local: preallocate files on linux with fallocate(2)

backend/local/preallocate_other.go

@@ -1,4 +1,4 @@
-//+build !windows
+//+build !windows,!linux
 
 package local
 

backend/local/preallocate_unix.go

@@ -0,0 +1,22 @@
+//+build linux
+
+package local
+
+import (
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+// preAllocate the file for performance reasons
+func preAllocate(size int64, out *os.File) error {
+	if size <= 0 {
+		return nil
+	}
+	err := unix.Fallocate(int(out.Fd()), unix.FALLOC_FL_KEEP_SIZE, 0, size)
+	// FIXME could be doing something here
+	// if err == unix.ENOSPC {
+	// 	log.Printf("No space")
+	// }
+	return err
+}