EFI system partition

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The EFI system partition (also called ESP or EFISYS) is an OS independent partition that acts as the storage place for the EFI bootloaders, applications and drivers to be launched by the UEFI firmware. It is mandatory for UEFI boot.

The UEFI specification mandates support for the FAT12, FAT16, and FAT32 filesystems (see UEFI specification version 2.7, section 13.3.1.1), but any conformant vendor can optionally add support for additional filesystems; for example, Apple Macs support (and by default use) their own HFS+ filesystem drivers.

Warning:
  • The EFI system partition must be a physical partition in the main partition table of the disk, not under LVM or software RAID etc.
  • If dual-booting with an existing installation of Windows on a UEFI/GPT system, avoid reformatting the ESP, as this includes the Windows .efi file required to boot it. In other words, use the existing partition as is and simply #Mount the partition.

Create the partition

The following two sections show how to create an EFI system partition (ESP).

Note: It is recommended to use GPT for UEFI boot, because some UEFI firmwares do not allow UEFI/MBR boot.

To avoid potential problems with some UEFI implementations, the ESP should be formatted with FAT32 and the size should be at least 512 MiB. 550 MiB is recommended to avoid MiB/MB confusion and accidentally creating FAT16 [1], although larger sizes are fine.

According to a Microsoft note[2], the minimum size for the EFI system partition (ESP) would be 100 MiB, though this is not stated in the UEFI Specification. Note that for Advanced Format 4K Native drives (4-KiB-per-sector) drives, the size is at least 256 MiB, because it is the minimum partition size of FAT32 drives (calculated as sector size (4KiB) x 65527 = 256 MiB), due to a limitation of the FAT32 file format.

GPT partitioned disks

EFI system partition on GPT is identified by the partition type GUID C12A7328-F81F-11D2-BA4B-00A0C93EC93B.

Choose one of the following methods to create an ESP for a GPT partitioned disk:

  • fdisk: Create a partition with partition type EFI System.
  • gdisk: Create a partition with partition type EF00.
  • GNU Parted: Create a partition with fat32 as the file system type and set/activate the esp flag on it.

Proceed to #Format the partition section below.

MBR partitioned disks

EFI system partition on MBR is identified by the partition type ID EF.

Choose one of the following methods to create an ESP for a MBR partitioned disk:

  • fdisk: Create a primary partition with partition type EFI (FAT-12/16/32).
  • GNU Parted: Create a primary partition with fat32 as the file system type and set/activate the esp flag on it.

Proceed to #Format the partition section below.

Format the partition

After creating the ESP, you must format it as FAT32:

# mkfs.fat -F32 /dev/sdxY

If you get the message WARNING: Not enough clusters for a 32 bit FAT!, reduce cluster size with mkfs.fat -s2 -F32 ... or -s1; otherwise the partition may be unreadable by UEFI. See mkfs.fat(8) for supported cluster sizes.

Mount the partition

The kernels and initramfs files need to be accessible by the boot loader or UEFI itself to successfully boot the system. Thus if you want to keep the setup simple, your boot loader choice limits the available mount points for EFI system partition.

The simplest scenarios for mounting EFI system partition are:

  • mount ESP to /efi and use a boot loader which has a driver for your root file system (eg. GRUB, rEFInd).
  • mount ESP to /boot. This is the preferred method when directly booting a EFISTUB kernel from UEFI.

Alternative mount points

If you do not use one of the simple methods from #Mount the partition, you will need to copy your boot files to ESP (referred to hereafter as esp).

# mkdir -p esp/EFI/arch
# cp -a /boot/vmlinuz-linux esp/EFI/arch/
# cp -a /boot/initramfs-linux.img esp/EFI/arch/
# cp -a /boot/initramfs-linux-fallback.img esp/EFI/arch/
Note: When using an Intel CPU, you may need to copy the Microcode to the boot-entry location.

Furthermore, you will need to keep the files on the ESP up-to-date with later kernel updates. Failure to do so could result in an unbootable system. The following sections discuss several mechanisms for automating it.

Note: If ESP is not mounted to /boot, make sure to not rely on the systemd automount mechanism (including that of systemd-gpt-auto-generator(8)). Always have it mounted manually prior to the any system or kernel update, otherwise you may not be able to mount it after the update, locking you in the currently running kernel with no ability to update the copy of kernel on ESP.

Alternatively preload the required kernel modules on boot, e.g.:

/etc/modules-load.d/vfat.conf
vfat
nls_cp437
nls_iso8859-1

Using bind mount

Instead of mounting the ESP itself to /boot, you can mount a directory of the ESP to /boot using a bind mount (see mount(8)). This allows pacman to update the kernel directly while keeping the ESP organized to your liking.

Note:
  • This requires a kernel and bootloader compatible with FAT32. This is not an issue for a regular Arch install, but could be problematic for other distributions (namely those that require symlinks in /boot/). See the forum post here.
  • You must use the root= kernel parameter in order to boot using this method.

Just like in #Alternative mount points, copy all boot files to a directory on your ESP, but mount the ESP outside /boot. Then bind mount the directory:

# mount --bind esp/EFI/arch /boot

After verifying success, edit your Fstab to make the changes persistent:

/etc/fstab
esp/EFI/arch /boot none defaults,bind 0 0

Using systemd

Systemd features event triggered tasks. In this particular case, the ability to detect a change in path is used to sync the EFISTUB kernel and initramfs files when they are updated in /boot/. The file watched for changes is initramfs-linux-fallback.img since this is the last file built by mkinitcpio, to make sure all files have been built before starting the copy. The systemd path and service files to be created are:

/etc/systemd/system/efistub-update.path
[Unit]
Description=Copy EFISTUB Kernel to EFI system partition

[Path]
PathChanged=/boot/initramfs-linux-fallback.img

[Install]
WantedBy=multi-user.target
WantedBy=system-update.target
/etc/systemd/system/efistub-update.service
[Unit]
Description=Copy EFISTUB Kernel to EFI system partition

[Service]
Type=oneshot
ExecStart=/usr/bin/cp -af /boot/vmlinuz-linux esp/EFI/arch/
ExecStart=/usr/bin/cp -af /boot/initramfs-linux.img esp/EFI/arch/
ExecStart=/usr/bin/cp -af /boot/initramfs-linux-fallback.img esp/EFI/arch/

Then enable and start efistub-update.path.

