[Yaffs] Why does YAFFS skip the first block?

[Charles Manning]

On Wednesday 22 June 2005 15:09, Coywolf Qi Hunt wrote:
> On 6/22/05, Charles Manning <manningc2@actrix.gen.nz> wrote:
> <snip>
>
> > There is nothing technically wrong with using block zero from a
> > memory/NAND perspective.  At the end of the day, file system integrit=
y is
> > governed by the performance of the worst block and not the best block=
.
> > Block zero is only guaranteed good at the time of shipping and can go=
 bad
> > with time.  Also, many/most systems use block zero for some other pur=
pose
> > (eg. boot data or bad block marker). Thus, relying on chunk zero bein=
g
> > good is pointless.
>
> Is it that the code in the block 0 can be executed in place for
> booting?  We may consider supporting partitions.


Some device (eg. Samsung) support loading from block zero.
>
> As to bad block marker issue, using a fixed block for that purpose is
> against wear leveling and not reliable.  Bad block information always
> can be gathered through oob area.

This is not always true.  Some devices have fixed bytes in the OOB area f=
or=20
marking the bad blocks. This is what YAFFS1 expects.

Some devices, particularly some of the newer 2k page devices, have differ=
ent=20
bad block marking strategies. In some of these, any non-FF blocks should =
be=20
considered bad. For these you need to build some sort of bad block table =
and=20
keep that somewhere.  Some people choose to use block zero for this purpo=
se,=20
because it is guaranteed good and therefore an easy place to store the ta=
ble.


In general, it would be better for YAFFS to allow block zero to be usable=
 by=20
YAFFS.  At the time the YAFFS code was written, I made the decision not t=
o=20
use block zero and these points were merely what I considered as part of =
the=20
rationale behind skipping block zero. To work around this is not very=20
difficult outside of YAFFS. I will likely, at some time, integrate the=20
workaround into YAFFS so that this all happens transparently inside YAFFS=
 and=20
thus make everybody happy.

-- CHarles