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GRUB2

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GRUB2 is a bootloader with support for many modern day computer systems. It is meant to replace the older GRUB bootloader (now referred to by upstream as "GRUB Legacy"). GRUB2 has an entirely separate codebase from GRUB Legacy, and boasts features such as a new shell-like syntax which permits advanced scripting capabilities. GRUB Legacy users are encouraged to migrate to GRUB2.

Upgrading to GRUB 2 might be necessary, as it allows:

  • booting from UEFI platforms;
  • booting from GPT partitioned drives without even needing an awkward hybrid MBR, even though one can use one for compatibility/portability reasons;
  • booting from a btrfs formatted /boot
  • booting directly from a btrfs mirror or raid set without needing an initramfs and early mount setup
  • booting from directly logical volume management such as LVM2 support;
  • booting from raid DM-RAID support for RAID [0?] 1, 4, 5, 6, 9 and 10 [or 1x?];
  • and booting from encrypted devices, probably LUKS, because of the inexistent documentation despite the cypher/hash modules being there: so no guidance on a howto for the moment.

For a shotgun approach, see instead GRUB2 Quick Start.

Installation

The sys-boot/grub package is slotted; both grub-0.97 and grub-2.00 may be installed at the same time.

If you want to install grub-0.97 or keep it installed after an upgrade, call emerge with the "0" slot on the end of the package atom. This will add sys-boot/grub:0 to your world file. For example:

root # emerge --noreplace sys-boot/grub:0

Only one version may be installed in your MBR at one time.

By default, GRUB2 will guess a default platform to target at build time. This may be overridden by setting the GRUB_PLATFORMS variable in make.conf.

File/etc/portage/make.conf

GRUB_PLATFORMS="emu gentoo_platforms_efi-32 gentoo_platforms_efi-64 pc"

The following platforms are supported depending on the target CPU.

Target
Platform i386 ia64 mips mipsel powerpc spac64 x86_64
arc No No No Yes No No No
coreboot Yes No No No No No 32-bit
efi Yes Yes No No No No Yes
emu Yes Yes Yes Yes Yes Yes Yes
ieee1275 Yes No No No Yes Yes 32-bit
loongson No No No Yes No No No
multiboot Yes No No No No No 32-bit
qemu Yes No No No No No 32-bit
qemu-mips No No Yes No No No No
pc Yes No No No No No 32-bit

Install the package using the normal emerge syntax.

root # emerge --ask sys-boot/grub:2

Optionally, install sys-boot/os-prober to have GRUB2 probe for other operating systems when running grub2-mkconfig.

root # emerge --ask sys-boot/os-prober

Configuration

GRUB2 does not require you to manually maintain your boot options configuration, instead it can optionally generate the file (/boot/grub/grub.cfg) using the command grub2-mkconfig. This utility will parse the scripts in /etc/grub.d and the settings file /etc/default/grub.

  • GRUB_FONT
Path to the PF2 font you want GRUB2 to use. To create a PF2 font, use the grub2-mkfont utility, example:
root # grub2-mkfont /usr/share/fonts/droid/DroidSansMonoSlashed.ttf --size=20 --output /boot/grub/fonts/DroidSansMonoSlashed20.pcf
and set GRUB_FONT=/boot/grub/fonts/DroidSansMonoSlashed20.pcf
  • GRUB_DEFAULT
Default menu entry selected on boot. May be a numeric index, a menu title, or "saved". Defaults to first detected entry.
  • GRUB_TIMEOUT
Delay before booting default menu entry. Set to 0 to wait indefinitely. Default is 5 seconds.
  • GRUB_DISABLE_RECOVERY
If true, recovery menu entries will not be generated. On Linux, recovery entries pass "single" on the kernel command line.
  • GRUB_CMDLINE_LINUX
Parameters to be passed on the kernel command line for all Linux menu entries.
  • GRUB_CMDLINE_LINUX_DEFAULT
Parameters to be passed on the kernel command line for non-recovery Linux menu entries.
  • GRUB_DISABLE_LINUX_UUID
If true, ${GRUB_DEVICE} is passed in the root parameter on the kernel command line.
If false, ${GRUB_DEVICE_UUID} is passed in the root parameter on the kernel command line when an initramfs is available.
  • GRUB_GFXPAYLOAD_LINUX
Controls the video mode in which the Linux kernel starts. May be set to "text", "keep", or a display resolution.
  • GRUB_DEVICE
The initial root device, i.e. the kernel's root= parameter. Set this if you want to override grub2-mkconfig's root device auto-detection; for example, GRUB_DEVICE=/dev/ram0 will force root=/dev/ram0 to be used in your kernel command line.

