MTD_PHRAM - what filesystem to use?

MTD_PHRAM - what filesystem to use?

[Sam Ravnborg]

Hi Jörn and other MTD folks.

At my new job we are planning to start usign a ARM9 based
design where we need to store data persistently.

Since the data are updated several times / minute the FLASH
devices are not suitable and therefore alternative solutions
are being looked at.

The most promising solutions semms to use battery
backed-up RAM.
>From Linux we could memorymap this area but using a filesystem
gives all sorts of extra bonusses so this is preferred.

I looked shortly at PRAMFS that seems to do the trick but PRMFS
seems unmaintained and not merged.
Then I stumbled over PHRAM.
I and pretty clear upon the basic parts lettign the RAM look
like any other FLASH device.
But then the question popped up.

What is the best filesystem to use on top of a PHRAM based
MTD device?
My first two ideas were to use either JFFS2 or ext2
(the latter with the blocklayer emulation).
But reading the documentation both looks like overkill.

The requirement so far is to gain maximum bebefit of the
avalable 100 kbytes of RAM.
There will be a limited number of files (< 500).

Any inputs?

	Sam

Re: MTD_PHRAM - what filesystem to use?

[Jörn Engel]

On Sun, 11 March 2007 21:05:08 +0100, Sam Ravnborg wrote:
> 
> At my new job we are planning to start usign a ARM9 based
> design where we need to store data persistently.
> 
> Since the data are updated several times / minute the FLASH
> devices are not suitable and therefore alternative solutions
> are being looked at.
> 
> The most promising solutions semms to use battery
> backed-up RAM.
> >From Linux we could memorymap this area but using a filesystem
> gives all sorts of extra bonusses so this is preferred.
> 
> I looked shortly at PRAMFS that seems to do the trick but PRMFS
> seems unmaintained and not merged.
> Then I stumbled over PHRAM.
> I and pretty clear upon the basic parts lettign the RAM look
> like any other FLASH device.
> But then the question popped up.
> 
> What is the best filesystem to use on top of a PHRAM based
> MTD device?
> My first two ideas were to use either JFFS2 or ext2
> (the latter with the blocklayer emulation).
> But reading the documentation both looks like overkill.
> 
> The requirement so far is to gain maximum bebefit of the
> avalable 100 kbytes of RAM.
> There will be a limited number of files (< 500).

If you decide on battery-backed RAM, the PHRAM/JFFS2 combo would be the
best existing solution.  Alternatively you could write a custom
filesystem.

With JFFS2 you should carefully decide on a suitable erasesize.  The
JFFS2 overhead can be minimized by tuning this number.  Most likely
something small, 4KiB or 8KiB is optimal.

Yet another alternative would be to use flash.  The math is quite
simple.  Example calculation:
Update interval: 10s
Updated data size: 10kB
Required device lifetime: 10a
JFFS2 overhead: 10%
Flash durability: 100k erases

Write rate is 10kB / 10s * 1.1 = 1.1kB/s.
Total written data in 10a is: 1.1kB/s * 31536000s/a * 10a = 346GB
Required flash size: 346GB / 100k = 3468kB

So with the example numbers, you would only need a 4MiB flash chip.
Those should be rather cheap.  But it remains your duty to redo the
calculation with correct numbers and check for any mistakes I may have
made.

Also make sure that your supplier guarantees 100k erases on the flash.
At 10k erases, you would need a device 10x bigger.  1M erases, if
available, are even better.

Jörn

-- 
You can't tell where a program is going to spend its time. Bottlenecks
occur in surprising places, so don't try to second guess and put in a
speed hack until you've proven that's where the bottleneck is.
-- Rob Pike

Re: MTD_PHRAM - what filesystem to use?

