Enhance EC balancing to separate parity and data shards (#8038)

* Enhance EC balancing to separate parity and data shards across racks

* Rename avoidRacks to antiAffinityRacks for clarity

* Implement server-level EC separation for parity/data shards

* Optimize EC balancing: consolidate helpers and extract two-pass selection logic

* Add comprehensive edge case tests for EC balancing logic

* Apply code review feedback: rename select_(), add divide-by-zero guard, fix comment

* Remove unused parameters from doBalanceEcShardsWithinOneRack and add explicit anti-affinity check

* Add disk-level anti-affinity for data/parity shard separation

- Modified pickBestDiskOnNode to accept shardId and dataShardCount
- Implemented explicit anti-affinity: 1000-point penalty for placing data shards on disks with parity (and vice versa)
- Updated all call sites including balancing and evacuation
- For evacuation, disabled anti-affinity by passing dataShardCount=0

weed/shell/command_ec_common.go

@@ -809,24 +809,48 @@ func countShardsByRack(vid needle.VolumeId, locations []*EcNode, diskType types.
 	})
 }
 
-// shardsByTypePerRack counts data shards (< dataShards) and parity shards (>= dataShards) per rack
-func shardsByTypePerRack(vid needle.VolumeId, locations []*EcNode, diskType types.DiskType, dataShards int) (dataPerRack, parityPerRack map[string][]erasure_coding.ShardId) {
-	dataPerRack = make(map[string][]erasure_coding.ShardId)
-	parityPerRack = make(map[string][]erasure_coding.ShardId)
+// shardsByType is a generic helper that counts data and parity shards per group
+func shardsByType(vid needle.VolumeId, locations []*EcNode, diskType types.DiskType, dataShards int, keyExtractor func(*EcNode) string) (dataPerGroup, parityPerGroup map[string][]erasure_coding.ShardId) {
+	dataPerGroup = make(map[string][]erasure_coding.ShardId)
+	parityPerGroup = make(map[string][]erasure_coding.ShardId)
 	for _, ecNode := range locations {
 		si := findEcVolumeShardsInfo(ecNode, vid, diskType)
-		rackId := string(ecNode.rack)
+		groupKey := keyExtractor(ecNode)
 		for _, shardId := range si.Ids() {
 			if int(shardId) < dataShards {
-				dataPerRack[rackId] = append(dataPerRack[rackId], shardId)
+				dataPerGroup[groupKey] = append(dataPerGroup[groupKey], shardId)
 			} else {
-				parityPerRack[rackId] = append(parityPerRack[rackId], shardId)
+				parityPerGroup[groupKey] = append(parityPerGroup[groupKey], shardId)
 			}
 		}
 	}
 	return
 }
 
+// shardsByTypePerRack counts data shards (< dataShards) and parity shards (>= dataShards) per rack
+func shardsByTypePerRack(vid needle.VolumeId, locations []*EcNode, diskType types.DiskType, dataShards int) (dataPerRack, parityPerRack map[string][]erasure_coding.ShardId) {
+	return shardsByType(vid, locations, diskType, dataShards, func(ecNode *EcNode) string {
+		return string(ecNode.rack)
+	})
+}
+
+// shardsByTypePerNode counts data shards and parity shards per node
+func shardsByTypePerNode(vid needle.VolumeId, locations []*EcNode, diskType types.DiskType, dataShards int) (dataPerNode, parityPerNode map[string][]erasure_coding.ShardId) {
+	return shardsByType(vid, locations, diskType, dataShards, func(ecNode *EcNode) string {
+		return ecNode.info.Id
+	})
+}
+
+func countShardsByNode(vid needle.VolumeId, locations []*EcNode, diskType types.DiskType) map[string]int {
+	return groupByCount(locations, func(ecNode *EcNode) (id string, count int) {
+		id = ecNode.info.Id
+		if si := findEcVolumeShardsInfo(ecNode, vid, diskType); si != nil {
+			count = si.Count()
+		}
+		return
+	})
+}
+
 func (ecb *ecBalancer) doBalanceEcShardsAcrossRacks(collection string, vid needle.VolumeId, locations []*EcNode) error {
 	racks := ecb.racks()
 	numRacks := len(racks)
@@ -852,20 +876,30 @@ func (ecb *ecBalancer) doBalanceEcShardsAcrossRacks(collection string, vid needl
 	rackToShardCount := countShardsByRack(vid, locations, ecb.diskType)
 
 	// First pass: Balance data shards across racks
-	if err := ecb.balanceShardTypeAcrossRacks(collection, vid, racks, rackEcNodesWithVid, dataPerRack, rackToShardCount, maxDataPerRack, "data"); err != nil {
+	if err := ecb.balanceShardTypeAcrossRacks(collection, vid, racks, rackEcNodesWithVid, dataPerRack, rackToShardCount, maxDataPerRack, "data", nil); err != nil {
 		return err
 	}
 
 	// Refresh locations after data shard moves and get parity distribution
 	locations = ecb.collectVolumeIdToEcNodes(collection)[vid]
-	_, parityPerRack := shardsByTypePerRack(vid, locations, ecb.diskType, dataShardCount)
+	dataPerRack, parityPerRack := shardsByTypePerRack(vid, locations, ecb.diskType, dataShardCount)
 	rackEcNodesWithVid = groupBy(locations, func(ecNode *EcNode) string {
 		return string(ecNode.rack)
 	})
 	rackToShardCount = countShardsByRack(vid, locations, ecb.diskType)
 
