Re: Cramfs Limitations

In message [off-list ref] you wrote:

quoted

Are you really surprised that opening a file for
writing fails  on  a read-only filesystem???

So only for this reason, I cannot choose Cramfs for my
application deployment??? Then I have nearly no other

Why not? Of course you can. You just cannot write to it.

choice but use JFFS2 for deployment, which performance
couldn't compare with RAMDISK.It's a pity that RAMDISK
cannot update some info permanently. EXT2 seems not a

You can use any filesystem, including one on a RAMDISK.

If the root filesystem happens to be  read-only,  you  just  need  to
provide  a  writable  filesystem  for  those  things that need to get
written. In case of data that may get lost  at  reboot  you  can  use
tmpfs, and for persistent storage you can use JFFS2.

If  you  are  concerned  about  updating  individual  files  in  your
read-only  filesystem  (instead  of updating the whole image (*)) you
can use an overlay filesystem. For example,  you  can  use  "mini_fo"
which  was  specifically  tailored  for  such  purposes;  please  see
http://atlas.denx.de/denx/e/news.php#MINI_FO

(*) Updating the whole image is not a bad idea -  this  way  you  can
    guarantee  that all files int he image are really compatible with
    each other; keeping the system in a consisten state is much  more
    difficult when individual files can get replaced at random.

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
"UNIX was not designed to stop you from doing stupid things,  because
that would also stop you from doing clever things."       - Doug Gwyn

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Wolfgang Denk [off-list ref] wrote��

Why not? Of course you can. You just cannot write to it.

But I know little on application code's behavior. I
cannot adjust it's write operation, I am afraid.
It's another folk's job to code the application.

quoted

choice but use JFFS2 for deployment, which performance couldn't
compare with RAMDISK.It's a pity that RAMDISK cannot update some
info permanently. EXT2 seems not a

You can use any filesystem, including one on a RAMDISK.

Good hints. I don't really want to give up RAMDISK for
it has a better performance than JFFS2 and EXT2.

If the root filesystem happens to be read-only, you just need to
provide a writable filesystem for those things that need to get
written. In case of data that may

Recently, I have been thinking about to build up a
sector on FLASH for R/W operation, like JFFS2. Then my
root fileystem still keeps as RAMDISK. So I can get a
fastest boot process and write sector while running.
In your opinion, this way should work. Right?

get lost at reboot you can use tmpfs, and for persistent storage you
can use JFFS2.

tmfs is new to me but interesting. I will try it.

Thanks for your further help a lot!

Sam

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