The directory /etc/grub.d holds the scripts grub2-mkconfig uses to generate grub.cfg. By default the contents of this directory should be similar to the following:

user $ ls /etc/grub.d/
00_header 10_linux 20_linux_xen 30_os-prober 40_custom 41_custom README

By default GRUB2 will use all scripts when first installed. If you want to disable any of them you only need to remove the executable bit. In the following example we disable everything but 00_header and 10_linux:

root # chmod -x /etc/grub.d/{20_linux_xen,30_os-prober,40_custom,41_custom}

When configured you run grub2-mkconfig with the -o argument pointing to the output file, which should usually be /boot/grub/grub.cfg:

root # grub2-mkconfig -o /boot/grub/grub.cfg
Generating grub.cfg ...

Found linux image: /boot/vmlinuz-3.3.0-gentoo

done

This will (re)generate GRUB2 configuration. Note that if grub2-mkconfig doesn't report anything as found then nothing was, and GRUB2 will offer no boot selections.

BIOS/MBR or BIOS/GPT

Installing in this mode is straight forward as it's just like the legacy GRUB with new GRUB2 additions. If you have a GPT partition table, you will need a small BIOS boot partition. 1 MiB may be enough but 2-4 MiB will definitely work. It will hold stage 2 of the bootloader and you don't need to format the partition with a filesystem - grub2-install will overwrite it anyway. You can mark a partition with the command line tool "parted" by typing (change 1 to the number of the partition you want to mark as a BIOS Boot partition!):

(parted) set 1 bios_grub on

More specifically the following type of layout will work for BIOS/GPT, and it is easier to use the curses based cgdisk utility from the gptfdisk ebuild. Create your first partition at sector 2048 (leaving the first 1 MiB as free space for MBR's etc). Give it 4096 to 8192 sectors (2-4 MiB) for size. Set the type code to 0xEF02 and give it a label like "gptbios" (effectively what the bios_grub command does in parted. Then add partitions for boot and root and everything else as you need. Boot is usually anywhere from 200-500mb.

If needed, start with mounting the /boot parition:

root # mount /boot

Run the grub2-install utility to copy the relevant files to /boot/grub. On the PC platform, this also installs a boot image to the master boot record (MBR) or a partition's boot sector. grub2-install accepts a --target option to specify which CPU/Platform to install. If unspecified, grub2-install will make a guess; on amd64/x86 it will use "i386-pc" by default:

root # grub2-install /dev/sda
Installation finished. No error reported.

UEFI/GPT

EFI system partition

For UEFI boot, your disk must have a dedicated "EFI system partition" containing a FAT filesystem. 200 MB should be large enough.

If you need to create an EFI system partition, first create it in gdisk or parted with the proper type. In gdisk, this is type ef00.

Create a FAT filesystem using mkdosfs and add it to fstab.

root # mkdosfs -F 32 -n efi-boot /dev/sda1
root #
mkdir /boot/efi
root #
echo 'LABEL=efi-boot /boot/efi vfat defaults 0 2' >> /etc/fstab
root #
mount /boot/efi

Install GRUB

root # grub2-install --target=x86_64-efi

This will install grub in /boot/grub, copy the core image to /boot/efi/EFI/gentoo/grubx64.efi, and call efibootmgr to add a boot entry.

efibootmgr may fail if the system is not already booted in EFI mode. If so, you will need to boot your system in EFI mode using an EFI shell or a suitable EFI boot media.

Creating EFI boot media

  • USB EFI bootable flashdrive can be created with (assuming FAT formatted drive, mounted under /media/flashdrive)
    root # grub2-install --target=x86_64-efi --efi-directory=/media/flashdrive --removable --modules=part_gpt
  • Kernel support: (re)compile kernel with CONFIG_EFI_VARS=y OR modprobe efivars. Copy kernel on /media/flashdrive/vmlinuz for convenience.
  • Reboot machine from the flashdrive in UEFI mode - motherboard/notebook manual should contain instructions on how to do this.
  • GRUB2 command line should appear. linux /vmlinuz root=/dev/sdaX [...add necessary parameters] [press ENTER] boot [press ENTER]
  • Look at /sys/firmware/efi/vars to see whether the EFI variables are accessible.
  • run grub2-install

Hybrid GPT/MBR

Dual booting with operating systems that are blind to a GPT-partitioned disk requires this tricky workaround. Playing with a hybrid MBR can be tricky and you should make no assumptions about the reliability of the instructions in this sub-section. Consider them rather as guidances for experimentation.