-	// Second pass: Balance parity shards across racks
-	if err := ecb.balanceShardTypeAcrossRacks(collection, vid, racks, rackEcNodesWithVid, parityPerRack, rackToShardCount, maxParityPerRack, "parity"); err != nil {
+	// Identify racks containing data shards to avoid for parity placement.
+	// We call this "antiAffinityRacks" because we want parity shards to have anti-affinity
+	// with racks that hold data shards, to ensure better fault tolerance.
+	antiAffinityRacks := make(map[string]bool)
+	for rackId, shards := range dataPerRack {
+		if len(shards) > 0 {
+			antiAffinityRacks[rackId] = true
+		}
+	}
+
+	// Second pass: Balance parity shards across racks, ignoring racks with data shards if possible
+	if err := ecb.balanceShardTypeAcrossRacks(collection, vid, racks, rackEcNodesWithVid, parityPerRack, rackToShardCount, maxParityPerRack, "parity", antiAffinityRacks); err != nil {
 		return err
 	}
 
@@ -882,6 +916,7 @@ func (ecb *ecBalancer) balanceShardTypeAcrossRacks(
 	rackToShardCount map[string]int,
 	maxPerRack int,
 	shardType string,
+	antiAffinityRacks map[string]bool,
 ) error {
 	// Find racks with too many shards of this type
 	shardsToMove := make(map[erasure_coding.ShardId]*EcNode)
@@ -908,7 +943,7 @@ func (ecb *ecBalancer) balanceShardTypeAcrossRacks(
 	// Move shards to racks that have fewer than maxPerRack of this type
 	for shardId, ecNode := range shardsToMove {
 		// Find destination rack with room for this shard type
-		destRackId, err := ecb.pickRackForShardType(racks, shardsPerRack, maxPerRack, rackToShardCount)
+		destRackId, err := ecb.pickRackForShardType(racks, shardsPerRack, maxPerRack, rackToShardCount, antiAffinityRacks)
 		if err != nil {
 			fmt.Printf("ec %s shard %d.%d at %s can not find a destination rack:\n%s\n", shardType, vid, shardId, ecNode.info.Id, err.Error())
 			continue
@@ -942,41 +977,145 @@ func (ecb *ecBalancer) balanceShardTypeAcrossRacks(
 	return nil
 }
 
-// pickRackForShardType selects a rack that has room for more shards of a specific type
-func (ecb *ecBalancer) pickRackForShardType(
-	rackToEcNodes map[RackId]*EcRack,
-	shardsPerRack map[string][]erasure_coding.ShardId,
-	maxPerRack int,
-	rackToShardCount map[string]int,
-) (RackId, error) {
-	var candidates []RackId
-	minShards := maxPerRack + 1
+// twoPassSelector implements two-pass selection with anti-affinity
+// Pass 1: Select from candidates NOT in antiAffinity set
+// Pass 2: Fallback to any valid candidate if Pass 1 yields no results
+type twoPassSelector[T any] struct {
+	candidates         []T
+	shardsPerTarget    map[string][]erasure_coding.ShardId
+	maxPerTarget       int
+	targetToShardCount map[string]int
+	antiAffinity       map[string]bool
+
+	// Functions to extract info from candidate
+	getKey       func(T) string
+	hasFreeSlots func(T) bool
+	checkLimit   func(T) bool // replica placement or other limits
+}
 
-	for rackId, rack := range rackToEcNodes {
-		if rack.freeEcSlot <= 0 {
+func (s *twoPassSelector[T]) selectCandidate() (T, error) {
+	var selected []T
+	minShards := s.maxPerTarget + 1
+
+	// Pass 1: Try candidates NOT in anti-affinity set
+	for _, candidate := range s.candidates {
+		if !s.hasFreeSlots(candidate) {
 			continue
 		}
-		currentCount := len(shardsPerRack[string(rackId)])
-		if currentCount >= maxPerRack {
+		key := s.getKey(candidate)
+		currentCount := len(s.shardsPerTarget[key])
+		if currentCount >= s.maxPerTarget {
 			continue
 		}
-		// For EC shards, replica placement constraint only applies when DiffRackCount > 0.
-		if ecb.replicaPlacement != nil && ecb.replicaPlacement.DiffRackCount > 0 && rackToShardCount[string(rackId)] >= ecb.replicaPlacement.DiffRackCount {
+		if !s.checkLimit(candidate) {
 			continue
 		}
+
+		// Skip anti-affinity targets in pass 1
+		if s.antiAffinity != nil && s.antiAffinity[key] {
+			continue
+		}
+
 		if currentCount < minShards {
-			candidates = nil
+			selected = nil
 			minShards = currentCount
 		}
 		if currentCount == minShards {
-			candidates = append(candidates, rackId)
+			selected = append(selected, candidate)
 		}
 	}
 
