GRUB2 and LVM - grub-install is "Unable to identify a filesystem"

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https://bbs.archlinux.org – Hi folks,   I'm installing a brand new Arch Virtualbox VM and I got stuck trying to install Grub2 with a LVM /boot.   The command:grub-install --target=i386-pc --recheck --debug /dev/sda   The error:/usr/sbin/grub-bios-setup: info: Looking for /dev/sda. /usr/sbin/grub-bios-setup: info: /dev/sda is a parent of /dev/sda. /usr/sbin/grub-bios-setup: info: /dev/dm-4 is an LVM. /usr/sbin/grub-bios-setup: info: the size of hostdisk//dev/sda is 20971520. /usr/sbin/grub-bios-setup: info: Scanning for DISKFILTER devices on disk hostdisk//dev/sda. /usr/sbin/grub-bios-se (HowTos)