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Motivation

Motivation. SSDs will become the primary storage devices on PC, but NTFS behavior may not suitable to flash memory especially on metadata files.

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Motivation

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  1. Motivation • SSDs will become the primary storage devices on PC, but NTFS behavior may not suitable to flash memory especially on metadata files. • When considering the ram consumption, the mapping unit of the SSDs may become large; this situation will degrade the performance since there are a lot of small writes come from metadata files access. • Design a transformation scheme in FTL to make the NTFS behavior compatible to SSDs. • Log-based mechanism and sequential access on flash memory. • Fast mount time and easy to recover.

  2. YAFFS • Every file has a file id, which is 18 bits. • File data is stored in chunks, which is the same size as flash page. • Chunk 0 is used to store file header. • Each flash contains file id and chunk number, which is 20 bits, in the spare area. • Each page contains 2-bit serial number for crash-recovery.

  3. YAFFS Slot = file id % 256 Hash Link

  4. JFFS2 • JFFS is a log-structured file system for NOR flash memory. • Changes to files and directories are "logged" to flash in nodes, of which there are two types: • inodes: a header with file metadata, followed by the file data (if any). A file can correspond to mulipleinodes. • dirent nodes: directory entries each holding a name and an inode number.

  5. Disadvantages of YAFFS and JFFS • When mounting the device, each page need to be scanned to establish the whole file system information. • There are a lot of structure need to be maintained in the ram, such as inodes, yaffs_objects. • They are both designed for embedded system, so there are a lot of limitations in the file system, such as file size, number of files. • They all need to mark invalid pages, which is not allowed in MLC flash.

  6. Ext2/3 File system layout • When modifying a file, file system need to update group descriptor table, block bitmap, inode bitmap, inode table, and journaling blocks; all of these accesses are small writes and go fixed location in the file system. File system journaling

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