waiting 30 seconds for device /dev/disk/bylabel/XXXX error: boot device didn't show up after 30 seconds

Q:

when grub2 boot archLinux 2011.08.19 iso file, the error is:

waiting 30 seconds for device /dev/disk/by-label/XXXX
error: boot device didn't show up after 30 seconds

A:

mkdir /cdrom
mount -t vfat /dev/sda1 /cdrom
modprobe loop
losetup /dev/loop6 /cdrom/boot/iso/archlinux-2011.08.19-core-i686.iso
ln -s /dev/loop6 /dev/disk/by-label/ARCH_201108
exit

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A(Update):

add earlymodules=loop as bootparam

menuentry "ArchLinux-2011.08.19-core-i686.iso" {
 set isofile="/boot/iso/archlinux-2011.08.19-core-i686.iso"
 set dev_label="MULTIBOOT"
 loopback loop $isofile
 linux (loop)/arch/boot/i686/vmlinuz archisolabel=ARCH_201108 img_dev=/dev/disk/by-label/$dev_label img_loop=$isofile earlymodules=loop
 initrd (loop)/arch/boot/i686/archiso.img
}

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