fix ec.balance failing to rebalance when all nodes share all volumes (#8796)

* fix ec.balance failing to rebalance when all nodes share all volumes (#8793)

Two bugs in doBalanceEcRack prevented rebalancing:

1. Sorting by freeEcSlot instead of actual shard count caused incorrect
   empty/full node selection when nodes have different total capacities.

2. The volume-level check skipped any volume already present on the
   target node. When every node has a shard of every volume (common
   with many EC volumes across N nodes with N shards each), no moves
   were possible.

Fix: sort by actual shard count, and use a two-pass approach - first
prefer moving shards of volumes not on the target (best diversity),
then fall back to moving specific shard IDs not yet on the target.

* add test simulating real cluster topology from issue #8793

Uses the actual node addresses and mixed max capacities (80 vs 33)
from the reporter's 14-node cluster to verify ec.balance correctly
rebalances with heterogeneous node sizes.

* fix pass comments to match 0-indexed loop variable

weed/shell/command_ec_common.go

@@ -1301,49 +1301,65 @@ func (ecb *ecBalancer) doBalanceEcRack(ecRack *EcRack) error {
 	hasMove := true
 	for hasMove {
 		hasMove = false
+		// Sort by shard count (ascending) so emptyNode has fewest shards, fullNode has most.
+		// Using freeEcSlot would be incorrect when nodes have different total capacities.
 		slices.SortFunc(rackEcNodes, func(a, b *EcNode) int {
-			return b.freeEcSlot - a.freeEcSlot
+			return ecNodeIdToShardCount[a.info.Id] - ecNodeIdToShardCount[b.info.Id]
 		})
 		emptyNode, fullNode := rackEcNodes[0], rackEcNodes[len(rackEcNodes)-1]
 		emptyNodeShardCount, fullNodeShardCount := ecNodeIdToShardCount[emptyNode.info.Id], ecNodeIdToShardCount[fullNode.info.Id]
 		if fullNodeShardCount > averageShardCount && emptyNodeShardCount+1 <= averageShardCount {
 
-			emptyNodeIds := make(map[uint32]bool)
+			// Build a map of volume ID -> shard bits on the empty node
+			emptyNodeVolumeShards := make(map[uint32]erasure_coding.ShardBits)
 			if emptyDiskInfo, found := emptyNode.info.DiskInfos[string(ecb.diskType)]; found {
 				for _, shards := range emptyDiskInfo.EcShardInfos {
-					emptyNodeIds[shards.Id] = true
+					emptyNodeVolumeShards[shards.Id] = erasure_coding.ShardBits(shards.EcIndexBits)
 				}
 			}
+
 			if fullDiskInfo, found := fullNode.info.DiskInfos[string(ecb.diskType)]; found {
-				for _, shards := range fullDiskInfo.EcShardInfos {
-					if _, found := emptyNodeIds[shards.Id]; found {
-						continue
-					}
-					si := erasure_coding.ShardsInfoFromVolumeEcShardInformationMessage(shards)
-					for _, shardId := range si.Ids() {
-						vid := needle.VolumeId(shards.Id)
-						// For balancing, strictly require matching disk type
-						// 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)
-						} else {
-							fmt.Printf("%s moves ec shards %d.%d to %s\n", fullNode.info.Id, shards.Id, shardId, emptyNode.info.Id)
+				// Pass 0: prefer moving shards of volumes not on the empty node (best diversity)
+				// Pass 1: move shards of shared volumes where the specific shard ID differs
+				for pass := 0; pass < 2 && !hasMove; pass++ {
+					for _, shards := range fullDiskInfo.EcShardInfos {
+						emptyBits, volumeOnEmpty := emptyNodeVolumeShards[shards.Id]
+						if pass == 0 && volumeOnEmpty {
+							continue // pass 0: skip volumes already on the empty node
 						}
-
-						err := moveMountedShardToEcNode(ecb.commandEnv, fullNode, shards.Collection, vid, shardId, emptyNode, destDiskId, ecb.applyBalancing, ecb.diskType)
-						if err != nil {
-							return err
+						if pass == 1 && !volumeOnEmpty {
+							continue // pass 1: only consider volumes already on the empty node
 						}
+						si := erasure_coding.ShardsInfoFromVolumeEcShardInformationMessage(shards)
+						for _, shardId := range si.Ids() {
+							if pass == 1 && emptyBits.Has(shardId) {
+								continue // Skip shard IDs already on the empty node
+							}
+							vid := needle.VolumeId(shards.Id)
+							// 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)
+							} else {
+								fmt.Printf("%s moves ec shards %d.%d to %s\n", fullNode.info.Id, shards.Id, shardId, emptyNode.info.Id)
+							}
 
