In moving my boot device to mirrored LVM under Debian jessie, I ended up unable to boot. I thought my problem was that I failed to run install-grub after adding GRUB_PRELOAD_MODULES=lvm to /etc/default/grub, but when I finally sorted out a working PXE image for rescue, I found that I was unable to install while mirrored. That appears to be in part because Debian has taken upgrades to LVM2 that change the default mirroring layout from "mirror" to "raid1" (mirror_segtype_default = "raid1" in /etc/lvm/lvm.conf; the lvm pages are also clear on this distinction and the defaults), which results in an inability to successfully probe /boot/grub, which breaks grub-install. The clearest indication I could find of this as likely root cause was a debug message from grub-probe "Unknown LVM format raid1". Looking at the source head, it appears that there's first a check for raidX in lines 580-583 of grub-core/disk/lvm.c but that the subsequent switch at lines 587-600 provides for raid[456] but not raid1, which isn't defined in include/grub/diskfilter.h.
I'm looking at doing a bit more debug to confirm this before assessing fix options, but am I on the right track thus far?
Cheers,
Bayard