Some operating systems lacking ability to boot from anything but BIOS/MBR include all versions of Microsoft Windows prior to Vista. Though it should be noted that support for 32-bit (as opposed to 64-bit) UEFI is less than perfect for certain machines and / or versions of Windows 7 (conduct your own research on this point).

GRUB2 supports booting from GPT and MBR partition tables with the part_gpt and part_msdos modules. However, a successful setup and GRUBs ability to boot correctly depends both on loaded partition modules and on correctly installing GRUB2. A determining factor in how GRUB is installed (BIOS/MBR vs BIOS/GPT) is the presence of a EE00 (GPT protective) partition as the first partition in the MBR.

With a hybrid MBR, GRUB2 will install just fine if and only if the first partition in the MBR table is an EE00 partition. An EF02 partition should be present anywhere in the disk and not necessarily in the MBR list to leave a few room to embed core.img in the disk.
Note
In this case one do not have to add/list an EE00 partition in the hybrid MBR list manually, gdisk will prompt you to list one first if booting with GRUB2 before listing the provided list (up to 3 partition) by the user.

Creating a hybrid MBR is easy with gdisk: just hit `r' (recovery and transformation) in the main, then hit `h' (CHS recompute) in that menu, and then enter a series of partition (less or equal to 3) in the order you want it to be listed in the MBR, and then enter the hexadecimal partition type without the `0x' prefix or accept the default with hitting repeatedly with `Y', and enable only a single boot flag as the following example.

root # gdisk /dev/sdg
GPT fdisk (gdisk) version 0.8.1

Partition table scan:
  MBR: MBR only
  BSD: not present
  APM: not present
  GPT: present

Found valid MBR and GPT. Which do you want to use?
 1 - MBR
 2 - GPT
 3 - Create blank GPT

Your answer: 2
root # gdisk /dev/sdg
...
Using GPT and creating fresh protective MBR.

Command (? for help): r
Recovery/transformation command (? for help): p
Disk /dev/sdg: 625142448 sectors, 298.1 GiB
Logical sector size: 512 bytes
Disk identifier (GUID): 744E8BF3-39A4-4908-8646-AC2E5661C8CF
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 625142414
Partitions will be aligned on 2048-sector boundaries
Total free space is 2014 sectors (1007.0 KiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048       566233087   270.0 GiB   8300  Linux filesystem
   2       566233088       625142414   28.1 GiB    0700  Microsoft basic data
Recovery/transformation command (? for help): h

WARNING! Hybrid MBRs are flaky and dangerous! If you decide not to use one,
just hit the Enter key at the below prompt and your MBR partition table will
be untouched.

Type from one to three GPT partition numbers, separated by spaces, to be
added to the hybrid MBR, in sequence: 2 1
Place EFI GPT (0xEE) partition first in MBR (good for GRUB)? (Y/N): N


Creating entry for GPT partition #2 (MBR partition #1)
Enter an MBR hex code (default 07): 
Set the bootable flag? (Y/N): Y

Creating entry for GPT partition #1 (MBR partition #2)
Enter an MBR hex code (default 83): 
...
Unused partition space(s) found. Use one to protect more partitions? (Y/N): N

Recovery/transformation command (? for help): o

Disk size is 625142448 sectors (298.1 GiB)
MBR disk identifier: 0x00000000
MBR partitions:

Number  Boot  Start Sector   End Sector   Status      Code
   1      *      566233088    625142414   primary     0x07
   2                  2048    566233087   primary     0x83
   4                     1         2047   primary     0xEE


Recovery/transformation command (? for help): w

And then check out if the operation was successful or not by listing the devices.

