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diff --git a/fs/yaffs2/README-linux b/fs/yaffs2/README-linux new file mode 100644 index 0000000..589ae8d --- /dev/null +++ b/fs/yaffs2/README-linux @@ -0,0 +1,201 @@ +Welcome to YAFFS, the first file system developed specifically for NAND flash. + +It is now YAFFS2 - original YAFFS (AYFFS1) only supports 512-byte page +NAND and is now deprectated. YAFFS2 supports 512b page in 'YAFFS1 +compatibility' mode (CONFIG_YAFFS_YAFFS1) and 2K or larger page NAND +in YAFFS2 mode (CONFIG_YAFFS_YAFFS2). + + +A note on licencing +------------------- +YAFFS is available under the GPL and via alternative licensing +arrangements with Aleph One. If you're using YAFFS as a Linux kernel +file system then it will be under the GPL. For use in other situations +you should discuss licensing issues with Aleph One. + + +Terminology +----------- +Page - NAND addressable unit (normally 512b or 2Kbyte size) - can + be read, written, marked bad. Has associated OOB. +Block - Eraseable unit. 64 Pages. (128K on 2K NAND, 32K on 512b NAND) +OOB - 'spare area' of each page for ECC, bad block marked and YAFFS + tags. 16 bytes per 512b - 64 bytes for 2K page size. +Chunk - Basic YAFFS addressable unit. Same size as Page. +Object - YAFFS Object: File, Directory, Link, Device etc. + +YAFFS design +------------ + +YAFFS is a log-structured filesystem. It is designed particularly for +NAND (as opposed to NOR) flash, to be flash-friendly, robust due to +journalling, and to have low RAM and boot time overheads. File data is +stored in 'chunks'. Chunks are the same size as NAND pages. Each page +is marked with file id and chunk number. These marking 'tags' are +stored in the OOB (or 'spare') region of the flash. The chunk number +is determined by dividing the file position by the chunk size. Each +chunk has a number of valid bytes, which equals the page size for all +except the last chunk in a file. + +File 'headers' are stored as the first page in a file, marked as a +different type to data pages. The same mechanism is used to store +directories, device files, links etc. The first page describes which +type of object it is. + +YAFFS2 never re-writes a page, because the spec of NAND chips does not +allow it. (YAFFS1 used to mark a block 'deleted' in the OOB). Deletion +is managed by moving deleted objects to the special, hidden 'unlinked' +directory. These records are preserved until all the pages containing +the object have been erased (We know when this happen by keeping a +count of chunks remaining on the system for each object - when it +reaches zero the object really is gone). + +When data in a file is overwritten, the relevant chunks are replaced +by writing new pages to flash containing the new data but the same +tags. + +Pages are also marked with a short (2 bit) serial number that +increments each time the page at this position is incremented. The +reason for this is that if power loss/crash/other act of demonic +forces happens before the replaced page is marked as discarded, it is +possible to have two pages with the same tags. The serial number is +used to arbitrate. + +A block containing only discarded pages (termed a dirty block) is an +obvious candidate for garbage collection. Otherwise valid pages can be +copied off a block thus rendering the whole block discarded and ready +for garbage collection. + +In theory you don't need to hold the file structure in RAM... you +could just scan the whole flash looking for pages when you need them. +In practice though you'd want better file access times than that! The +mechanism proposed here is to have a list of __u16 page addresses +associated with each file. Since there are 2^18 pages in a 128MB NAND, +a __u16 is insufficient to uniquely identify a page but is does +identify a group of 4 pages - a small enough region to search +exhaustively. This mechanism is clearly expandable to larger NAND +devices - within reason. The RAM overhead with this approach is approx +2 bytes per page - 512kB of RAM for a whole 128MB NAND. + +Boot-time scanning to build the file structure lists only requires +one pass reading NAND. If proper shutdowns happen the current RAM +summary of the filesystem status is saved to flash, called +'checkpointing'. This saves re-scanning the flash on startup, and gives +huge boot/mount time savings. + +YAFFS regenerates its state by 'replaying the