-						ecNodeIdToShardCount[emptyNode.info.Id]++
-						ecNodeIdToShardCount[fullNode.info.Id]--
-						hasMove = true
-						break
+							err := moveMountedShardToEcNode(ecb.commandEnv, fullNode, shards.Collection, vid, shardId, emptyNode, destDiskId, ecb.applyBalancing, ecb.diskType)
+							if err != nil {
+								return err
+							}
+
+							ecNodeIdToShardCount[emptyNode.info.Id]++
+							ecNodeIdToShardCount[fullNode.info.Id]--
+							hasMove = true
+							break
+						}
+						if hasMove {
+							break
+						}
 					}
-					break
 				}
 			}
 		}

weed/shell/command_ec_test.go

@@ -285,3 +285,140 @@ func TestCommandEcBalanceMultipleVolumesEvenDistribution(t *testing.T) {
 		t.Logf("Volume %d - Data: %v, Parity: %v", vid, dataPerRack, parityPerRack)
 	}
 }
+
+// TestCommandEcBalanceAllNodesShareAllVolumes reproduces the scenario from issue #8793:
+// When every node has a shard of every volume, ec.balance was unable to move any shards
+// because it skipped volumes that already existed on the target node at the volume level.
+func TestCommandEcBalanceAllNodesShareAllVolumes(t *testing.T) {
+	// 4 nodes, all in same rack, 2 volumes with 14 shards each.
+	// Distribute shards so every node has shards of both volumes, but unevenly:
+	// dn1: vol1 shards 0-4, vol2 shards 0-4 => 10 shards
+	// dn2: vol1 shards 5-9, vol2 shards 5-9 => 10 shards
+	// dn3: vol1 shards 10-12, vol2 shards 10-12 => 6 shards
+	// dn4: vol1 shard 13, vol2 shard 13 => 2 shards
+	// Total: 28 shards, average = 7 per node
+	ecb := &ecBalancer{
+		ecNodes: []*EcNode{
+			newEcNode("dc1", "rack1", "dn1", 100).
+				addEcVolumeAndShardsForTest(1, "c1", []erasure_coding.ShardId{0, 1, 2, 3, 4}).
+				addEcVolumeAndShardsForTest(2, "c1", []erasure_coding.ShardId{0, 1, 2, 3, 4}),
+			newEcNode("dc1", "rack1", "dn2", 100).
+				addEcVolumeAndShardsForTest(1, "c1", []erasure_coding.ShardId{5, 6, 7, 8, 9}).
+				addEcVolumeAndShardsForTest(2, "c1", []erasure_coding.ShardId{5, 6, 7, 8, 9}),
+			newEcNode("dc1", "rack1", "dn3", 100).
+				addEcVolumeAndShardsForTest(1, "c1", []erasure_coding.ShardId{10, 11, 12}).
+				addEcVolumeAndShardsForTest(2, "c1", []erasure_coding.ShardId{10, 11, 12}),
+			newEcNode("dc1", "rack1", "dn4", 100).
+				addEcVolumeAndShardsForTest(1, "c1", []erasure_coding.ShardId{13}).
+				addEcVolumeAndShardsForTest(2, "c1", []erasure_coding.ShardId{13}),
+		},
+		applyBalancing: false,
+		diskType:       types.HardDriveType,
+	}
+
+	ecb.balanceEcVolumes("c1")
+	ecb.balanceEcRacks()
+
+	// Count total shards per node after balancing
+	for _, node := range ecb.ecNodes {
+		count := 0
+		if diskInfo, found := node.info.DiskInfos[string(types.HardDriveType)]; found {
+			for _, ecsi := range diskInfo.EcShardInfos {
+				count += erasure_coding.GetShardCount(ecsi)
+			}
+		}
+		// Average is 7, so all nodes should be at 7 (ceil(28/4) = 7)
+		if count > 7 {
+			t.Errorf("node %s has %d shards after balancing, expected at most 7", node.info.Id, count)
+		}
+		t.Logf("node %s: %d shards", node.info.Id, count)
+	}
+}
+
+// TestCommandEcBalanceIssue8793Topology simulates the real cluster from issue #8793:
