expected: 1,
+		},
+	}
+
+	for i := range testCases {
+		tc := testCases[i]
+		t.Run("test "+fmt.Sprint(i), func(t *testing.T) {
+			t.Parallel()
+			if out := repair.MaxRingParallel(tc.Ring, tc.DCs); out != tc.Expected {
+				t.Fatalf("Expected %d, got %d", tc.Expected, out)
+			}
+		})
+	}
+}