-	if len(candidates) == 0 {
+	// Pass 2: Fallback if no candidates found
+	if len(selected) == 0 {
+		minShards = s.maxPerTarget + 1
+		for _, candidate := range s.candidates {
+			if !s.hasFreeSlots(candidate) {
+				continue
+			}
+			key := s.getKey(candidate)
+			currentCount := len(s.shardsPerTarget[key])
+			if currentCount >= s.maxPerTarget {
+				continue
+			}
+			if !s.checkLimit(candidate) {
+				continue
+			}
+
+			if currentCount < minShards {
+				selected = nil
+				minShards = currentCount
+			}
+			if currentCount == minShards {
+				selected = append(selected, candidate)
+			}
+		}
+	}
+
+	if len(selected) == 0 {
+		var zero T
+		return zero, errors.New("no valid candidate available")
+	}
+	return selected[rand.IntN(len(selected))], nil
+}
+
+// pickRackForShardType selects a rack that has room for more shards of a specific type
+func (ecb *ecBalancer) pickRackForShardType(
+	rackToEcNodes map[RackId]*EcRack,
+	shardsPerRack map[string][]erasure_coding.ShardId,
+	maxPerRack int,
+	rackToShardCount map[string]int,
+	antiAffinityRacks map[string]bool,
+) (RackId, error) {
+	// Convert map to slice for iteration
+	var rackCandidates []struct {
+		id   RackId
+		rack *EcRack
+	}
+	for id, rack := range rackToEcNodes {
+		rackCandidates = append(rackCandidates, struct {
+			id   RackId
+			rack *EcRack
+		}{id, rack})
+	}
+
+	selector := &twoPassSelector[struct {
+		id   RackId
+		rack *EcRack
+	}]{
+		candidates:         rackCandidates,
+		shardsPerTarget:    shardsPerRack,
+		maxPerTarget:       maxPerRack,
+		targetToShardCount: rackToShardCount,
+		antiAffinity:       antiAffinityRacks,
+		getKey: func(c struct {
+			id   RackId
+			rack *EcRack
+		}) string {
+			return string(c.id)
+		},
+		hasFreeSlots: func(c struct {
+			id   RackId
+			rack *EcRack
+		}) bool {
+			return c.rack.freeEcSlot > 0
+		},
+		checkLimit: func(c struct {
+			id   RackId
+			rack *EcRack
+		}) bool {
+			// For EC shards, replica placement constraint only applies when DiffRackCount > 0.
+			if ecb.replicaPlacement != nil && ecb.replicaPlacement.DiffRackCount > 0 {
+				return rackToShardCount[string(c.id)] < ecb.replicaPlacement.DiffRackCount
+			}
+			return true
+		},
+	}
+
+	selected, err := selector.selectCandidate()
+	if err != nil {
 		return "", errors.New("no rack available for shard type balancing")
 	}
-	return candidates[rand.IntN(len(candidates))], nil
+	return selected.id, nil
 }
 
 func (ecb *ecBalancer) pickRackToBalanceShardsInto(rackToEcNodes map[RackId]*EcRack, rackToShardCount map[string]int) (RackId, error) {
@@ -1032,51 +1171,74 @@ func (ecb *ecBalancer) balanceEcShardsWithinRacks(collection string) error {
 
 		// see the volume's shards are in how many racks, and how many in each rack
 		rackToShardCount := countShardsByRack(vid, locations, ecb.diskType)
-		rackEcNodesWithVid := groupBy(locations, func(ecNode *EcNode) string {
-			return string(ecNode.rack)
-		})
-
-		for rackId, _ := range rackToShardCount {
 
+		for rackId := range rackToShardCount {
 			var possibleDestinationEcNodes []*EcNode
 			for _, n := range racks[RackId(rackId)].ecNodes {
 				if _, found := n.info.DiskInfos[string(ecb.diskType)]; found {
 					possibleDestinationEcNodes = append(possibleDestinationEcNodes, n)
 				}
 			}
-			sourceEcNodes := rackEcNodesWithVid[rackId]
-			averageShardsPerEcNode := ceilDivide(rackToShardCount[rackId], len(possibleDestinationEcNodes))
 			ewg.Add(func() error {
-				return ecb.doBalanceEcShardsWithinOneRack(averageShardsPerEcNode, collection, vid, sourceEcNodes, possibleDestinationEcNodes)
+				return ecb.doBalanceEcShardsWithinOneRack(collection, vid, possibleDestinationEcNodes)
 			})
 		}
 	}
 	return ewg.Wait()
 }
 
-func (ecb *ecBalancer) doBalanceEcShardsWithinOneRack(averageShardsPerEcNode int, collection string, vid needle.VolumeId, existingLocations, possibleDestinationEcNodes []*EcNode) error {
-	for _, ecNode := range existingLocations {
+func (ecb *ecBalancer) doBalanceEcShardsWithinOneRack(collection string, vid needle.VolumeId, possibleDestinationEcNodes []*EcNode) error {
+	// Use configured EC scheme
+	dataShardCount := ecb.getDataShardCount()
 
-		si := findEcVolumeShardsInfo(ecNode, vid, ecb.diskType)
-		overLimitCount := si.Count() - averageShardsPerEcNode
+	// Get current distribution of data shards per node
+	dataPerNode, parityPerNode := shardsByTypePerNode(vid, possibleDestinationEcNodes, ecb.diskType, dataShardCount)
 
-		for _, shardId := range si.Ids() {
+	// Calculate max shards per node for each type
+	numNodes := len(possibleDestinationEcNodes)
+	if numNodes == 0 {
+		return nil
+	}
 
-			if overLimitCount <= 0 {
-				break
-			}
+	// Calculate totals based on actual shards present in the rack (subset of all shards)
+	totalData := 0
+	for _, shards := range dataPerNode {
+		totalData += len(shards)
+	}
+	totalParity := 0
+	for _, shards := range parityPerNode {
+		totalParity += len(shards)
+	}
 
