Q: file system for embedded

Q: file system for embedded

[Ran Shalit]

Hello,

I hope you can help main understand issue of file system for embedded linux.
I read free-electrons "Embedded Linux system development training" and
some of the files recommended there such as cramfs, are aceesing the
flash during execution of apllications. As far as I understand
accessing the flash after initilization is not good in embedded
system.
Is there something here that I am missing ?

Thanks, Ran

Re: Q: file system for embedded

[Christophe Aeschlimann]

Hi,

Le 13.06.2012 23:31, Ran Shalit a écrit :

> Hello,
> 
> I hope you can help main understand issue of file system for embedded linux.
> I read free-electrons "Embedded Linux system development training" and
> some of the files recommended there such as cramfs, are aceesing the
> flash during execution of apllications. As far as I understand
> accessing the flash after initilization is not good in embedded
> system.


Define not good... some applications requires write accesses to the
flash. (how would you program music in your MP3 player ? Where do you
store crash dumps so they are persistent across reboot ? Where do you
store your user configurable options, etc.)

There are different kinds of flash memory but each have a limited number
of program/erase cycle so you have to use them sparingly. (e.g. make
sure the write accesses are distributed across the chip that's called
"wear levelling", detect when bits have toggled and correct the error
using some extra redundancy stored in the out-of-band area of each block
(e.g. ECC). If the error cannot be repaired mark the block as unusable.

When you access the bare memory chips then these issues are solved in
the different layers of MTD [1] and the specific flash file system [2] :
JFFS2, UBIFS, etc.

If you don't access the bare memory chips e.g. you use eMMC or an SSD
these problems are left to a dedicated controller/firmware.

[1] http://www.linux-mtd.infradead.org/index.html
[2] http://elinux.org/File_Systems

Best regards,

-- 
Christophe Aeschlimann

Embedded Software Engineer & IT Manager
ACN Advanced Communications Networks S.A.
2000 - Neuchâtel, Switzerland
Tel. +41 32 724 74 31

Re: Q: file system for embedded

[Ran Shalit]

On Thu, Jun 14, 2012 at 11:34 AM, Christophe Aeschlimann
<c.aeschlimann@acn-group.ch> wrote:
> Hi,
>
> Le 13.06.2012 23:31, Ran Shalit a écrit :
>
>> Hello,
>>
>> I hope you can help main understand issue of file system for embedded linux.
>> I read free-electrons "Embedded Linux system development training" and
>> some of the files recommended there such as cramfs, are aceesing the
>> flash during execution of apllications. As far as I understand
>> accessing the flash after initilization is not good in embedded
>> system.
>
>
> Define not good... some applications requires write accesses to the
> flash. (how would you program music in your MP3 player ? Where do you
> store crash dumps so they are persistent across reboot ? Where do you
> store your user configurable options, etc.)
>
> There are different kinds of flash memory but each have a limited number
> of program/erase cycle so you have to use them sparingly. (e.g. make
> sure the write accesses are distributed across the chip that's called
> "wear levelling", detect when bits have toggled and correct the error
> using some extra redundancy stored in the out-of-band area of each block
> (e.g. ECC). If the error cannot be repaired mark the block as unusable.
>
> When you access the bare memory chips then these issues are solved in
> the different layers of MTD [1] and the specific flash file system [2] :
> JFFS2, UBIFS, etc.
>
> If you don't access the bare memory chips e.g. you use eMMC or an SSD
> these problems are left to a dedicated controller/firmware.
>
> [1] http://www.linux-mtd.infradead.org/index.html
> [2] http://elinux.org/File_Systems
>
> Best regards,
>
> --
> Christophe Aeschlimann
>

Hi Christophe,

Thanks for the reply.
I have no need to write to flash during the main application
execution: I will program the configuration and executable before
start of execution, and after initialization there is no need to write
again to flash.
What I still do not understand is why cramfs, ubifs are recommended
for embedded if they access the flash.
Is accessing the flash not a problem in embedded linux ? What does it
mean using mlockall with cramfs ?
Regards, Ran

Re: Q: file system for embedded

[Christophe Aeschlimann]

Le 14.06.2012 14:58, Ran Shalit a écrit :

 
> Hi Christophe,
> 
> Thanks for the reply.
> I have no need to write to flash during the main application
> execution: I will program the configuration and executable before
> start of execution, and after initialization there is no need to write
> again to flash.


How are you going to program the flash ? Which initialization ?

> What I still do not understand is why cramfs, ubifs are recommended
> for embedded if they access the flash.


For the reasons I gave in my previous email. I never used cramfs but I
see that it is a read-only compressed format (like supported in JFFS2)
which decompresses your data on-the-fly before read/execution hence
allowing to store more information if your flash is small.

> Is accessing the flash not a problem in embedded linux ? 

I'm not sure what you mean here. It's not a problem, but it requires
some care.

> What does it mean using mlockall with cramfs ?


Well if you don't have swap enabled mlockall has no effect since there
is nowhere to swap anyway. But I'm not sure I understand your question.

> Regards, Ran


Regards,

-- 
Christophe Aeschlimann

Embedded Software Engineer & IT Manager
ACN Advanced Communications Networks S.A.
2000 - Neuchâtel, Switzerland
Tel. +41 32 724 74 31