sysboot: add a section on how to determine the used bootloader

[pve-docs.git] / system-booting.adoc

[[sysboot]]
Host Bootloader
---------------
ifdef::wiki[]
:pve-toplevel:
endif::wiki[]

{pve} currently uses one of two bootloaders depending on the disk setup
selected in the installer.

For EFI Systems installed with ZFS as the root filesystem `systemd-boot` is
used. All other deployments use the standard `grub` bootloader (this usually
also applies to systems which are installed on top of Debian).

[[sysboot_determine_bootloader_used]]
Determine which bootloader is used
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[thumbnail="screenshot/boot-grub.png", float="left"]

The simplest and most reliable way to determine which bootloader is used, is to
watch the boot process of the {pve} node.

You will either see the blue box of `grub` or the simple black on white
`systemd-boot`.

[thumbnail="screenshot/boot-systemdboot.png"]

Determining the bootloader from a running system might not be 100% accurate. The
safest way is to run the following command:


----
# efibootmgr -v
----

If it returns a message that EFI variables are not supported, `grub` is used in
BIOS/Legacy mode.

If the output contains a line that looks similar to the following, `grub` is
used in UEFI mode.

----
Boot0005* proxmox	[...] File(\EFI\proxmox\grubx64.efi)
----

If the output contains a line similar to the following, `systemd-bood` is used.

----
Boot0006* Linux Boot Manager	[...] File(\EFI\systemd\systemd-bootx64.efi)
----


[[sysboot_installer_part_scheme]]
Partitioning scheme used by the installer
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The {pve} installer creates 3 partitions on the bootable disks selected for
installation. The bootable disks are:

* For Installations with `ext4` or `xfs` the selected disk

* For ZFS installations all disks belonging to the first `vdev`:
** The first disk for RAID0
** All disks for RAID1, RAIDZ1, RAIDZ2, RAIDZ3
** The first two disks for RAID10

The created partitions are:

* a 1 MB BIOS Boot Partition (gdisk type EF02)

* a 512 MB EFI System Partition (ESP, gdisk type EF00)

* a third partition spanning the set `hdsize` parameter or the remaining space
    used for the chosen storage type

`grub` in BIOS mode (`--target i386-pc`) is installed onto the BIOS Boot
Partition of all bootable disks for supporting older systems.


[[sysboot_grub]]
Grub
~~~~

`grub` has been the de-facto standard for booting Linux systems for many years
and is quite well documented
footnote:[Grub Manual https://www.gnu.org/software/grub/manual/grub/grub.html].

The kernel and initrd images are taken from `/boot` and its configuration file
`/boot/grub/grub.cfg` gets updated by the kernel installation process.

Configuration
^^^^^^^^^^^^^
Changes to the `grub` configuration are done via the defaults file
`/etc/default/grub` or config snippets in `/etc/default/grub.d`. To regenerate
the `/boot/grub/grub.cfg` after a change to the configuration run:

----
`update-grub`.
----

[[sysboot_systemd_boot]]
Systemd-boot
~~~~~~~~~~~~

`systemd-boot` is a lightweight EFI bootloader. It reads the kernel and initrd
images directly from the EFI Service Partition (ESP) where it is installed.
The main advantage of directly loading the kernel from the ESP is that it does
not need to reimplement the drivers for accessing the storage.  In the context
of ZFS as root filesystem this means that you can use all optional features on
your root pool instead of the subset which is also present in the ZFS
implementation in `grub` or having to create a separate small boot-pool
footnote:[Booting ZFS on root with grub https://github.com/zfsonlinux/zfs/wiki/Debian-Stretch-Root-on-ZFS].

In setups with redundancy (RAID1, RAID10, RAIDZ*) all bootable disks (those
being part of the first `vdev`) are partitioned with an ESP. This ensures the
system boots even if the first boot device fails.  The ESPs are kept in sync by
a kernel postinstall hook script `/etc/kernel/postinst.d/zz-pve-efiboot`. The
script copies certain kernel versions and the initrd images to `EFI/proxmox/`
on the root of each ESP and creates the appropriate config files in
`loader/entries/proxmox-*.conf`. The `pve-efiboot-tool` script assists in
managing both the synced ESPs themselves and their contents.

