balance: extract the bytes-aware density metric to a shared package (#10174)

* balance: extract the bytes-aware density metric to weed/topology/balancer

The shell's volume.balance ranks servers by a bytes-aware density (used volume
equivalents over free capacity). Move that math into the shared balancer package
(VolumeDensity / DensityRatio / DensityNextRatio) so the maintenance worker can
adopt the same metric next. Shell behavior is unchanged.

* balance: rank a server with no free capacity as the fullest

DensityRatio/DensityNextRatio divided by capacity, so a server past its slot
limit (negative capacity) returned a negative ratio and sorted as the emptiest
under ascending consumers — the opposite of reality. Treat any non-positive
capacity (full, or overfull mid-run after receiving volumes) as the fullest
(+Inf) so it is a move source, never a target. Covered by negative-capacity and
ordering tests.

weed/shell/command_volume_balance.go

@@ -23,10 +23,7 @@ import (
 	"github.com/seaweedfs/seaweedfs/weed/storage/needle"
 )
 
-const (
-	thresholdVolumeSize      = 1.01
-	countZeroSelectedVolumes = 0.5
-)
+const thresholdVolumeSize = 1.01
 
 func init() {
 	Commands = append(Commands, &commandVolumeBalance{})
@@ -338,8 +335,7 @@ func capacityByMinVolumeDensity(diskType types.DiskType, volumeSizeLimitMb uint6
 		if volumeSizeLimitMb == 0 {
 			volumeSizeLimitMb = util.VolumeSizeLimitGB * util.KiByte
 		}
-		usedVolumeCount := volumeSizes / (volumeSizeLimitMb * util.MiByte)
-		return float64(diskInfo.MaxVolumeCount - int64(usedVolumeCount)), usedVolumeCount
+		return balancer.VolumeDensity(diskInfo.MaxVolumeCount, volumeSizes, volumeSizeLimitMb*util.MiByte)
 	}
 }
 
@@ -365,8 +361,7 @@ func capacityByActualDataUsage(diskType types.DiskType, volumeSizeLimitMb uint64
 		if volumeSizeLimitMb == 0 {
 			volumeSizeLimitMb = util.VolumeSizeLimitGB * util.KiByte
 		}
-		usedVolumeCount := volumeSizes / (volumeSizeLimitMb * util.MiByte)
-		return 1, usedVolumeCount
+		return 1, balancer.UsedVolumeEquivalents(volumeSizes, volumeSizeLimitMb*util.MiByte)
 	}
 }
 
@@ -399,22 +394,11 @@ func capacityByFreeVolumeCount(diskType types.DiskType) CapacityFunc {
 }
 
 func (n *Node) localVolumeDensityRatio(capacityFunc DensityFunc) float64 {
-	capacity, selectedVolumes := capacityFunc(n.info)
-	if capacity == 0 {
-		return 0
-	}
-	if selectedVolumes == 0 {
-		return countZeroSelectedVolumes / capacity
-	}
-	return float64(selectedVolumes) / capacity
+	return balancer.DensityRatio(capacityFunc(n.info))
 }
 
 func (n *Node) localVolumeDensityNextRatio(capacityFunc DensityFunc) float64 {
-	capacity, selectedVolumes := capacityFunc(n.info)
-	if capacity == 0 {
-		return 0
-	}
-	return float64(selectedVolumes+1) / capacity
+	return balancer.DensityNextRatio(capacityFunc(n.info))
 }
 
 func (n *Node) localVolumeRatio(capacityFunc CapacityFunc) float64 {