Note
I answered most safe questions to `N' because without that `/fdisk -l /dev/sdg' will list the MBR partion list just fine but listing the block devices with a `ls /dev' wouldn't list the partions and more annoying: the partions wouldn't be listed in DOS/BIOS.
Note
I had to manually remove the EE00 (GPT protective) partition in the MBR list for that drive with fdisk, because fdisk listed that partition as being the third one. And I couldn't get the BIOS/DOS recognize the partition table with its presence. This is quite weird (as the previous output shows) because the MBR partition table was correctly recognized when the hybrid MBR was freshly created. I had to reload GPT partition table from a file and re-hybridize the MBR a second time after a few experiments.

And finally install GRUB as usual. Everything should be fine at this point. See troubleshooting section for possible issues not listed on the previous Notes and comments.

Chainloading

There's a truly improved chainload mode compared to GRUB Legacy: the new iso (or loop) chainload mechanism. Actually, one could still chainload ISO images with GRUB Legacy, and I did, but there were no way to pass kernel cmdline arguments. In either case, the ISO image in question should have been built with that in mind like SysRescueCd or on a lesser extend Gentoo liveCD, otherwise booting will just fail badly in the physical media check/test. Gentoo liveCD is handy because it has a minimal shell which let mount the squashed image to the right place and then hit Esc to continue the boot process. That's a handy way to install an OS with everything in RAM, especially for such light liveCD, without listening to a whining drive on each command.

ISO images

To chainload an ISO with custom or defaults kernel cmdline arguments, something like the following can be easily added:

Code

menuentry "SYSRESCUECD" {
	set iso=/systemrescuecd-x86-3.8.1.iso
	loopback loop ${iso}
	linux  (loop)/isolinux/rescue64 nomodeset vga=791 docache setkmap=fr isoloop=${iso}
	initrd (loop)/isolinux/initram.igz
}

For a permanent and automatic entry to grub.cfg, a custom file could be added to /etc/grub.d.

File/etc/grub.d/50_isofile

#!/bin/sh
exec tail -n +3 $0

menuentry "GRUB4DOS" {
	linux /grub4dos-0.4.4/grub.exe --config-file=/menu.lst
}

menuentry "SYSRESCUECD" {
	set iso=/systemrescuecd-x86-3.8.1.iso
	loopback loop ${iso}
	linux  (loop)/isolinux/rescue64 nomodeset vga=791 docache setkmap=fr isoloop=${iso}
	initrd (loop)/isolinux/initram.igz
}

menuentry "STG3-AMD64" {
	set cmdline="dokeymap looptype=squashfs loop=/image.squashfs cdroot"
	loopback loop /stg3-amd64-<DATE>.iso
	linux  (loop)/isolinux/gentoo $cmdline root=/dev/ram0 init=/linuxrc initrd=gentoo.igz
	initrd (loop)/isolinux/gentoo.igz
}

Make the script executable:

root # chmod +x /etc/grub.d/50_isofile

And lastly regenerate your grub.cfg with grub2-mkconfig.

Another Bootloader

Chainloading other bootloader is fairly easy if there's support in GRUB2 for that specific bootloader. Otherwise, one could use a man in the middle or another bootloader in other words to do the task if necessary. In the latter case, one could use GRUB4DOS as it has functionalities similar to GRUB Legacy. This is required when loading another bootloader like TrueCrypt become problematic in a single multiboot disk, otherwise something as simple as the following is enough to boot another disk with whatever bootloader.

Code

menuentry "WHATEVER" {
     insmod part_msdos
     insmod chain
     chainloader (hd1,1)+1
}

TrueCrypt

Well, let's make a long story short: GRUB2 cannot neither chainload a disk with TrueCrypt in the MBR neither the rescue CD image. This is perfectly possible, feasible and works just fine with GRUB Legacy. It even works with GRUB4DOS! This could be used as workaround. See bug #385619 for more info and the above GRUB4DOS menu entry as a starting point. The infamous `invalid signature' await the more adventurous ones.

Simply put, one should stick to GRUB Legacy or to another bootloader or else... use GRUB4DOS as workaround for this. GRUB4DOS has a very similar GRUB Legacy interface and menu.lst entry which can be used to chainload TrueCrypt bootloader or rescue CD to boot from an encrypted partition. An executable with grldr in the `/' root of the booting device is required, plus a menu.lst. See below for a menu entry example. Chainloading TrueCrypt bootloader in another disk is straightforward like any other bootloader.

