need arguments to use JFFS2

need arguments to use JFFS2

[Chantara Thlang]

Hi everybody,

In the web, I am not sure to see an article which clearly express the
advantage of JFFS2.
Can we find any comparison document between all existing Flash FS ? Does it
exist ??
Please could someone give me some arguments to use JFFS2 instead of all
others Flash FS such as CRAMFS ?

One of the reason could it be in the Flash management : optimization ?

Thanks in advance,

Chantara

Re: need arguments to use JFFS2

[Wolfgang Denk]

Dear Chantara,

in message <049e01c29d0f$b3c52ff0$221faf9d@iphase.com> you wrote:
> 
> In the web, I am not sure to see an article which clearly express the
> advantage of JFFS2.

It makes little  sense  to  dicuss  "advantages"  without  specifying
requirements.

> Please could someone give me some arguments to use JFFS2 instead of all
> others Flash FS such as CRAMFS ?

One significant advantage is that JFFS2 provides a  _writable_  flash
filesystem,  while cramfs is read-only. But as stated before: this is
an advantage only when you need a writable filesystem.

> One of the reason could it be in the Flash management : optimization ?

Yes.

Best regards,

Wolfgang Denk

-- 
Software Engineering:  Embedded and Realtime Systems,  Embedded Linux
Phone: (+49)-8142-4596-87  Fax: (+49)-8142-4596-88  Email: wd@denx.de
Every program has at least one bug and can be shortened by  at  least
one instruction - from which, by induction, one can deduce that every
program can be reduced to one instruction which doesn't work.

Re: need arguments to use JFFS2

[David Woodhouse]

thlang@iphase.com said:
> Please could someone give me some arguments to use JFFS2 instead of
> all others Flash FS such as CRAMFS ? 

cramfs is read-only. If you don't want to write to your file system, you 
should probably be using it instead of JFFS2. 

The only other flash file system for Linux is YAFFS, for use on NAND flash.
I don't know as much about YAFFS as I should -- the choice between YAFFS and
JFFS2 on NAND is basically a tradeoff between the improved speed and mount
time of YAFFS, and the increased storage capacity which JFFS2 compression 
gives, I think. Charles will probably offer more enlightenment.

Other than that, you only have the brain-dead 'translation layer' crap which
is a kind of journalling pseudo-filesystem used to emulate a virtual hard
drive, on which you need to use put a 'normal' journalling file system. This
two-layer approach made some sense in the days of DOS, where you could just
provide an INT 13h Disk BIOS IRQ handler and pretend to be a normal hard
drive to DOS, but it makes far less sense now. You end up with power-loss
safety being implemented in _both_ your 'normal' file system and the
underlying pseudo-fs, the pseudo-fs has no way of knowing which blocks on
the virtual hard drive are marked as 'free' by the higher file system and so
it keep garbage-collecting stale data from one place on the flash to
another, and at least at the moment, it means you don't get compression --
although that's not a fundamental design flaw like the others. 


--
dwmw2

Re: need arguments to use JFFS2

[Charles Manning]

It is virtually impossible to make any firm statements without understanding 
your needs a bit better.

A few others have already covered cramfs and JFFSx. I can tell you a bit 
about YAFFS.

YAFFS only works with NAND flash. YAFFS does not provide compression (like 
JFFS does). YAFFS is faster than JFFS2 in most situations and uses less RAM.
Mileage will vary though. Compression is not necessarily a huge advantage for 
JFFSx. Some people loop-mount a cramfs image stored on YAFFS to get the 
benefits of compression as well as the faster speed for other files.

If you have NAND, then most likely YAFFS will serve you better for larger 
flash arrays (say 16MB and bigger) and JFFS2 for smaller arrays.

YAFFS will do nothing for you if you only have NOR flash.

If you're designing a new board then consider including NAND flash for 
storage (rather than NOR). NAND is cheaper, smaller (physically), bigger (in 
bytes) and faster. You can then use YAFFS or JFFS2 (even both - by 
partitioning the device).


-- Charles