PosFFS2: 임베디드 리눅스 기반 스냅샷지원 새로운 플래시 메모리 파일 시스템

Abstract

Flash memory is widely used as a storage device for mobile devices because of its lightweight and non-volatility. Flash memory comes in two favors: NOR and NAND. NOR is byte-addressable and designed for code storage. NAND is block-oriented and designed for high capacity data storage. So NAND flash is more suitable flash memory for high performance mobile device than NOR. Mobile Device computing power and its storage capacity are rapidly increasing. Because existing Linux based NAND flash memory file system such as JFFS2 or YAFFS2 doesn’t exhibit satisfactory performance and doesn’t support snapshot function for data backup. In this paper, we present a new flash memory file system supporting snapshot in Embedded Linux. We design and implement prototype of our file system named PosFFS2 based on PosFFS. Unlike hard disk, NAND flash memory does not support in-place-update. In PosFFS2, a modified B-tree data structure is used for efficient block mapping and manages physical address of contiguous data on high capacity flash memory. By using the modified B-tree, PosFFS2 produces better performance and less memory usage than typical Flash memory file systems like JFFS2 or YAFFS2. As mobile devices are supporting various functions and growing its performance, user data is getting increase in mobile device’s storage. For this reason, PosFFS2 supports a snapshot for efficient user data backup. PosFFS2’s snapshot function is implemented using NAND flash memory’s out-place-update characteristic. When a user updates files, a snapshot image is automatically generated and managed by PosFFS2. Finally, PosFFS2 provides a garbage collection mechanism for snapshot images when storage’s free space is insufficient.