The following kernel versions are configured by default:

* the currently running kernel
* the version being newly installed on package updates
* the two latest already installed kernels
* the latest version of the second-to-last kernel series (e.g. 4.15, 5.0), if applicable
* any manually selected kernels (see below)

The ESPs are not kept mounted during regular operation, in contrast to `grub`,
which keeps an ESP mounted on `/boot/efi`. This helps to prevent filesystem
corruption to the `vfat` formatted ESPs in case of a system crash, and removes
the need to manually adapt `/etc/fstab` in case the primary boot device fails.

[[sysboot_systemd_boot_config]]
Configuration
^^^^^^^^^^^^^

`systemd-boot` is configured via the file `loader/loader.conf` in the root
directory of an EFI System Partition (ESP). See the `loader.conf(5)` manpage
for details.

Each bootloader entry is placed in a file of its own in the directory
`loader/entries/`

An example entry.conf looks like this (`/` refers to the root of the ESP):

----
title    Proxmox
version  5.0.15-1-pve
options   root=ZFS=rpool/ROOT/pve-1 boot=zfs
linux    /EFI/proxmox/5.0.15-1-pve/vmlinuz-5.0.15-1-pve
initrd   /EFI/proxmox/5.0.15-1-pve/initrd.img-5.0.15-1-pve
----


.Manually keeping a kernel bootable

Should you wish to add a certain kernel and initrd image to the list of
bootable kernels use `pve-efiboot-tool kernel add`.

For example run the following to add the kernel with ABI version `5.0.15-1-pve`
to the list of kernels to keep installed and synced to all ESPs:

----
pve-efiboot-tool kernel add 5.0.15-1-pve
----

`pve-efiboot-tool kernel list` will list all kernel versions currently selected
for booting:

----
# pve-efiboot-tool kernel list
Manually selected kernels:
5.0.15-1-pve

Automatically selected kernels:
5.0.12-1-pve
4.15.18-18-pve
----

Run `pve-efiboot-tool remove` to remove a kernel from the list of manually
selected kernels, for example:

----
pve-efiboot-tool kernel remove 5.0.15-1-pve
----

NOTE: It's required to run `pve-efiboot-tool refresh` to update all EFI System
Partitions (ESPs) after a manual kernel addition or removal from above.

[[sysboot_systemd_boot_setup]]
.Setting up a new partition for use as synced ESP

To format and initialize a partition as synced ESP, e.g., after replacing a
failed vdev in an rpool, or when converting an existing system that pre-dates
the sync mechanism, `pve-efiboot-tool` from `pve-kernel-helpers` can be used.

WARNING: the `format` command will format the `<partition>`, make sure to pass
in the right device/partition!

For example, to format an empty partition `/dev/sda2` as ESP, run the following:

----
pve-efiboot-tool format /dev/sda2
----

To setup an existing, unmounted ESP located on `/dev/sda2` for inclusion in
{pve}'s kernel update synchronization mechanism, use the following:

----
pve-efiboot-tool init /dev/sda2
----

Afterwards `/etc/kernel/pve-efiboot-uuids` should contain a new line with the
UUID of the newly added partition. The `init` command will also automatically
trigger a refresh of all configured ESPs.

[[sysboot_systemd_boot_refresh]]
.Updating the configuration on all ESPs

To copy and configure all bootable kernels and keep all ESPs listed in
`/etc/kernel/pve-efiboot-uuids` in sync you just need to run:

----
 pve-efiboot-tool refresh
----
(The equivalent to running `update-grub` on systems being booted with `grub`).

This is necessary should you make changes to the kernel commandline, or want to
sync all kernels and initrds.

NOTE: Both `update-initramfs` and `apt` (when necessary) will automatically
trigger a refresh.

[[sysboot_edit_kernel_cmdline]]
Editing the kernel commandline
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can modify the kernel commandline in the following places, depending on the
bootloader used:

.Grub

The kernel commandline needs to be placed in the variable
`GRUB_CMDLINE_LINUX_DEFAULT` in the file `/etc/default/grub`. Running
`update-grub` appends its content to all `linux` entries in
`/boot/grub/grub.cfg`.

.Systemd-boot

The kernel commandline needs to be placed as one line in `/etc/kernel/cmdline`.
To apply your changes, run `pve-efiboot-tool refresh`, which sets it as the
`option` line for all config files in `loader/entries/proxmox-*.conf`.