-			fmt.Printf("%s has %d overlimit, moving ec shard %d.%d\n", ecNode.info.Id, overLimitCount, vid, shardId)
+	maxDataPerNode := ceilDivide(totalData, numNodes)
+	maxParityPerNode := ceilDivide(totalParity, numNodes)
 
-			err := ecb.pickOneEcNodeAndMoveOneShard(ecNode, collection, vid, shardId, possibleDestinationEcNodes)
-			if err != nil {
-				return err
-			}
+	// Track total shard count per node
+	nodeToShardCount := countShardsByNode(vid, possibleDestinationEcNodes, ecb.diskType)
+
+	// First pass: Balance data shards across nodes
+	if err := ecb.balanceShardTypeAcrossNodes(collection, vid, possibleDestinationEcNodes, dataPerNode, nodeToShardCount, maxDataPerNode, "data", nil); err != nil {
+		return err
+	}
+
+	// Refresh locations after data shard moves
+	// We need to re-scan because moving shards changes node states
+	dataPerNode, parityPerNode = shardsByTypePerNode(vid, possibleDestinationEcNodes, ecb.diskType, dataShardCount)
+	nodeToShardCount = countShardsByNode(vid, possibleDestinationEcNodes, ecb.diskType)
 
-			overLimitCount--
+	// Identify nodes containing data shards to avoid for parity placement
+	antiAffinityNodes := make(map[string]bool)
+	for nodeId, shards := range dataPerNode {
+		if len(shards) > 0 {
+			antiAffinityNodes[nodeId] = true
 		}
 	}
 
+	// Second pass: Balance parity shards across nodes, avoiding nodes with data shards if possible
+	if err := ecb.balanceShardTypeAcrossNodes(collection, vid, possibleDestinationEcNodes, parityPerNode, nodeToShardCount, maxParityPerNode, "parity", antiAffinityNodes); err != nil {
+		return err
+	}
+
 	return nil
 }
 
@@ -1144,7 +1306,9 @@ func (ecb *ecBalancer) doBalanceEcRack(ecRack *EcRack) error {
 					for _, shardId := range si.Ids() {
 						vid := needle.VolumeId(shards.Id)
 						// For balancing, strictly require matching disk type
-						destDiskId := pickBestDiskOnNode(emptyNode, vid, ecb.diskType, true)
+						// For balancing, strictly require matching disk type and apply anti-affinity
+						dataShardCount := ecb.getDataShardCount()
+						destDiskId := pickBestDiskOnNode(emptyNode, vid, ecb.diskType, true, shardId, dataShardCount)
 
 						if destDiskId > 0 {
 							fmt.Printf("%s moves ec shards %d.%d to %s (disk %d)\n", fullNode.info.Id, shards.Id, shardId, emptyNode.info.Id, destDiskId)
@@ -1265,8 +1429,11 @@ func diskDistributionScore(ecNode *EcNode, vid needle.VolumeId) int {
 
 // pickBestDiskOnNode selects the best disk on a node for placing a new EC shard
 // It prefers disks of the specified type with fewer shards and more free slots
+// When shardId is provided and dataShardCount > 0, it applies anti-affinity:
+// - For data shards (shardId < dataShardCount): prefer disks without parity shards
+// - For parity shards (shardId >= dataShardCount): prefer disks without data shards
 // If strictDiskType is false, it will fall back to other disk types if no matching disk is found
-func pickBestDiskOnNode(ecNode *EcNode, vid needle.VolumeId, diskType types.DiskType, strictDiskType bool) uint32 {
+func pickBestDiskOnNode(ecNode *EcNode, vid needle.VolumeId, diskType types.DiskType, strictDiskType bool, shardId erasure_coding.ShardId, dataShardCount int) uint32 {
 	if len(ecNode.disks) == 0 {
 		return 0 // No disk info available, let the server decide
 	}
@@ -1276,21 +1443,47 @@ func pickBestDiskOnNode(ecNode *EcNode, vid needle.VolumeId, diskType types.Disk
 	var fallbackDiskId uint32
 	fallbackScore := -1
 
+	// Determine if we're placing a data or parity shard
+	isDataShard := dataShardCount > 0 && int(shardId) < dataShardCount
+
 	for diskId, disk := range ecNode.disks {
 		if disk.freeEcSlots <= 0 {
 			continue
 		}
 
-		// Check if this volume already has shards on this disk
+		// Check existing shards on this disk for this volume
 		existingShards := 0
+		hasDataShards := false
+		hasParityShards := false
 		if si, ok := disk.ecShards[vid]; ok {
 			existingShards = si.Count()
+			// Check what type of shards are on this disk
+			if dataShardCount > 0 {
+				for _, existingShardId := range si.Ids() {
+					if int(existingShardId) < dataShardCount {
+						hasDataShards = true
+					} else {
+						hasParityShards = true
+					}
+				}
+			}
 		}
 
 		// Score: prefer disks with fewer total shards and fewer shards of this volume
 		// Lower score is better
 		score := disk.ecShardCount*10 + existingShards*100
 
+		// Apply anti-affinity penalty if applicable
+		if dataShardCount > 0 {
+			if isDataShard && hasParityShards {
+				// Penalize placing data shard on disk with parity shards
+				score += 1000
+			} else if !isDataShard && hasDataShards {
+				// Penalize placing parity shard on disk with data shards
+				score += 1000
+			}
+		}
+
 		if disk.diskType == string(diskType) {
 			// Matching disk type - this is preferred
 			if bestScore == -1 || score < bestScore {
@@ -1314,19 +1507,20 @@ func pickBestDiskOnNode(ecNode *EcNode, vid needle.VolumeId, diskType types.Disk
 }
 
