On Sat, Jun 26, 2021, 14:06 Johannes Köhler <koehler.johannes@xxxxxxxxxxxxxx> wrote:
Hi systemd maintainer, again!
with my last post i got a hint to
follow the netiquette. My netiquette
with now, was: read manpages and html
searches before asking stupid questions... :)
So to say, i am happy about private messages for adding
concrete rules to that basic...
During my tour through systemd there spawned
some new questions:
_systemd-nspawn_
I think i realized: nspawn is using a machine and an
image to create a virtual machine container, to run
as process or thread within systemd.
1. Can i use systemd-nspawn like QEMU?
nspawn creates a container – *not* a virtual machine. It is much more like LXC or runC than qemu.
Containers only virtualize certain things, such as filesystem or process tree, but they still share the host kernel, and the host will see each individual process running inside a container.
(Usually containers even share the host filesystem, like fancy chroots, although I think nspawn also supports using loop-mounted disk images.)
2. When iam installing an systemd linux distribution
does this result in an systemd container with my
linux installation (kernel, filesystem, etc.)?
Containers don't virtualize the kernel. But they *do* have their own init, so if the guest distro comes with systemd then your container will run systemd.
Further, - runs systemd-nspawn service
before my linux kernel image is running?
(because: my .host is a machine and an image, also)
No. As mentioned, nspawn containers are not virtual machines, they're just fancy chroots that are created using "namespace" features in the host kernel. So they cannot possibly run before the kernel.
And the ".host" isn't really a container at all – it's only a convenience shortcut in systemd's tools (to allow things like 'machinectl shell'), but it doesn't mean that the host is virtualized in any way.
_/var/lib/machines_
When trying to mount a btrfs subvolume as user home directory
on boot, i got an html howto describing this by using
the service var-lib-machines.mount. Later, i realized i
can use mounts in /etc/fstab with btrfs subvolumes directly.
1. What is the /var/lib/machines directory for?
In the documentation of systemd it gets described with
the "usecase" to keep machine images?
It's the default location that's used for `machinectl start` and systemd-nspawn@.service.
What is doing
the service var-lib-machines.mount? Like I told, i used it
to mount a btrfs subvolume to my linux filesystem (home)
during boot sequence. Since that, the service is starting
on boot because it is still enabled but for mounting
my home subvolume while fstab is doing the same...
.mount units are not services; they literally represent actual mounts.
* If you manually mount something, systemd shows a virtual .mount unit for it.
* If you create a real .mount unit and start it, systemd mounts the thing that's described in the .mount unit. That's all it does.
As for fstab... Fstab isn't actually doing anything on its own. During boot, all your fstab entries are *converted* to .mount units and *those* are used to actually mount things. So it is normal to see fstab entries duplicated to .mount units.
https://cloudnull.io/2017/12/btrfs-subvolume-mounts-yes-you-can/
Is that a useless howto?
THX and sincerely
-kefko
--
Wonderful vim doku:
"When a mapping triggers itself, it will run forever"
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