NAND vs SD on GTA02

[Gennady Kupava]

Hi, Michael,

В Сбт, 24/09/2011 в 00:42 +0000, Michael Sokolov пишет:
> Hello again,
> 
> As I was browsing through the Om community documentation, I saw a note
> somewhere (I think it was somewhere in the wiki, but I have a hard time
> finding it again) saying that the native NAND flash on the GTA02 should
> not be used for anything but the bootloader, and that the OS/distro
> should live on the micro-SD card instead.  No explanation was given as
> to why.  I wonder, would anyone here happen to know the rationale for
> that recommendation?  And does the Om community actually follow that
> recommendation?  Where do most GTA02 users keep their OS/distro: NAND
> or micro-SD?

Community is set of independent people, they may have opposite opinions
on some questions, and someone may write his opinion onto wiki. Others
may ignore it or even just do not want to contradict person who wrote
it. This way you may get anything on wiki. Personally I prefer to add
exploration :)

Personally I were using mixed NAND/sd environment - just /usr and /home
moved to sd. NAND is 100% reliable and much faster (especially with
UBI), so it is worth using it.

> Unless I'm missing an elephant somewhere, my common sense suggests that
> JFFS2 on the native NAND ought to be much more convenient: SD is a
> serial bus, and it hangs off the Glamo on the GTA02, whereas the native
> NAND is, well, native, it's right there inside the MCU.  It also seems
> to me that the native NAND ought to be an outright win in terms of power
> management: NAND is powered from the IO_1V8 rail which is fed from a
> switching converter in the PMIC, whereas SD is powered via an HCLDO at
> 3.3 V.  The IO_1V8 supply has to stay on all the time anyway as it keeps
> the SDRAM in self-refresh, so whatever tiny idle current the NAND draws
> is going to be the same whether one uses it to store a file system or
> not.  But for the SD one needs 3.3 instead of 1.8 V, the linear regulator
> is inherently more wasteful than a switcher, and what happens when the
> battery voltage falls below 3.3 V?  If I've read the PMIC datasheet
> correctly, one ought to be able to keep running a GTA02 on battery until
> it goes down to almost 2.8 V: I reason it ought to be doable with NAND,
> but not with SD.

Interesting point. I noticed that all my last GSM failures with GTA were
related to simple fact that CPU itself were working, but GSM do not
because simple battery discharge. 

May be it is possible to get similar problems with glamo? May be all
glamo problems related to power supply? We have very strange glamo init
procedure on boot.

> The only truly valid reason I've been able to come up with for using SD
> instead of NAND is flash wear concerns.  As all good embedded system
> engineers know, NAND flash is finicky: you've got read disturb, program
> disturb, and a limited number of erase cycles in each block.  Of course
> the actual physics of NAND flash is exactly the same whether you use it
> directly or if it's hidden from you behind an interface like SD.  The
> actual "magic smoke" in those SD cards, USB sticks etc is the exact same
> NAND which people dislike using directly; the only difference is that
> with native NAND you as a Linux developer get to implement the necessary
> algorithms yourself (JFFS2 et al) in a fully visible, free / open source
> manner, whereas with SD cards etc someone else has done that work, in
> the form of a closed source circuit which you have no visibility into.
> I tend to agree with Linux MTD maintainer dwmw2's preference of doing it
> all yourself with JFFS2 et al.



> 
> Of course the underlying problem of flash wear exhaustion remains there:
> no matter how good your flash management algorithms are, eventually the
> available number of erase/program cycles will be exhausted for more
> blocks than the maximum number of bad blocks one can tolerate.  What do
> we do then?  This concern is the ONLY area where I can see an advantage
> with SD: once the micro-SD card's flash wear cycles have been exhausted
> (forgetting for the moment that you can't actually depend on the card
> reporting this condition to you), you simply throw that card in the
> trash and get a fresh one.  OTOH, if you exhaust the available erase/
> program cycles for the NAND flash inside that oddball Samsung chip on
> the GTA02, getting a replacement chip and getting it swapped out on the
> board would be problematic.
Right, problem is not in layers, but in the fact that internal one is
not interchangeable.

> Hence my question to the community: is the flash wear-out concern I've
> just outlined the primary reason for the recommendation of using SD
> instead of NAND to hold the OS/distro, or was that recommendation driven
> by some other, completely unrelated concerns?  Has anyone here actually
> exhaused the available number of flash erase/program cycles, on any HW
> platform, ever?  (I personally haven't, in my ~10 y of doing embedded
> Linux work on various systems: I know from the physics and from device
> datasheets that the number is finite, I just haven't had a personal
> experience exhausting it.)

Personally mine shown no signs of wearing out. I were using it quite
heavily with qtmoko for several years.

> Just trying to figure out whether I should use NAND or SD to store the
> OS/distro on my GTA02 (when I get it), and have a rationale for that
> choice.

Use NAND. If it wears out in few years, you can just buy one more GTA02
cheap as dirt. Also you can switch to uSD at any time, so if you NAND
will fail, you can just start using uSD.

> 
> And if I do end up wearing out the NAND flash in the Samsung ARM chip on
> my GTA02...
> 
> Dr. H. Nikolaus Schaller <hns at goldelico.com> wrote:
> 
> > For us, the GTA02 devices have mostly become sources for plastic parts [...]
> 
> So when you cannibalize a good GTA02 for its plastic parts (for use with
> a GTA04 board or whatever), what happens to the good GTA02 PCBA?  I hope
> you don't throw them out, do you?  Perhaps those boards could be made
> available to those hapless GTA02 users who have worn out their NAND
> flash?  Developing a new from-scratch distro generally involves a lot
> more reloading of the flash than a normal user would do...

I heard of nobody with weared-out flash so far, so no need to worry i
think.

> And while we are on that subject, I wonder, how many of these new-in-box
> GTA02 devices are still left?  At what rate is this legacy stock being
> depleted?  (Unless you consider that a business secret, of course...
> But methinks the community has a right to know.)
> 

I hope anyone interested can buy GTA04 soon, so no need to worry about
GTA02 stock. I guess you'll be able to find used one if you need.

Gennady.