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+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
+-------------