 // pickEcNodeAndDiskToBalanceShardsInto picks both a destination node and specific disk
-func (ecb *ecBalancer) pickEcNodeAndDiskToBalanceShardsInto(vid needle.VolumeId, existingLocation *EcNode, possibleDestinations []*EcNode) (*EcNode, uint32, error) {
+func (ecb *ecBalancer) pickEcNodeAndDiskToBalanceShardsInto(vid needle.VolumeId, shardId erasure_coding.ShardId, existingLocation *EcNode, possibleDestinations []*EcNode) (*EcNode, uint32, error) {
 	node, err := ecb.pickEcNodeToBalanceShardsInto(vid, existingLocation, possibleDestinations)
 	if err != nil {
 		return nil, 0, err
 	}
 
-	// For balancing, strictly require matching disk type
-	diskId := pickBestDiskOnNode(node, vid, ecb.diskType, true)
+	// For balancing, strictly require matching disk type and apply anti-affinity
+	dataShardCount := ecb.getDataShardCount()
+	diskId := pickBestDiskOnNode(node, vid, ecb.diskType, true, shardId, dataShardCount)
 	return node, diskId, nil
 }
 
 func (ecb *ecBalancer) pickOneEcNodeAndMoveOneShard(existingLocation *EcNode, collection string, vid needle.VolumeId, shardId erasure_coding.ShardId, possibleDestinationEcNodes []*EcNode) error {
-	destNode, destDiskId, err := ecb.pickEcNodeAndDiskToBalanceShardsInto(vid, existingLocation, possibleDestinationEcNodes)
+	destNode, destDiskId, err := ecb.pickEcNodeAndDiskToBalanceShardsInto(vid, shardId, existingLocation, possibleDestinationEcNodes)
 	if err != nil {
 		fmt.Printf("WARNING: Could not find suitable target node for %d.%d:\n%s", vid, shardId, err.Error())
 		return nil
@@ -1441,3 +1635,114 @@ func compileCollectionPattern(pattern string) (*regexp.Regexp, error) {
 	}
 	return regexp.Compile(pattern)
 }
+
+// balanceShardTypeAcrossNodes spreads shards of a specific type (data or parity) evenly across nodes
+func (ecb *ecBalancer) balanceShardTypeAcrossNodes(
+	collection string,
+	vid needle.VolumeId,
+	possibleDestinationEcNodes []*EcNode,
+	shardsPerNode map[string][]erasure_coding.ShardId,
+	nodeToShardCount map[string]int,
+	maxPerNode int,
+	shardType string,
+	antiAffinityNodes map[string]bool,
+) error {
+	// Map ID to EcNode for lookup
+	nodeMap := make(map[string]*EcNode)
+	for _, n := range possibleDestinationEcNodes {
+		nodeMap[n.info.Id] = n
+	}
+
+	// Find nodes with too many shards of this type
+	shardsToMove := make(map[erasure_coding.ShardId]*EcNode)
+	for nodeId, shards := range shardsPerNode {
+		if len(shards) <= maxPerNode {
+			continue
+		}
+		// Pick excess shards to move
+		excess := len(shards) - maxPerNode
+		ecNode := nodeMap[nodeId]
+		if ecNode == nil {
+			continue
+		}
+
+		for i := 0; i < excess && i < len(shards); i++ {
+			shardId := shards[i]
+			// Verify node has this shard
+			si := findEcVolumeShardsInfo(ecNode, vid, ecb.diskType)
+			if si.Has(shardId) {
+				shardsToMove[shardId] = ecNode
+			}
+		}
+	}
+
+	// Move shards to nodes that have fewer than maxPerNode of this type
+	for shardId, ecNode := range shardsToMove {
+		// Find destination node with room for this shard type
+		destNode, err := ecb.pickNodeForShardType(possibleDestinationEcNodes, shardsPerNode, maxPerNode, nodeToShardCount, antiAffinityNodes)
+		if err != nil {
+			fmt.Printf("ec %s shard %d.%d at %s can not find a destination node:\n%s\n", shardType, vid, shardId, ecNode.info.Id, err.Error())
+			continue
+		}
+
+		err = ecb.pickOneEcNodeAndMoveOneShard(ecNode, collection, vid, shardId, []*EcNode{destNode})
+		if err != nil {
+			return err
+		}
+
+		// Update tracking
+		destNodeId := destNode.info.Id
+		shardsPerNode[destNodeId] = append(shardsPerNode[destNodeId], shardId)
+
+		// Remove from source node
+		srcNodeId := ecNode.info.Id
+		for i, s := range shardsPerNode[srcNodeId] {
+			if s == shardId {
+				shardsPerNode[srcNodeId] = append(shardsPerNode[srcNodeId][:i], shardsPerNode[srcNodeId][i+1:]...)
+				break
+			}
+		}
+		nodeToShardCount[destNodeId] += 1
+		nodeToShardCount[srcNodeId] -= 1
+		destNode.freeEcSlot -= 1
+		ecNode.freeEcSlot += 1
+	}
+
+	return nil
+}
+
+// pickNodeForShardType selects a node that has room for more shards of a specific type
+func (ecb *ecBalancer) pickNodeForShardType(
+	nodes []*EcNode,
+	shardsPerNode map[string][]erasure_coding.ShardId,
+	maxPerNode int,
+	nodeToShardCount map[string]int,
+	antiAffinityNodes map[string]bool,
+) (*EcNode, error) {
+	selector := &twoPassSelector[*EcNode]{
+		candidates:         nodes,
+		shardsPerTarget:    shardsPerNode,
+		maxPerTarget:       maxPerNode,
+		targetToShardCount: nodeToShardCount,
+		antiAffinity:       antiAffinityNodes,
+		getKey: func(n *EcNode) string {
+			return n.info.Id
+		},
+		hasFreeSlots: func(n *EcNode) bool {
+			return n.freeEcSlot > 0
+		},
+		checkLimit: func(n *EcNode) bool {
+			// For EC shards, replica placement constraint only applies when SameRackCount > 0.
+			if ecb.replicaPlacement != nil && ecb.replicaPlacement.SameRackCount > 0 {
+				return nodeToShardCount[n.info.Id] < ecb.replicaPlacement.SameRackCount+1
+			}
+			return true
+		},
+	}
+
+	selected, err := selector.selectCandidate()
+	if err != nil {
+		return nil, errors.New("no node available for shard type balancing")
+	}
+	return selected, nil
+}

