ext2 for read-only file system on UBI

[Artem Bityutskiy]

Hello Hamish,

On Mon, 2008-05-19 at 16:21 +1000, Hamish Moffatt wrote:
> My embedded device has a read-only root file system which is only
> replaced by writing a whole image (dd, flashcp, ubiupdatevol etc).
> 
> Using gluebi and mtdblock, I think I can put a traditional block file
> system (eg ext2) on top of NAND flash. What are the disadvantages of
> this? (For the read-only application only.)

For read-only it should be ok, although I am not sure mtdblock will like
non power of 2 eraseblock sizes. But this should be trivial to fix.

> Background: I've got older hardware which uses ext2 on top of compact
> flash in IDE mode, and new hardware which has replaced the compact flash
> with NAND. I'd like to share an ext2 image between the two if possible.

Should be possible for R/O. Different sizes of eraseblocks may add extra
work though.

> The read-write file systems use ubifs. I'm only considering this for the
> read-only volumes. One obvious disadvantage is lack of compression. Are
> there others? Do I still get the reliability of ubi? 

No, should be fine. Well, you'll still have WL across whole NAND chip,
yes. You'll still have bit-flip handling.

> ie what does "UBI is not an FTL" mean in practice?

In practice it means that /dev/ubiX_Y are not block devices, where you
may directly put ext2 and the like.

-- 
Best regards,
Artem Bityutskiy (Битюцкий Артём)