Code

title Windows7-TrueCrypt-BIOS/MBR
find --set-root /truecrypt_rescue_image.iso
map --mem /truecrypt_rescue_image.iso (hd32)
map (hd0) (hd1)
map (hd1) (hd0)
map --hook
root (hd32)
chainloader (hd32)
Note
The remapping may not be necessary for you if you have a single boot disk system. The mapping are necessary before chainlading TrueCrypt bootloader in that case because the boot disk (here hd1) should appear as the primary boot device (hd0) otherwise TrueCrypt would just fail to boot because... it did not find the right disk.

Another workaround cited on the bug is to boot from TrueCrypt as the main boot loader and then hit ESC[ape] to chainload the following partition if any or the following disk.

MSDOS/Windows

If MSDOS/Windows is on another disk partitioned with MSDOS partition layout with its bootloader within the MBR, then one can boot with a simple menu entry as the previous example in Chainloading other bootloader section. This later section describes how to probe and manually specify a Windows partition for booting. This later case is usually encountered with multiple Windows operating systems are installed on a single hard drive.

Microsoft Windows 8 versions is no longer installed using MSDOS partitions and the install CDROM needs to be booted using BIOS and not the UEFI boot method in order for Windows 8 to install into MSDOS partitions. I find it's usually best, do both, first manually create the MSDOS partitions and then manually boot the Windows 8 CDROM using a BIOS provided option, which provides a BIOS or UEFI boot option.

If you're trying to decide whether to use MSDOS or GPT/UEFI, might be best to stick with the simple MSDOS option until you absolutely need the extra disk space, as MSDOS is limited to two to three Terabyte hard drives, as well as three partitions. (NOTE: When installing Windows 8 into GPT partitions, unlike Linux and other operating systems and according to Microsoft documentation as of this date, Windows 8 requires a UEFI partition in order to use GPT partition layouts!)

MSDOS Partions

Note
Complications will arise if MSDOS/Windows is in another partition of the disk and the partition is encrypted with TrueCrypt for example, or simply if a hybrid MBR is involved.

The simplest way to dual boot MSDOS/Windows is using BIOS/MBR combination. Simply backup the whole MBR to be safe or simply the bootloader before installing MSDOS/Windows as that thing will overwrite the bootloader for sure. And then simple menu entry could added to grub.cfg.

Code

menuentry "Windows 7 BIOS/MBR" {
     insmod part_msdos
     insmod ntldr
     insmod ntfs
     ntldr (hd0,msdos1)/bootmgr
}

Or the following for MSDOS/Windows XP

Code

menuentry "Windows XP BIOS/MBR" {
     insmod part_msdos
     insmod ntldr
     insmod ntfs
     ntldr (hd0,msdos1)/ntldr
}

An entry with GPT/hybrid MBR doesn't not to the list to the previous usual BIOS/MBR examples. Booting multiple MSDOS/Windows could be achieved with remapping and/hiding partion with partool.

Code

menuentry "Windows 7 BIOS/MBR" {
     insmod part_msdos
     insmod chain
     parttool hd1,msdos1 hidden+ boot-
     parttool hd1,msdos2 hidden- boot+
     chainloader (hd1,msdos2)+1
}
Note
Remapping the disk with parttoll in that manner is only required if multiple MSDOS/Windows systems are installed on the same boot device. Otherwise, remapping the devices to set the primary boot disk to another disk could be achieved with something as the following example.
Code

menuentry "Windows 7 BIOS/MBR" {
     insmod part_msdos
     insmod chain
     drivemap hd0 hd1
     chainloader (hd1,msdos2)+1
}
Probing

To have GRUB2 automatically find the Windows partitions and assign as the root partition, using the following commands. The Windows partition must be first mounted under Linux. See notes at the end of this section concerning missing C:\bootmgr and C:\Boot files and folders, and make sure they exist before trying to boot Windows.

Probing feature requires an additional package not initially pulled in when installing GRUB2.

root # emerge sys-boot/os-prober
Code

# grub2-probe --target=hints_string /mnt/windows7/bootmgr
--hint-bios=hd1,msdos1 --hint-efi=hd1,msdos1 --hint-baremetal=ahci1,msdos1
Code

# grub2-probe --target=fs_uuid /mnt/windows7/bootmgr
2ABF87DC395CFC02

From the output provided by the above two commands, the 'search' line within the below grub2.cfg configuration file can be then be constructed. Remaping the drive and partition as the first hard drive and first partition will make Windows XP or Windows 8 freer of silent errors while loading. Also note from previous ntldr command usage, specifying the $root prefix is no longer needed and if used will generate boot errors.