Tip: For Secure Boot with your own keys, you can set up the service to also sign the image using sbsigntools:
ExecStart=/usr/bin/sbsign --key /path/to/db.key --cert /path/to/db.crt --output esp/EFI/arch/vmlinuz-linux /boot/vmlinuz-linux

Using incron

incron can be used to run a script syncing the EFISTUB Kernel after kernel updates.

/usr/local/bin/efistub-update
#!/bin/sh
cp -af /boot/vmlinuz-linux esp/EFI/arch/
cp -af /boot/initramfs-linux.img esp/EFI/arch/
cp -af /boot/initramfs-linux-fallback.img esp/EFI/arch/
Note: The first parameter /boot/initramfs-linux-fallback.img is the file to watch. The second parameter IN_CLOSE_WRITE is the action to watch for. The third parameter /usr/local/bin/efistub-update is the script to execute.
/etc/incron.d/efistub-update.conf
/boot/initramfs-linux-fallback.img IN_CLOSE_WRITE /usr/local/bin/efistub-update

In order to use this method, enable the incrond.service.

Using mkinitcpio hook

Mkinitcpio can generate a hook that does not need a system level daemon to function. It spawns a background process which waits for the generation of vmlinuz, initramfs-linux.img, and initramfs-linux-fallback.img before copying the files.

Add efistub-update to the list of hooks in /etc/mkinitcpio.conf.

/etc/initcpio/install/efistub-update
#!/usr/bin/env bash
build() {
	/usr/local/bin/efistub-copy $$ &
}

help() {
	cat <<HELPEOF
This hook waits for mkinitcpio to finish and copies the finished ramdisk and kernel to the ESP
HELPEOF
}
/usr/local/bin/efistub-copy
#!/usr/bin/env bash

if [[ $1 -gt 0 ]]
then
	while [ -e /proc/$1 ]
	do
		sleep .5
	done
fi

rsync -a /boot/ esp/

echo "Synced /boot with ESP"

Using mkinitcpio hook (2)

Another alternative to the above solutions, that is potentially cleaner because there are less copies and does not need a system level daemon to function. The logic is reversed, the initramfs is directly stored in the EFI partition, not copied in /boot/. Then the kernel and any other additional files are copied to the ESP partition, thanks to a mkinitcpio hook.

Edit the file /etc/mkinitcpio.d/linux.preset :

/etc/mkinitcpio.d/linux.preset
# mkinitcpio preset file for the 'linux' package

# Directory to copy the kernel, the initramfs...
ESP_DIR="esp/EFI/arch"

ALL_config="/etc/mkinitcpio.conf"
ALL_kver="/boot/vmlinuz-linux"

PRESETS=('default' 'fallback')

#default_config="/etc/mkinitcpio.conf"
default_image="${ESP_DIR}/initramfs-linux.img"
default_options="-A esp-update-linux"

#fallback_config="/etc/mkinitcpio.conf"
fallback_image="${ESP_DIR}/initramfs-linux-fallback.img"
fallback_options="-S autodetect"

Then create the file /etc/initcpio/install/esp-update-linux which need to be executable :

/etc/initcpio/install/esp-update-linux
# Directory to copy the kernel, the initramfs...
ESP_DIR="esp/EFI/arch"

build() {
	cp -af /boot/vmlinuz-linux "${ESP_DIR}/"
	# If ucode is used uncomment this line
	#cp -af /boot/intel-ucode.img "${ESP_DIR}/"
}

help() {
	cat <<HELPEOF
This hook copies the kernel to the ESP partition
HELPEOF
}

To test that, just run:

# rm /boot/initramfs-linux-fallback.img
# rm /boot/initramfs-linux.img
# mkinitcpio -p linux

Using pacman hook

A last option relies on the pacman hooks that are run at the end of the transaction.

The first file is a hook that monitors the relevant files, and it is run if they were modified in the former transaction.

/etc/pacman.d/hooks/999-kernel-efi-copy.hook
[Trigger]
Type = File
Operation = Install
Operation = Upgrade
Target = boot/vmlinuz*
Target = usr/lib/initcpio/*
Target = boot/intel-ucode.img

[Action]
Description = Copying linux and initramfs to EFI directory...
When = PostTransaction
Exec = /usr/local/bin/kernel-efi-copy.sh

The second file is the script itself. Create the file and make it executable:

/usr/local/bin/kernel-efi-copy.sh
#!/usr/bin/env bash
#
# Copy kernel and initramfs images to EFI directory
#

ESP_DIR="esp/EFI/arch"

for file in /boot/vmlinuz*
do
        cp -af "$file" "$ESP_DIR/$(basename "$file").efi"
        [[ $? -ne 0 ]] && exit 1
done

for file in /boot/initramfs*
do
        cp -af "$file" "$ESP_DIR/"
        [[ $? -ne 0 ]] && exit 1
done

[[ -e /boot/intel-ucode.img ]] && cp -af /boot/intel-ucode.img "$ESP_DIR/"

exit 0

Known issues

ESP on RAID

It is possible to make the ESP part of a RAID1 array, but doing so brings the risk of data corruption, and further considerations need to be taken when creating the ESP. See [3] and [4] for details.

See also