EFI system partition

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Revision as of 16:26, 9 February 2018 by Nl6720 (talk | contribs) (Using bind mount: use "esp" pseudo-variable)
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The EFI System Partition (also called ESP or EFISYS) is a FAT32 formatted physical partition (in the main partition table of the disk, not under LVM or software RAID etc.) from where the UEFI firmware launches the UEFI bootloader and application.

It is an OS independent partition that acts as the storage place for the EFI bootloaders and applications to be launched by the EFI firmware. It is mandatory for UEFI boot.

Warning: If dual-booting with an existing installation of Windows on a UEFI/GPT system, avoid reformatting the UEFI partition, 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 EFIs, ESP size should be at least 512 MiB. 550 MiB is recommended to avoid confusion with 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

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

  • fdisk/gdisk: Create a partition with partition type EFI System (EFI System in fdisk or EF00 in gdisk). Proceed to #Format the partition section below.
  • GNU Parted: Create a FAT32 partition and in Parted set/activate the boot flag (not legacy_boot flag) on that partition. Proceed to #Mount the partition section below.

MBR partitioned disks

Create a partition with partition type EFI System using fdisk. Proceed to #Format the partition.

Format the partition

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

# mkfs.fat -F32 /dev/sdxY

If you used GNU Parted above, it should already be formatted.

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 /boot/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.

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.

  • 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:

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:

Description=Copy EFISTUB Kernel to EFI System Partition


Description=Copy EFISTUB Kernel to EFI System Partition

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 esp/EFI/arch/vmlinuz_linux

Using incron

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

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.
/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.

#!/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
#!/usr/bin/env bash

wait $PPID

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/

echo "Synced kernel 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 :

# mkinitcpio preset file for the 'linux' package

# Directory to copy the kernel, the initramfs...


PRESETS=('default' 'fallback')

default_options="-A esp-update-linux"

fallback_options="-S autodetect"

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

# Directory to copy the kernel, the initramfs...

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

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.

Type = File
Operation = Install
Operation = Upgrade
Target = boot/vmlinuz*
Target = usr/lib/initcpio/*
Target = boot/intel-ucode.img

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/bin/env bash
# Copy kernel and initramfs images to EFI directory


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

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

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

exit 0

Known issues


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