weed/shell/command_ec_common_avoid_test.go

@@ -0,0 +1,377 @@
+package shell
+
+import (
+	"testing"
+
+	"github.com/seaweedfs/seaweedfs/weed/pb/master_pb"
+	"github.com/seaweedfs/seaweedfs/weed/storage/erasure_coding"
+	"github.com/seaweedfs/seaweedfs/weed/storage/needle"
+	"github.com/seaweedfs/seaweedfs/weed/storage/super_block"
+	"github.com/seaweedfs/seaweedfs/weed/storage/types"
+)
+
+func TestPickRackForShardType_AntiAffinityRacks(t *testing.T) {
+	// Setup topology with 3 racks, each with 1 node, enough free slots
+	topo := &master_pb.TopologyInfo{
+		Id: "test_topo",
+		DataCenterInfos: []*master_pb.DataCenterInfo{
+			{
+				Id: "dc1",
+				RackInfos: []*master_pb.RackInfo{
+					buildRackWithEcShards("rack0", "node0:8080", 100, nil),
+					buildRackWithEcShards("rack1", "node1:8080", 100, nil),
+					buildRackWithEcShards("rack2", "node2:8080", 100, nil),
+				},
+			},
+		},
+	}
+
+	ecNodes, _ := collectEcVolumeServersByDc(topo, "", types.HardDriveType)
+	ecb := &ecBalancer{
+		ecNodes:  ecNodes,
+		diskType: types.HardDriveType,
+	}
+
+	racks := ecb.racks()
+	rackToShardCount := make(map[string]int)
+	shardsPerRack := make(map[string][]erasure_coding.ShardId)
+	maxPerRack := 2
+
+	// Case 1: Avoid rack0
+	antiAffinityRacks := map[string]bool{"rack0": true}
+
+	// Try multiple times to ensure randomness doesn't accidentally pass
+	for i := 0; i < 20; i++ {
+		picked, err := ecb.pickRackForShardType(racks, shardsPerRack, maxPerRack, rackToShardCount, antiAffinityRacks)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if picked == "rack0" {
+			t.Errorf("picked avoided rack rack0")
+		}
+	}
+
+	// Case 2: Fallback - avoid all racks
+	avoidAll := map[string]bool{"rack0": true, "rack1": true, "rack2": true}
+	picked, err := ecb.pickRackForShardType(racks, shardsPerRack, maxPerRack, rackToShardCount, avoidAll)
+	if err != nil {
+		t.Fatalf("fallback failed: %v", err)
+	}
+	if picked == "" {
+		t.Errorf("expected some rack to be picked in fallback")
+	}
+}
+
+func TestPickRackForShardType_EdgeCases(t *testing.T) {
+	t.Run("NoFreeSlots", func(t *testing.T) {
+		topo := &master_pb.TopologyInfo{
+			Id: "test_topo",
+			DataCenterInfos: []*master_pb.DataCenterInfo{
+				{
+					Id: "dc1",
+					RackInfos: []*master_pb.RackInfo{
+						buildRackWithEcShards("rack0", "node0:8080", 0, nil), // maxVolumes=0
+						buildRackWithEcShards("rack1", "node1:8080", 0, nil),
+					},
+				},
+			},
+		}
+
+		ecNodes, _ := collectEcVolumeServersByDc(topo, "", types.HardDriveType)
+		ecb := &ecBalancer{
+			ecNodes:  ecNodes,
+			diskType: types.HardDriveType,
+		}
+
+		racks := ecb.racks()
+		_, err := ecb.pickRackForShardType(racks, make(map[string][]erasure_coding.ShardId), 2, make(map[string]int), nil)
+		if err == nil {
+			t.Error("expected error when no free slots, got nil")
+		}
+	})
+
+	t.Run("AllRacksAtMaxCapacity", func(t *testing.T) {
+		topo := &master_pb.TopologyInfo{
+			Id: "test_topo",
+			DataCenterInfos: []*master_pb.DataCenterInfo{
+				{
+					Id: "dc1",
+					RackInfos: []*master_pb.RackInfo{
+						buildRackWithEcShards("rack0", "node0:8080", 100, nil),
+						buildRackWithEcShards("rack1", "node1:8080", 100, nil),
+					},
+				},
+			},
+		}
+
+		ecNodes, _ := collectEcVolumeServersByDc(topo, "", types.HardDriveType)
+		ecb := &ecBalancer{
+			ecNodes:  ecNodes,
+			diskType: types.HardDriveType,
+		}
+
+		racks := ecb.racks()
+		shardsPerRack := map[string][]erasure_coding.ShardId{
+			"rack0": {0, 1}, // 2 shards
+			"rack1": {2, 3}, // 2 shards
+		}
+		maxPerRack := 2
+
+		_, err := ecb.pickRackForShardType(racks, shardsPerRack, maxPerRack, make(map[string]int), nil)
+		if err == nil {
+			t.Error("expected error when all racks at max capacity, got nil")
+		}
+	})
+
+	t.Run("ReplicaPlacementLimit", func(t *testing.T) {
