[Triumf-linux-managers] TRIUMF customizations to Scientific Linux distributions

[Andrew Daviel]

On Thu, 8 Jan 2009, Rod Nussbaumer wrote:

> Andrew Daviel wrote:
>> On Thu, 8 Jan 2009, Rod Nussbaumer wrote:
>> 
>>> Recently, I've been working on cross-development of Linux applications for 
>>> an ARM CPU based single board computer. The target SBC uses a SD flash 
>>> memory for storage and as a boot media. To prevent wear and tear from 
>>> write cycles, the SD flash memory uses a wear-leveling filesystem, JFS.
>> 
>> I believe that is JFFS2, not the IBM journalling filesystem "JFS"
>
> That's what I first thought, too. Not so...
>> 
>> My Nokia tablet mounts the / partition using JFFS2, but the SD cards use 
>> SCSI emulation and appear as SCSI disks. On which one can use VFAT 
>> (default), ext2/ext3 etc. The emulation layer takes care of the wear 
>> levelling.
>
> I've never heard that before. What purpose do specific wear-leveling 
> filesystems serve, then? Perhaps the vendor (Technologic Systems 
> <http://www.embeddedarm.com>) chose JFS for another good reason.

http://en.wikipedia.org/wiki/Wear_levelling

Bits in flash memory can only be written so many times before they stop
working. The directory nodes in a normal FS like EXT2 (or, AFAIK, JFS) 
get a hammering as the directory is rewritten every time a file is 
accessed. JFFS attempts to get around this by using different areas of 
the chip each time. There is a jffs2 module in the Linux kernel tree.

http://lists.maemo.org/pipermail/maemo-developers/2008-May/051353.html 
"SD cards have a controller inside that hides the access to the raw flash 
and deals with things like write levelling, that JFFS2 would otherwise 
have to do in software"

(The Nokia N8* series are also ARM based. There is quite a nice 
cross-development environment using gcc and emulation etc.)

-- 
Andrew Daviel, TRIUMF, Canada
Tel. +1 (604) 222-7376  (Pacific Time)
Network Security Manager