perf(weed/topology): size the new-volume list from the actual delta (#10613)

A reconnecting volume server reports every volume it has as new, so newVolumes
grew from nil to one entry per volume, reallocating and copying its way there.
Sizing it to len(actualVolumes) instead would allocate the whole list on every
steady-state heartbeat, where nothing is new.

After the deletion pass everything left on the node is also in this heartbeat,
so the difference is exactly what the node is about to gain: all of them on a
reconnect, none in steady state.

First registration of 550k volumes  1041.7 MB -> 667.4 MB

weed/topology/data_node.go

@@ -86,6 +86,7 @@ func (dn *DataNode) UpdateVolumes(actualVolumes []storage.VolumeInfo) (newVolume
 	dn.Lock()
 	defer dn.Unlock()
 
+	keptCount := 0
 	for _, c := range dn.children {
 		disk := c.(*Disk)
 		for _, v := range disk.RemoveVolumesNotIn(actualVolumeIds) {
@@ -102,6 +103,13 @@ func (dn *DataNode) UpdateVolumes(actualVolumes []storage.VolumeInfo) (newVolume
 			}
 			disk.UpAdjustDiskUsageDelta(types.ToDiskType(v.DiskType), deltaDiskUsage)
 		}
+		keptCount += disk.VolumeCount()
+	}
+	// Everything still on the node is also in this heartbeat, so the remainder
+	// is what the node is about to gain. A steady-state heartbeat gains nothing
+	// and must not allocate here; a reconnecting server gains all of them.
+	if addedCount := len(actualVolumes) - keptCount; addedCount > 0 {
+		newVolumes = make([]storage.VolumeInfo, 0, addedCount)
 	}
 	for _, v := range actualVolumes {
 		isNew, isChanged := dn.doAddOrUpdateVolume(v)

weed/topology/disk.go

@@ -212,6 +212,12 @@ func (d *Disk) GetVolumes() (ret []storage.VolumeInfo) {
 	return ret
 }
 
+func (d *Disk) VolumeCount() int {
+	d.RLock()
+	defer d.RUnlock()
+	return len(d.volumes)
+}
+
 // RemoveVolumesNotIn drops the volumes whose ids are absent from keep and
 // returns them, so a heartbeat can be diffed without first copying the whole
 // volume map out.