On 12/5/2010 1:10 PM, Chuck Guzis wrote:
Okay, a faintly related question to this thing with SD
cards.
Are USB pen drives any better at leveling wear than SDHCs? The
reason I ask is that there's a USB 2.0 flash driver available for the
inexpensive AVR90USB162 chip that might also work as a SCSI-emulated
device. Any sort of buffering isn't possible, as the chip only has
512 bytes of SRAM. But there are many other inexpensive USB-capable
microcontrollers as well.
--Chuck
I had been wondering the same thing. I did a bit of internet searching
and found that it appears that the controller inside the USB stick does
do some degree of wear leveling. I thought the following URLs were
pretty interesting.
http://www.corsair.com/_faq/FAQ_flash_drive_wear_leveling.pdf
http://www.pcreview.co.uk/forums/thread-1941484.php
http://blog.ironkey.com/?p=595
I had not thought of the fact that wear leveling has security
implications as indicated here. Surely this is not an issue with
vintage systems though.
http://www.truecrypt.org/docs/?s=wear-leveling
Then I wondered about CF and SD, and found that they appear to be wear
leveled internally as well.
http://superuser.com/questions/17350/whats-the-life-expectancy-of-an-sd-card
I must say that all this makes me feel much better about their potential
use as hard drive substitutes in vintage systems. I also learned that
some of the USB drives have a RAM buffer (512 bytes or so), so one might
not even need to worry much about buffering data in a bridge device. I
do wonder what algorithm they use to decide when to write the RAM to
flash - probably varies some by manufacturer. I do think the bridge
should provide a "flash drive busy" indicator to discourage removal of
the flash device while a write is in progress. The built in LED in most
USB drives might suffice for them, but I don't believe I've ever seen
that in a CF or SD card.
Later,
Charlie C.