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.

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.

YAFFS

Hash Link

Slot = file id % 256

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 muliple inodes.

– dirent nodes: directory entries each holding a name and an inode number.

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.

Ext2/3File system layout

File system journaling

• 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.