UBI: Single versus Multiple Images

[Artem Bityutskiy]

On Tue, 2012-06-05 at 06:21 -0700, Doug Kehn wrote:
> Hi Everyone,
> 
> I have the following NAND flash MTD layout (presently JFFS2):
> 
> * Boot-loader
> * Kernel
> * Root file-system (rootfs)
> 
> * Data
> 
> I'm going to switch from JFFS2 to UBI/UBIFS.  I'm wondering if it is
> better to create a single UBI image containing both rootfs and data
> volumes or to create separate UBI images (each with a single volume)
> or is the answer it depends?

Are you actually talking about how to partition your flash - whether to
have one partition or several?

> The data volume will be used for logging data.  The volume won't
> completely fill as old data will be purged to make room for new data.
> For the single image multiple volume case, if I understand the 
> documentation correctly, UBI will use all PEB from both volumes for
> mapping per-volume LEB, 
> correct?  If my understanding is correct, then it's possible, after
> enough time, maximum PEB erase count will be reached and both rootfs
> and data volumes will be read-only?  If the goal is to keep the rootfs
> volume writable, even if the data volume become read-only, then would
> it be better to create multiple UBI images?  Or is my understanding
> all wrong?

I do not really understand the questions. UBI will do wear-leveling
across the mtd device it is attached to. If you have one MTD partition
which spans entire flash, you'll have wear-leveling across entire flash.
You will ave /dev/ubi0 represinting the UBI device,
and /dev/ubi0_0, /dev/ubi0_1, etc for each volume for this UBI device
number 0.

If you partition your flash, then each partition will be managed
independently, and you'll have wear-leveling per-partition. So one
partition may wear out faster than another. You'll
have /dev/ubi0, /dev/ubi1, etc for each partition. Then if you create a
volume in each UBI device, you'll have /dev/ubi0_0, /dev/ubi1_0, etc for
each volume.

-- 
Best Regards,
Artem Bityutskiy
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