support Fallocate on linux

weed/storage/volume_create.go

@@ -0,0 +1,17 @@
+// +build !linux
+
+package storage
+
+import (
+	"os"
+
+	"github.com/chrislusf/seaweedfs/weed/glog"
+)
+
+func createVolumeFile(fileName string, preallocate int64) (file *os.File, e error) {
+	file, e = os.OpenFile(fileName, os.O_RDWR|os.O_CREATE, 0644)
+	if preallocate > 0 {
+		glog.V(0).Infof("Preallocated disk space for %s is not supported", fileName)
+	}
+	return file, e
+}