Matthias Weingart wrote:
On Fri, Jan 23, 2004 at 09:59:09AM -0500, J.C. Wren wrote:
Be aware that FAT is an extremely poor format for MMC if you plan to do
much writing. Compact flash cards have automagic wear leveling
algorithms that prevent premature wearing of sectors. MMC has no such
facility.
MMC does have wear leveling.
This is from MMCProdMn_5.1.pdf (sandisk):
"MultiMediaCards do a read after write under margin conditions to verify
that the data is written correctly. In the rare case that a bit is found to
be defective, MultiMediaCards replace this bad bit with a spare bit within
the sector header. If necessary, MultiMediaCards will even replace the
entire sector with a spare sector. This is completely transparent to the
host and does not consume any user data space."
M.
This is contrary to EVERYTHING I know about MMC cards, and my
experiences with them. I can't say I have more experience with any
particular brand of card than others, but perhaps this is a Sandisk
feature. One of the points of MMC was to make it smaller and cheaper
than CF, and one way was by simplifying the controller. I've also read
a number of reports of people trashing MMC cards using them for FAT file
systems that saw a lot of activity.
Pretty odd that they'd be able to do bit level remapping, or even byte
level. Sector level I could expect. I wonder how large the pool of
spare bits and sectors are.
--jc