+		topo := &master_pb.TopologyInfo{
+			Id: "test_topo",
+			DataCenterInfos: []*master_pb.DataCenterInfo{
+				{
+					Id: "dc1",
+					RackInfos: []*master_pb.RackInfo{
+						buildRackWithEcShards("rack0", "node0:8080", 100, nil),
+						buildRackWithEcShards("rack1", "node1:8080", 100, nil),
+					},
+				},
+			},
+		}
+
+		ecNodes, _ := collectEcVolumeServersByDc(topo, "", types.HardDriveType)
+		rp, _ := super_block.NewReplicaPlacementFromString("012") // DiffRackCount = 1
+		ecb := &ecBalancer{
+			ecNodes:          ecNodes,
+			diskType:         types.HardDriveType,
+			replicaPlacement: rp,
+		}
+
+		racks := ecb.racks()
+		rackToShardCount := map[string]int{
+			"rack0": 1, // At limit
+			"rack1": 0,
+		}
+
+		picked, err := ecb.pickRackForShardType(racks, make(map[string][]erasure_coding.ShardId), 5, rackToShardCount, nil)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if picked != "rack1" {
+			t.Errorf("expected rack1 (not at limit), got %v", picked)
+		}
+	})
+
+	t.Run("PreferFewerShards", func(t *testing.T) {
+		topo := &master_pb.TopologyInfo{
+			Id: "test_topo",
+			DataCenterInfos: []*master_pb.DataCenterInfo{
+				{
+					Id: "dc1",
+					RackInfos: []*master_pb.RackInfo{
+						buildRackWithEcShards("rack0", "node0:8080", 100, nil),
+						buildRackWithEcShards("rack1", "node1:8080", 100, nil),
+						buildRackWithEcShards("rack2", "node2:8080", 100, nil),
+					},
+				},
+			},
+		}
+
+		ecNodes, _ := collectEcVolumeServersByDc(topo, "", types.HardDriveType)
+		ecb := &ecBalancer{
+			ecNodes:  ecNodes,
+			diskType: types.HardDriveType,
+		}
+
+		racks := ecb.racks()
+		shardsPerRack := map[string][]erasure_coding.ShardId{
+			"rack0": {0, 1}, // 2 shards
+			"rack1": {2},    // 1 shard
+			"rack2": {},     // 0 shards
+		}
+
+		// Should pick rack2 (fewest shards)
+		picked, err := ecb.pickRackForShardType(racks, shardsPerRack, 5, make(map[string]int), nil)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if picked != "rack2" {
+			t.Errorf("expected rack2 (fewest shards), got %v", picked)
+		}
+	})
+}
+
+func TestPickNodeForShardType_AntiAffinityNodes(t *testing.T) {
+	// Setup topology with 1 rack, 3 nodes
+	topo := &master_pb.TopologyInfo{
+		Id: "test_topo",
+		DataCenterInfos: []*master_pb.DataCenterInfo{
+			{
+				Id: "dc1",
+				RackInfos: []*master_pb.RackInfo{
+					{
+						Id: "rack0",
+						DataNodeInfos: []*master_pb.DataNodeInfo{
+							buildDataNode("node0:8080", 100),
+							buildDataNode("node1:8080", 100),
+							buildDataNode("node2:8080", 100),
+						},
+					},
+				},
+			},
+		},
+	}
+
+	ecNodes, _ := collectEcVolumeServersByDc(topo, "", types.HardDriveType)
+	ecb := &ecBalancer{
+		ecNodes:  ecNodes,
+		diskType: types.HardDriveType,
+	}
+
+	nodeToShardCount := make(map[string]int)
+	shardsPerNode := make(map[string][]erasure_coding.ShardId)
+	maxPerNode := 2
+
+	// Case 1: Avoid node0
+	antiAffinityNodes := map[string]bool{"node0:8080": true}
+
+	for i := 0; i < 20; i++ {
+		picked, err := ecb.pickNodeForShardType(ecNodes, shardsPerNode, maxPerNode, nodeToShardCount, antiAffinityNodes)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if picked.info.Id == "node0:8080" {
+			t.Errorf("picked avoided node node0")
+		}
+	}
+}
+
+func TestPickNodeForShardType_EdgeCases(t *testing.T) {
+	t.Run("NoFreeSlots", func(t *testing.T) {
+		topo := &master_pb.TopologyInfo{
+			Id: "test_topo",
+			DataCenterInfos: []*master_pb.DataCenterInfo{
+				{
+					Id: "dc1",
+					RackInfos: []*master_pb.RackInfo{
+						{
+							Id: "rack0",
+							DataNodeInfos: []*master_pb.DataNodeInfo{
+								buildDataNode("node0:8080", 0), // No capacity
+							},
+						},
+					},
+				},
+			},
+		}
+
+		ecNodes, _ := collectEcVolumeServersByDc(topo, "", types.HardDriveType)
+		ecb := &ecBalancer{
+			ecNodes:  ecNodes,
