By not using root on lvm on raid, I naively thought that it'll avoid
boot problems etc, but I was very wrong.
1. partman wanted to update the partition table on /dev/md2,
which seemed to lead to big problems (like cat /proc/mdstat
hang, then d-i hang/crash) So I guess lvm on raid is not (well?)
supported by current d-i. (Resolved by manually creating PV, VG, LVs)
If it's unsupported, it's my fault. But then I think partman should
make it impossible to create lvm on raid on partitions.
2. I also had problems because /boot and / were on different partitions
(grub and lilo both failed). I used a separate /boot to avoid those
kind of problems ; bad idea apparently. Resolved by reinstalling
(from scratch) with /boot and / on the same partitions.
3. On another box, an initrd bug prevented the box from loading
(could not find VG, because of missing ide/dm-mod modules)
(Resolved by manually editing /etc/mkinitramfs/modules
and dpkg-reconfigure linux-image-2.6...)