How to install GRUB to a whole ext4 disk without partition table

bootgrub-legacy

Currently I have the entire disk /dev/sda formatted as ext4, and installed Gentoo.

(There is no MBR, no partition at all. )

But finally, I can't install GRUB on it, because it seems like GRUB needs to write to MBR. 

# grub-install /dev/sda
Unknown partition table signature
Unknown partition table signature
Unknown partition table signature
Unknown partition table signature
Unknown partition table signature
The file /boot/grub/stage1 not read correctly.

neither does grub work, 

# grub
grub> root (hd0)
 Filesystem type unknown, using whole disk

grub> setup (hd0)
 Error 17: Cannot mount selected partition

Any way can I install GRUB into the /dev/sda without MBR?

P.S. The /boot directory and grub.conf files: 

# tree /boot 
/boot
|-- boot -> .
|-- grub
|   |-- default
|   |-- device.map
|   |-- e2fs_stage1_5
|   |-- fat_stage1_5
|   |-- ffs_stage1_5
|   |-- grub.conf
|   |-- iso9660_stage1_5
|   |-- jfs_stage1_5
|   |-- menu.lst -> grub.conf
|   |-- minix_stage1_5
|   |-- reiserfs_stage1_5
|   |-- splash.xpm.gz
|   |-- stage1
|   |-- stage2
|   |-- stage2_eltorito
|   |-- ufs2_stage1_5
|   |-- vstafs_stage1_5
|   `-- xfs_stage1_5
`-- kernel-2.6.36-gentoo-r5

# cat /boot/grub/grub.conf
default 0
timeout 30
splashimage=(hd0)/boot/grub/splash.xpm.gz

title Gentoo Linux 2.6.36-r5
root (hd0)
kernel /boot/kernel-2.6.36-gentoo-r5 root=/dev/sda

title Gentoo Linux 2.6.36-r5 Rescue
kernel /boot/kernel-2.6.36-gentoo-r5 root=/dev/sda init=/bin/bb

# cat /boot/grub/devices
(fd0)   /dev/fd0
(hd0)   /dev/sda

Best Answer

The BIOS reads the first sector (512 bytes) of the disk and branches into it. If your disk contains PC-style partitions, the first sector also contains the partition table. If your disk contains a single filesystem, the first sector contains whatever the filesystem decides to put there. In the case of ext[234] (and many other filesystems), the first sector¹ is reserved for the bootloader (and is initially zeroed out). You can install Grub on /dev/sda.

That being said, there are occasional BIOSes that refuse to boot from a device that don't contain a partition table. (But there are also BIOSes that refuse to boot from some external devices if they do contain a partition table!) If you have one of these BIOSes, you'll have to create a partition table.

Even if a partition table is not necessary, it's recommended. You only waste a few kilobytes, and gain readability under many non-Linux OSes and less surprise for any co-sysadmin. If you accidentally plug your disk into a machine running Windows, it might suggest you to reformat the disk if it doesn't see a partition table, whereas it'll just complain it can't read the data if it sees a partition table with a partition type it doesn't recognize.

¹ In fact, the first block, I think, where a block is 1kB, 2kB or 4kB depending on the options passed to mkfs.

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