+			diskType: types.HardDriveType,
+		}
+
+		_, err := ecb.pickNodeForShardType(ecNodes, make(map[string][]erasure_coding.ShardId), 2, make(map[string]int), nil)
+		if err == nil {
+			t.Error("expected error when no free slots, got nil")
+		}
+	})
+
+	t.Run("ReplicaPlacementSameRackLimit", func(t *testing.T) {
+		topo := &master_pb.TopologyInfo{
+			Id: "test_topo",
+			DataCenterInfos: []*master_pb.DataCenterInfo{
+				{
+					Id: "dc1",
+					RackInfos: []*master_pb.RackInfo{
+						{
+							Id: "rack0",
+							DataNodeInfos: []*master_pb.DataNodeInfo{
+								buildDataNode("node0:8080", 100),
+								buildDataNode("node1:8080", 100),
+							},
+						},
+					},
+				},
+			},
+		}
+
+		ecNodes, _ := collectEcVolumeServersByDc(topo, "", types.HardDriveType)
+		rp, _ := super_block.NewReplicaPlacementFromString("021") // SameRackCount = 1
+		ecb := &ecBalancer{
+			ecNodes:          ecNodes,
+			diskType:         types.HardDriveType,
+			replicaPlacement: rp,
+		}
+
+		nodeToShardCount := map[string]int{
+			"node0:8080": 3, // Exceeds SameRackCount + 1
+			"node1:8080": 0,
+		}
+
+		picked, err := ecb.pickNodeForShardType(ecNodes, make(map[string][]erasure_coding.ShardId), 5, nodeToShardCount, nil)
+		if err != nil {
+			t.Fatalf("unexpected error: %v", err)
+		}
+		if picked.info.Id != "node1:8080" {
+			t.Errorf("expected node1 (not at limit), got %v", picked.info.Id)
+		}
+	})
+}
+
+func TestShardsByType(t *testing.T) {
+	vid := needle.VolumeId(123)
+
+	// Create mock nodes with shards
+	nodes := []*EcNode{
+		{
+			info: &master_pb.DataNodeInfo{
+				Id: "node1",
+				DiskInfos: map[string]*master_pb.DiskInfo{
+					string(types.HardDriveType): {
+						EcShardInfos: []*master_pb.VolumeEcShardInformationMessage{
+							{
+								Id:          uint32(vid),
+								EcIndexBits: uint32((1 << 0) | (1 << 1) | (1 << 10) | (1 << 11)), // data: 0,1 parity: 10,11
+							},
+						},
+					},
+				},
+			},
+			rack: "rack1",
+		},
+	}
+
+	t.Run("Standard10Plus4", func(t *testing.T) {
+		dataPerRack, parityPerRack := shardsByTypePerRack(vid, nodes, types.HardDriveType, 10)
+
+		if len(dataPerRack["rack1"]) != 2 {
+			t.Errorf("expected 2 data shards, got %d", len(dataPerRack["rack1"]))
+		}
+		if len(parityPerRack["rack1"]) != 2 {
+			t.Errorf("expected 2 parity shards, got %d", len(parityPerRack["rack1"]))
+		}
+	})
+
+	t.Run("NodeGrouping", func(t *testing.T) {
+		dataPerNode, parityPerNode := shardsByTypePerNode(vid, nodes, types.HardDriveType, 10)
+
+		if len(dataPerNode["node1"]) != 2 {
+			t.Errorf("expected 2 data shards on node1, got %d", len(dataPerNode["node1"]))
+		}
+		if len(parityPerNode["node1"]) != 2 {
+			t.Errorf("expected 2 parity shards on node1, got %d", len(parityPerNode["node1"]))
+		}
+	})
+}
+
+func buildDataNode(nodeId string, maxVolumes int64) *master_pb.DataNodeInfo {
+	return &master_pb.DataNodeInfo{
+		Id: nodeId,
+		DiskInfos: map[string]*master_pb.DiskInfo{
+			string(types.HardDriveType): {
+				Type:           string(types.HardDriveType),
+				MaxVolumeCount: maxVolumes,
+				VolumeCount:    0,
+			},
+		},
+	}
+}

weed/shell/command_volume_server_evacuate.go

@@ -227,7 +227,8 @@ func (c *commandVolumeServerEvacuate) moveAwayOneEcVolume(commandEnv *CommandEnv
 			}
 			vid := needle.VolumeId(ecShardInfo.Id)
 			// For evacuation, prefer same disk type but allow fallback to other types
-			destDiskId := pickBestDiskOnNode(emptyNode, vid, diskType, false)
+			// No anti-affinity needed for evacuation (dataShardCount=0)
+			destDiskId := pickBestDiskOnNode(emptyNode, vid, diskType, false, shardId, 0)
 			if destDiskId > 0 {
 				fmt.Fprintf(writer, "moving ec volume %s%d.%d %s => %s (disk %d)\n", collectionPrefix, ecShardInfo.Id, shardId, thisNode.info.Id, emptyNode.info.Id, destDiskId)
 			} else {