Code

menuentry 'Microsoft Windows 7 or Windows 8 (on sdb1)' --class windows {
        insmod part_msdos
        insmod ntfs
        insmod search_fs_uuid
        insmod ntldr
        search --fs-uuid --set=root --hint-bios=hd1,msdos1 --hint-efi=hd1,msdos1 --hint-baremetal=ahci1,msdos1 2ABF87DC395CFC02
        drivemap (hd1,msdos1) (hd0,msdos1)
        #Or, "drivemap (hd1,msdos2) (hd0,msdos1)" for those with Windows installed on sdb2)
        ntldr /bootmgr
}
Important
If you do not have the C:/bootmgr and the C:/Boot folders for which the bootmgr file is linked to, you will likely need to install these files using an Administrator escalated cmd.exe shell (or command prompt) with something similar to the following:
Code

bcdboot c:\Windows /s c:

An indication you do not have this Boot folder (and is common with Windows 8 as the Boot folder doesn't seem to be generated by default except for the single c:\bootmgr file), is a boot error message concerning a missing bootmgr file immediately following attempting to boot one of the previously mentioned grub.cfg entries.

GPT Partitions

A UEFI dual boot could be achieved with the following, if, for example, the default bootloader was overwritten with GRUB2 or simply that bootmgr doesn't do the trick for different reasons.

Code

menuentry "Windows 7 UEFI/GPT" {
    insmod part_gpt
    insmod search_fs_uuid
    insmod chain
    search --fs-uuid --no-floppy --set{{=}}root 28cf-35de
    chainloader ($root)/EFI/MICROSOFT/BOOT/bootmgfw.efi
}

Extended features

Enabling resume swap for suspend

Open up /etc/default/grub with root privileges and add

Code

GRUB_CMDLINE_LINUX="resume=/dev/sdXY"

Where XY is the swap partition location, which can be found by

root # fdisk -l

It looks like you are using UUID instead and that's fine. /etc/default/grub only affects the current operating system so don't worry about every Linux OS using GRUB to start using that swap. After finishing your edits, run

root # grub2-mkconfig -o /boot/grub/grub.cfg

(substitute grub.cfg with whatever file grub reads at boot, e.g. it may be named /boot/grub/grub.efi) to update your grub startup information with what you changed in /etc/default/grub.

Chroot

If you have a separate /boot partition and plan to install GRUB2, be sure to mount only the target root partition before chrooting, but not the /boot partition as well. Mount /boot once inside the chroot environment. This is required because grub2-mkconfig will not detect /boot as a separate partition and will assume /boot and root are on the same partition if /boot was mounted before chrooting.

Booting from LVM, RAID and/or Encrypted Volumes

Note
GRUB2 booting to volumes that are using these technologies requires that, when partitioning, the size allotted for the MBR/post-MBR gap be greater than the conventional 32 kilobytes (63 sectors of 512 bytes). This is needed do to the enlarged size of the core.img file which GRUB embeds in the post-MBR gap region. It is recommended that the size before the first physical partition be 1 megabyte (or 2048 sectors of 512 bytes).

For the following two code blocks, observe that the first (only 63 sectors of 512 bytes) will not allow GRUB2 to work with these technologies... though the latter (2048 sectors of 512 blocks) is adequate.

Code


Disk /dev/sda: 160.0 GB, 160000000000 bytes, 312500000 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000cfe45

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *          63    48821534    24410736   8e  Linux LVM

Code


Disk /dev/sda: 320.1 GB, 320072933376 bytes, 625142448 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0009d633

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *        2048      499711      248832   8e  Linux LVM

Booting from LVM Logical Volumes

GRUB2 supports booting from an LVM partition. However, one must set the device-mapper USE flag in order to activate this feature:

File/etc/portage/package.use

sys-boot/grub:2 device-mapper

Reemerge if needed. Next tell GRUB2 to preload its "lvm" module,

root # echo "GRUB_PRELOAD_MODULES=lvm" >> /etc/default/grub

And (re)generate grub.cfg with grub2-mkconfig.

Booting from RAID Array

The documentation on this subject, along with booting from encrypted physical volumes, is close to null. As I do not boot from a RAID array, anyone who managed to boot from a complex RAID array is welcomed to complete this section.

