avoid unexpected compact size (#5272)

https://github.com/seaweedfs/seaweedfs/issues/5215

weed/storage/volume_vacuum.go

@@ -423,7 +423,10 @@ func (v *Volume) copyDataBasedOnIndexFile(srcDatName, srcIdxName, dstDatName, da
 	if dstDatBackend, err = backend.CreateVolumeFile(dstDatName, preallocate, 0); err != nil {
 		return err
 	}
-	defer dstDatBackend.Close()
+	defer func() {
+		dstDatBackend.Sync()
+		dstDatBackend.Close()
+	}()
 
 	oldNm := needle_map.NewMemDb()
 	defer oldNm.Close()
@@ -484,7 +487,20 @@ func (v *Volume) copyDataBasedOnIndexFile(srcDatName, srcIdxName, dstDatName, da
 	if err != nil {
 		return err
 	}
-
+	dstDatSize, _, err := dstDatBackend.GetStat()
+	if err != nil {
+		return err
+	}
+	if v.nm.ContentSize() > v.nm.DeletedSize() {
+		expectedContentSize := v.nm.ContentSize() - v.nm.DeletedSize()
+		if expectedContentSize > uint64(dstDatSize) {
+			return fmt.Errorf("volume %s unexpected new data size: %d does not match size of content minus deleted: %d",
+				v.Id.String(), dstDatSize, expectedContentSize)
+		}
+	} else {
+		glog.Warningf("volume %s content size: %d less deleted size: %d, new size: %d",
+			v.Id.String(), v.nm.ContentSize(), v.nm.DeletedSize(), dstDatSize)
+	}
 	err = newNm.SaveToIdx(datIdxName)
 	if err != nil {
 		return err