|
Yaffs (Yet Another Flash File System) was designed and written by Charles Manning, of Whitecliffs, New Zealand, for the company Aleph One. Yaffs1 was the first version of this file system and was designed for the then-current NAND chips with 512 byte page size (+ 16 byte spare (OOB;Out-Of-Band) area). Work started in 2002, and it was first released later that year. The initial work was sponsored by Toby Churchill Ltd, and Brightstar Engineering. These older chips also generally allow 2 or 3 write cycles per page, which YAFFS takes advantage of - i.e. dirty pages are marked by writing to a specific spare area byte. Newer NAND flash chips have larger pages, first 2K pages (+ 64 bytes OOB), later 4K, with stricter write requirements. Each page within an erase block (128 kilobytes) must be written to in sequential order, and each page must be written only once. YAFFS2 was designed to accommodate these newer chips. It was based on the YAFFS1 source code, with the major difference being that internal structures are not fixed to assume 512 byte sizing, and a block sequence number is placed on each written page. In this way older pages can be logically overwritten without violating the "write once" rule. It was released in late 2003. YAFFS is a robust log-structured file system that holds data integrity as a high priority. A secondary YAFFS goal is high performance. YAFFS will typically outperform most alternatives. It is also designed to be portable and has been used on Linux, WinCE, pSOS, eCos, ThreadX, and various special-purpose OSes. A variant 'YAFFS/Direct' is used in situations where there is no OS, embedded OSes or bootloaders: it has the same core filesystem but simpler interfacing to both the higher and lower level code and the NAND flash hardware. The YAFFS codebase is licensed both under the GPL and under per-product licenses available from Aleph One. ==YAFFS1== There is no special procedure to initialize a YAFFS filesystem beyond simply erasing the flash memory. When a bad block is encountered, YAFFS follows the smart media scheme of marking the fifth byte of the block's spare area. Blocks marked as such remain unallocated from then on. To write file data, YAFFS initially writes a whole page (chunk in YAFFS terminology) that describes the file metadata, such as timestamps, name, path, etc. The new file is assigned a unique object ID number; every data chunk within the file will contain this unique object ID within the spare area. YAFFS maintains a tree structure in RAM memory of the physical location of these chunks. When a chunk is no longer valid (the file is deleted, or parts of the file are overwritten), YAFFS marks a particular byte in the spare area of the chunk as ‘dirty’. When an entire block (32 pages) is marked as dirty, YAFFS can erase the block and reclaim the space. When the filesystem's free space is low, YAFFS consolidates a group of good pages onto a new block. YAFFS then reclaims the space used by dirty pages within each of the original blocks. When a YAFFS system mounts a NAND flash device, it must visit each block to check for valid data by scanning its spare area. With this information it then reconstitutes the memory-resident tree data structure. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「YAFFS」の詳細全文を読む スポンサード リンク
|