For the N8x0 the way to create flashable rootfs's - that is, rootfs's usable in flasher-3.5 -r rootfs.jffs2 -f -R was through these two steps:
mkfs.jffs2 -d $ROOTFS_DIRECTORY -l -n -e 128KiB
sumtool -l -n -e 128KiB -o rootfs.jffs2 -i rootfs.jffs2.raw
Now that the N900 uses ubifs for rootfs instead, how do you do create a flashable rootfs?
You need to make a file, ubinize.cfg:
Then, you run these two commands - you have to grab mtd-utils - Ubuntu Karmic has mtd-utils with ubifs support.
mkfs.ubifs -m 2048 -e 129024 -c 2047 -R 4MiB -r $ROOTFS_DIRECTORY -v /full/path/to/base.ubi.img
ubinize -o /full/path/to/ubi.img ubinize.cfg -m 2048 -p 128KiB -s 512
You can now run flasher-3.5 -r ubi.img -f -R.
What can this information be used for?
* Generate a full snapshot of your NAND rootfs and restore it with flasher after trying out something stupid that failed.
* Flash alternative OS'es onto your N900 NAND.
* Possibilities in rescue menu as in my previous post about bootmenu.sh hook - dump my rootfs to SD and I'll fix it on my PC and reflash it back to my N900.