[Sam Ravnborg]

On Sun, Mar 11, 2007 at 09:31:22PM +0100, Jörn Engel wrote:
> On Sun, 11 March 2007 21:05:08 +0100, Sam Ravnborg wrote:
> > 
> > At my new job we are planning to start usign a ARM9 based
> > design where we need to store data persistently.
> > 
> > Since the data are updated several times / minute the FLASH
> > devices are not suitable and therefore alternative solutions
> > are being looked at.
> > 
> > The most promising solutions semms to use battery
> > backed-up RAM.
> > >From Linux we could memorymap this area but using a filesystem
> > gives all sorts of extra bonusses so this is preferred.
> > 
> > I looked shortly at PRAMFS that seems to do the trick but PRMFS
> > seems unmaintained and not merged.
> > Then I stumbled over PHRAM.
> > I and pretty clear upon the basic parts lettign the RAM look
> > like any other FLASH device.
> > But then the question popped up.
> > 
> > What is the best filesystem to use on top of a PHRAM based
> > MTD device?
> > My first two ideas were to use either JFFS2 or ext2
> > (the latter with the blocklayer emulation).
> > But reading the documentation both looks like overkill.
> > 
> > The requirement so far is to gain maximum bebefit of the
> > avalable 100 kbytes of RAM.
> > There will be a limited number of files (< 500).
> 
> If you decide on battery-backed RAM, the PHRAM/JFFS2 combo would be the
> best existing solution.  Alternatively you could write a custom
> filesystem.

Thanks.
It could be a nice challenge to write my own filesystem
just for this - on the other hand JFFS2 despite the overhead
should fulfill my demand so I will most probarly stay with that.


> With JFFS2 you should carefully decide on a suitable erasesize.  The
> JFFS2 overhead can be minimized by tuning this number.  Most likely
> something small, 4KiB or 8KiB is optimal.
> 
> Yet another alternative would be to use flash.  The math is quite
> simple.  Example calculation:
> Update interval: 10s
> Updated data size: 10kB
> Required device lifetime: 10a
> JFFS2 overhead: 10%
> Flash durability: 100k erases
> 
> Write rate is 10kB / 10s * 1.1 = 1.1kB/s.
> Total written data in 10a is: 1.1kB/s * 31536000s/a * 10a = 346GB
> Required flash size: 346GB / 100k = 3468kB
> 
> So with the example numbers, you would only need a 4MiB flash chip.
> Those should be rather cheap.  But it remains your duty to redo the
> calculation with correct numbers and check for any mistakes I may have
> made.
> 
> Also make sure that your supplier guarantees 100k erases on the flash.
> At 10k erases, you would need a device 10x bigger.  1M erases, if
> available, are even better.
Thanks again.
When I have more exact numbers I will try to redo your math.

I assume that from a performance point-of-view the FLASH based version
suffer more than the RAM one - since it is simple to 'erase' RAM.

	Sam

Re: MTD_PHRAM - what filesystem to use?

[Jörn Engel]

On Sun, 11 March 2007 22:08:35 +0100, Sam Ravnborg wrote:
> > 
> > Write rate is 10kB / 10s * 1.1 = 1.1kB/s.
> > Total written data in 10a is: 1.1kB/s * 31536000s/a * 10a = 346GB
> > Required flash size: 346GB / 100k = 3468kB
> 
> I assume that from a performance point-of-view the FLASH based version
> suffer more than the RAM one - since it is simple to 'erase' RAM.

Not likely.  With the example numbers, 1.1kB/s will be peanuts compared
to flash write bandwidth.  Erases happen ahead-of-time in JFFS2, so
those won't matter much either.  Reads won't matter at all, if you are
looking at 100kB or so - all of that can easily be cached.

If you require a significantly higher write bandwidth, the required
flash size will equally increase.  Who knows, battery backed RAM may be
cheaper than flash due to the required size before the bandwidth becomes
a problem.

Good luck with your design, Sam!

Jörn

-- 
If you're willing to restrict the flexibility of your approach,
you can almost always do something better.
-- John Carmack