Note
Booting from RAID is only relevant for BIOS [boot]. EFI users shall roll out their custom initramfs or use an initramfs package available out there. Depending on your setup, you may use a BIOS/MBR or BIOS/GPT combination. Here is an example to a BIOS/GPT setup with `/boot' and `root' [`/'] on RAID-1 [1] and Xen on RAID-5.
Note
A note here about grub2-mkdevicemap and UUID re-setting has been removed. grub2-mkdevicemap is no longer distributed. The UUID problem doesn't appear to exist. Installing to the two devices making a RAID1 array seemed to work with no problem; i.e grub2-install /dev/sda && grub2-install /dev/sdb (after creating the expected ef02/bios boot partition).

Booting from a RAID array is very similar to booting from a LVM Logical Volume aside from RAID specific terminology and syntax of RAID partitioned volume.

Code

...
     insmod raid
#    and load the related `mdraid' module `mdraid09' for RAID arrays with version 0.9 metadata, and `mdraid1x' for arrays with version 1.x metadata.
     insmod mdraid09
     set root=(md0p1)
#    or the following for an unpartitioned RAID array
     set root=(md0)
...
Note
RAID user might merge the live ebuild to get the latest bit of fixes and updates.
Note
There's a nVidia (dm_nv) specific module for RAID 3/5, so something like `insmod dm_nv' is necessary for related hardware.

I can guess that this should work with a simple software RAID setup. However, I have no idea or what command, if any exit at the moment of writing, to use to assemble an array such as `mdadm --assemble --scan /dev/md0' that an initramfs could take care of.

Booting from LUKS Physical Volume

Tell GRUB2 to look for cryptodisks

root # echo "GRUB_CRYPTODISK_ENABLE=y" >> /etc/default/grub

and (re)generate grub.cfg with grub2-mkconfig.

Booting from network (PXE)

Using framebuffer display

To have GRUB use a framebuffer graphical display, emerge GRUB with the truetype USE flag enabled. This will install a default font as well as a font conversion utility.

Then, you can configure /etc/default/grub.conf. For example:

File/etc/default/grub.confFramebuffer related settings

# Set resolution and color depth
GRUB_GFXMODE=1366x768x32
 
# Keep resolution when loading the kernel
GRUB_GFXPAYLOAD_LINUX=keep
 
# Set a background image
GRUB_BACKGROUND="/boot/grub/bg.png"
 
# Use a custom font, converted using grub2-mkfont(1)
GRUB_FONT="/boot/grub/fonts/roboto.pf2"

To find out what modes your graphics card supports, use the following commands on the GRUB shell:

(grub) insmod all_video
(grub)
videoinfo

Troubleshooting

First off, checkout if you have an EFI02 (GPT disk) or bios_grub ([G]parted) for BIOS/GPT installation or enough gap before the first partion of the disk or the boot partition for BIOS/MBR installation before firing up `grub2-install' command. This will spare you some time.

Secondly, checkout if /boot/grub/grub.cfg was correctly generated with `grub2-mkconfig' or generate one yourself with at least a `menuentry'. Refers to previous section for more information. And avoid writing `Boots up to prompt' section in the wiki because of failing to do that simple check before rebooting!

GPT hybrid MBR workaround

GRUB2 require an EE00 partition type first in the MBR partition list entry, but that may pose a problem to boot other operating systems in the same disk especially MSDOS/Windows, to recognize a GPT partitioned disk with or without hybrid MBR. A workaround would be simply removing any protective EE00 partition, with fdisk for example but not gdisk! (it won't work with gdisk as the fake partition is just there to protect GPT main table), and boot in BIOS/MBR mode. Just remember to not use other disk tools but gdisk (it should work with [g]parted) to resize after that, else, use gdisk to add protective EE00 partitions to protect at least GPT main and backup partition before using any non GPT aware tools if necessary.

There's another issue, MSDOS/Windows may not find any suitable partition to boot from if there's a EE00 partition protecting GPT main partition table (sector 1-2047). This is happened to me and I had to remove the protective EE00 protective partition before getting a valid MBR partition list in [MS]DOS tools (to format a partition for example, the tools recognized a GPT partitioned disk at least). And then use gdisk to recompute the CHS value for hybrid MBR (in the recovery and transformation menu, or in the expert main menu).

