[systemd-devel] bootctl: default mount point for the ESP partition.

[Simon McVittie]

On 01/09/15 17:21, Goffredo Baroncelli wrote:
> I discovered that bootctl assume as default mount point for the ESP
> partition the /boot directory. Instead it seems to me that the most part
> of distributions prefers /boot/efi.

For some context, the reasoning for /boot/efi is:

In some distributions (presumably including the (Fedora-based?) ones
where this feature was developed), /boot is traditionally treated as
mutable and unpackaged, like /var; so the packages include the kernel in
/usr or /lib or something, and copy it into /boot. The cost of this is
one extra copy of the kernel on-disk, which used to be a significant
amount of space, although on modern disks it doesn't really matter.

In other distributions (like Debian and its derivatives), the OS
packages for kernels traditionally include /boot/vmlinux-whatever and
/boot/config-whatever as packaged files; the corresponding initramfs is
generated at install time and placed alongside them. In these
distributions, there is no second copy of the kernel in /usr or /lib.
This saves some space, although, again, the amount of space saved is
negligible on current hardware.

On BIOS systems, either way was fine, because /boot was either part of
the root filesystem, or a separate small ext2 partition near the
beginning of the drive (to work around BIOS disk-addressability
limitations) for use by lilo or grub or whatever. Similarly, on non-x86,
the bootloader would traditionally either load the kernel from a
non-filesystem like a raw flash partition, or from the root filesystem
or a separate /boot partition; either would be fine.

However, it starts to matter with the move to EFI (on x86 and
elsewhere), or on non-EFI systems where the bootloader also reads a FAT
partition (such as the Raspberry Pi). If the OS packages for kernels
include files in /boot, then having a non-Unix filesystem for /boot
becomes a problem, because the package manager (e.g. dpkg) can't use the
same hardlink-based atomic-overwrite that it would use for robustness on
a proper Unix filesystem. The ESP is FAT, which is not a full Unix
filesystem, so you can't do that sort of thing. That's why Debian and
its derivatives mount the ESP at /boot/efi instead of /boot, and why
<http://sjoerd.luon.net/posts/2015/02/debian-jessie-on-rpi2/> mounts the
ESP-equivalent at /boot/firmware.

AIUI, /boot/efi also makes it a bit easier to have the ESP remain
unmounted or read-only when not in active use, which is good for its own
robustness; a system crash corrupting an unmounted partition is less
likely than corrupting a mounted filesystem.

In principle, it should be possible for the ESP to never be mounted at
all, copying files in with mtools (or equivalent) when required.

-- 
Simon McVittie
Collabora Ltd. <http://www.collabora.com/>