tape' - i.e. by +scanning the chunks in their allocation order (i.e. block sequence ID +order), which is usually different form the media block order. Each +block is still only read once - starting from the end of the media and +working back. + +YAFFS tags in YAFFS1 mode: + +18-bit Object ID (2^18 files, i.e. > 260,000 files). File id 0- is not + valid and indicates a deleted page. File od 0x3ffff is also not valid. + Synonymous with inode. +2-bit serial number +20-bit Chunk ID within file. Limit of 2^20 chunks/pages per file (i.e. + > 500MB max file size). Chunk ID 0 is the file header for the file. +10-bit counter of the number of bytes used in the page. +12 bit ECC on tags + +YAFFS tags in YAFFS2 mode: + 4 bytes 32-bit chunk ID + 4 bytes 32-bit object ID + 2 bytes Number of data bytes in this chunk + 4 bytes Sequence number for this block + 3 bytes ECC on tags + 12 bytes ECC on data (3 bytes per 256 bytes of data) + + +Page allocation and garbage collection + +Pages are allocated sequentially from the currently selected block. +When all the pages in the block are filled, another clean block is +selected for allocation. At least two or three clean blocks are +reserved for garbage collection purposes. If there are insufficient +clean blocks available, then a dirty block ( ie one containing only +discarded pages) is erased to free it up as a clean block. If no dirty +blocks are available, then the dirtiest block is selected for garbage +collection. + +Garbage collection is performed by copying the valid data pages into +new data pages thus rendering all the pages in this block dirty and +freeing it up for erasure. I also like the idea of selecting a block +at random some small percentage of the time - thus reducing the chance +of wear differences. + +YAFFS is single-threaded. Garbage-collection is done as a parasitic +task of writing data. So each time some data is written, a bit of +pending garbage collection is done. More pages are garbage-collected +when free space is tight. + + +Flash writing + +YAFFS only ever writes each page once, complying with the requirements +of the most restricitve NAND devices. + +Wear levelling + +This comes as a side-effect of the block-allocation strategy. Data is +always written on the next free block, so they are all used equally. +Blocks containing data that is written but never erased will not get +back into the free list, so wear is levelled over only blocks which +are free or become free, not blocks which never change. + + + +Some helpful info +----------------- + +Formatting a YAFFS device is simply done by erasing it. + +Making an initial filesystem can be tricky because YAFFS uses the OOB +and thus the bytes that get written depend on the YAFFS data (tags), +and the ECC bytes and bad block markers which are dictated by the +hardware and/or the MTD subsystem. The data layout also depends on the +device page size (512b or 2K). Because YAFFS is only responsible for +some of the OOB data, generating a filesystem offline requires +detailed knowledge of what the other parts (MTD and NAND +driver/hardware) are going to do. + +To make a YAFFS filesystem you have 3 options: + +1) Boot the system with an empty NAND device mounted as YAFFS and copy + stuff on. + +2) Make a filesystem image offline, then boot the system and use + MTDutils to write an image to flash. + +3) Make a filesystem image offline and use some tool like a bootloader to + write it to flash. + +Option 1 avoids a lot of issues because all the parts +(YAFFS/MTD/hardware) all take care of their own bits and (if you have +put things together properly) it will 'just work'. YAFFS just needs to +know how many bytes of the OOB it can use. However sometimes it is not +practical. + +Option 2 lets MTD/hardware take care of the ECC so the filesystem +image just had to know which bytes to use for YAFFS Tags. + +Option 3 is hardest as the image creator needs to know exactly what +ECC bytes, endianness and algorithm to use as well as which bytes are +available to YAFFS. + +mkyaffs2image creates an image suitable for option 3 for the +particular case of yaffs2 on 2K page NAND with default MTD layout. + +mkyaffsimage creates an equivalent image for 512b page NAND (i.e. +yaffs1 format). + +Bootloaders +----------- + +A bootloader using YAFFS needs to know how MTD is laying out the OOB +so that it can skip bad blocks. + +YAFFS Tracing +------------- |