Repopulating EFI Boot Manager entries

In case the Boot Manager entries need to be restored, you can readd them without invoking grub-install. Load the efivars module and install efibootmgr:

root # modprobe efivars
root # emerge --ask sys-boot/efibootmgr

Then add an boot menu entry:

root # efibootmgr --create --gpt --disk /dev/sda --part 1 --write-signature --label "GRUB2" --loader "\\EFI\\GRUB2\\grub.efi"

Installation errors

Attempting to install GRUB to a disk or a partition

  • If you're getting this error:
Code

Attempting to install GRUB to a partition disk or to a partition.  This is a BAD idea.
Embedding is not possible.  GRUB can only be installed in this setup by using blocklists. 
However, blocklists are UNRELIABLE and their use is discouraged.

this could mean that:

  • 1st: A /boot partition is not mounted, simply run as root `mount /boot'.
  • 2nd: If you're trying to install GRUB2 in traditional BIOS/GPT setup, this could mean more likely that there isn't any EF02 or bios_boot to embed core.img. A little partition of a few hundreds kilobytes may be enough, although it's : good to align it's partition to something like 2048 sectors to avoid performance penalty, so a 1 MiB partition should do.
  • If you're trying to install GRUB2 in a partition e.g. /dev/sda5 in BIOS/MBR setup (who did try it to BIOS/GPT?), simply add `--force' cmdline switch to get going. You should be fine with either enough gap before the first sector of the first partition or else use chattr command (as described in BIOS/MBR installation) to get going or else... destroy your partition boundary and possibly your LUKS or LVM2 header if any.
  • If grub2-install does not print any errors message or if the message is about scanning disk files and you have several platforms enabled in /etc/portage/make.conf and nothing is installed in the disk. Then, you should edit `/sbin/grub2-install' `platform=qemu' to `platform=pc', for example, or the appropriate platform in that case. This happened to me, and I had to look arround to found out, at last, that the platform shall be rightly set when installing grub:2.
Note
This is true for sys-boot/grub-1.99x only... for sys-boot/grub-2.x one should edit /etc/grub.d/00_header, transform="s,grub,grub2-qemu," to transform="s,grub,grub2," for example, to install rightly with grub2-$platform-install.

No post-MBR gap

root # grub2-bios-setup ..
grub-setup: warn: This msdos-style partition label has no post-MBR gap; embedding won't be possible!
grub2-bios-setup: warn: Embedding is not possible. GRUB can only be installed in this setup by using blocklists.
            However, blocklists are UNRELIABLE and its use is discouraged.
grub2-bios-setup: error: If you really want blocklists, use --force.

This error means, that grub could not find the usual gap between the MBR and first partition of a disk. It is possible that the disk does not have this gap. Some tools format disks (e.g. LiveUSB disk installer) so that the first partition starts at sector 1 instead of 2048. Repartitioning the disk can fix this. Some visualization-based blockdevices can also cause problems. See this post from the Arch Linux forum for more informations.

Embedding area is unusually small

If you receive this error, it means that there is not enough room at the beginning of your disk before the first partition:

root # grub2-install /dev/sda
grub2-install: warn: Your embedding area is unusually
small. core.img won't fit in it.
grub2-install: warn: Embedding is not possible. GRUB can
only be installed in this setup by using blocklists. However,
blocklists are UNRELIABLE and their use is discouraged.
grub2-install: error: will not proceed with blocklists.

In general this is going to be a bit of trouble to fix. You will need to resize one or more partitions to leave some free space at the beginning of your drive. The gParted liveCD is recommended.

If you are lucky however, your swap partition is the first one on the disk, e.g. /dev/sda1. In that case, you can simply wipe out your swap space and repartition it on the fly.

Begin by disabling the swap partition and launching fdisk:

root # swapoff /dev/sda1
root #
fdisk /dev/sda

This should bring you to the fdisk prompt, where the following commands can be used to shrink the first partition on /dev/sda:

  • 'x' - extra functionality
  • 'b' - move beginning of data in a partition
  • '1' - Choose the first partition
  • '4096' - New beginning of data (1-8000369, default 63)
  • 'w' - write table to disk and exit

A number smaller than 4096 may work, but 4096 worked for the author.

Next, reformat the swap partition, and re-enable it:

root # mkswap /dev/sda1
root #
swapon /dev/sda1

You should now be able to install grub onto /dev/sda.

External resources