[LEDE-DEV] [RFC] rootfs_data on different MTD outside firmware image

[Adrian Panella]

Hi, some Linksys devices (i.e WRT1900AC, EA4500, EA8500,...) have two 
different partitions for dual boot, and an additional partition that 
Linksys uses for system config (sysconf).
Each of these partitions is of a considerable size (23-37 mb, varying 
between devices).

As far as I could see, the ports already in Owrt/LEDE (Kirkwook & Mvebu) 
use only one partition at a time for the overlayfs, so total 23-37mb 
shared among squashfs rom and ubifs overlay.
In some cases the third partition (sysconf) is mounted, but in /mmt, not 
taking part in the overlay, and so not directly useful for installing 
additional packages.

I believe that a way to better profit all this available space would be 
to leave one partition for the rom squashfs alone (23-37mb there) and 
share the 3rd partition between alternative boots as the ubifs overlay 
(another 23-37mb here). In total we double the space up to a full 74mb 
for packages, reducing the need for extroot.


Have you found any serious disadvantage on this approach, and that's why 
it is not implemented in mvebu/kirkwood? If so, which one?

If we leave ubifs outside the image, and only squash in one MTD, does it 
add any benefit to have squash image on top of an UBI layer? Erase 
counters would be lost between firmware flashes anyway and no other 
write would occur in between. On the other hand, in the third MTD (i.e. 
sysconf) the erase counters could be preserved between firmware updates, 
as the ubi block doesn't need to be recreated each time, and only a 
ubiformat could be performed on flash.

I'm planning on switching ipq806x/EA8500 to this scheme, and would 
appreciate opinions first.