+// 14 nodes (9 with max=80, 5 with max=33), all in one rack, with mixed capacities.
+// Each EC volume has 1 shard per node. Nodes have uneven totals (some have extra volumes).
+func TestCommandEcBalanceIssue8793Topology(t *testing.T) {
+	// Simulate 22 EC volumes across 14 nodes (each volume has 14 shards, 1 per node).
+	// Give nodes 0-3 an extra volume each (vol 23-26, all 14 shards) to create imbalance.
+	// Before balancing: nodes 0-3 have 22+14=36 shards each, nodes 4-13 have 22 shards each.
+	// Total = 4*36 + 10*22 = 144+220 = 364. Average = ceil(364/14) = 26.
+
+	type nodeSpec struct {
+		id      string
+		maxSlot int
+	}
+	nodes := []nodeSpec{
+		{"192.168.0.12:8332", 80}, {"192.168.0.12:8333", 80}, {"192.168.0.12:8334", 80},
+		{"192.168.0.12:8335", 80}, {"192.168.0.12:8336", 80}, {"192.168.0.12:8337", 80},
+		{"192.168.0.12:8338", 80}, {"192.168.0.12:8339", 80}, {"192.168.0.12:8340", 80},
+		{"192.168.0.12:8341", 33}, {"192.168.0.12:8342", 33}, {"192.168.0.12:8343", 33},
+		{"192.168.0.25:8350", 33}, {"192.168.0.25:8351", 33},
+	}
+
+	ecNodes := make([]*EcNode, len(nodes))
+	for i, ns := range nodes {
+		ecNodes[i] = newEcNode("home", "center", ns.id, ns.maxSlot)
+	}
+
+	// 22 shared volumes: each node gets exactly 1 shard (shard i for node i)
+	for vid := uint32(1); vid <= 22; vid++ {
+		for i := range ecNodes {
+			ecNodes[i].addEcVolumeAndShardsForTest(vid, "cldata", []erasure_coding.ShardId{erasure_coding.ShardId(i)})
+		}
+	}
+
+	// 4 extra volumes only on first 4 nodes (all 14 shards each) to create imbalance
+	for extra := uint32(0); extra < 4; extra++ {
+		vid := 23 + extra
+		nodeIdx := int(extra)
+		allShards := make([]erasure_coding.ShardId, 14)
+		for s := 0; s < 14; s++ {
+			allShards[s] = erasure_coding.ShardId(s)
+		}
+		ecNodes[nodeIdx].addEcVolumeAndShardsForTest(vid, "cldata", allShards)
+	}
+
+	ecb := &ecBalancer{
+		ecNodes:        ecNodes,
+		applyBalancing: false,
+		diskType:       types.HardDriveType,
+	}
+
+	// Log initial state
+	for _, node := range ecb.ecNodes {
+		count := 0
+		if diskInfo, found := node.info.DiskInfos[string(types.HardDriveType)]; found {
+			for _, ecsi := range diskInfo.EcShardInfos {
+				count += erasure_coding.GetShardCount(ecsi)
+			}
+		}
+		t.Logf("BEFORE node %s (max %d): %d shards", node.info.Id, node.freeEcSlot+count, count)
+	}
+
+	ecb.balanceEcVolumes("cldata")
+	ecb.balanceEcRacks()
+
+	// Verify: no node should exceed the average
+	totalShards := 0
+	shardCounts := make(map[string]int)
+	for _, node := range ecb.ecNodes {
+		count := 0
+		if diskInfo, found := node.info.DiskInfos[string(types.HardDriveType)]; found {
+			for _, ecsi := range diskInfo.EcShardInfos {
+				count += erasure_coding.GetShardCount(ecsi)
+			}
+		}
+		shardCounts[node.info.Id] = count
+		totalShards += count
+	}
+	avg := ceilDivide(totalShards, len(ecNodes))
+
+	for _, node := range ecb.ecNodes {
+		count := shardCounts[node.info.Id]
+		t.Logf("AFTER  node %s: %d shards (avg %d)", node.info.Id, count, avg)
+		if count > avg {
+			t.Errorf("node %s has %d shards, expected at most %d (avg)", node.info.Id, count, avg)
+		}
+	}
+}