The compressed ready-only-memory (ROM) file system (CramFS) has been widely used in Linux-based devices. However, it is a read-only filesystem which can not meet the requirement of todays smart mobile devices because most of them require updating the software on the air (OTA). This paper presents a design and implementation of write-enabled CramFS which only introduces minor architecture changes and very limited space cost. The performance benchmark with other popular write-enabled filesystems also proves the feasibility of this solution.