A note on licencing
-------------------
-YAFFS is available under the GPL and via alternative licensing
+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.
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).
+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.
+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
+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
+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.
-
+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
+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
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
+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.
+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.
+working back.
YAFFS tags in YAFFS1 mode:
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
+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
+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.
+when free space is tight.
Flash writing
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.
+are free or become free, not blocks which never change.
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.
+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.
-----------
A bootloader using YAFFS needs to know how MTD is laying out the OOB
-so that it can skip bad blocks.
+so that it can skip bad blocks.
YAFFS Tracing
-------------
projects / yaffs2.git / blobdiff