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List:       systemd-devel
Subject:    [systemd-devel] elegant way to use a single mount unit file for multiple paths
From:       Marcel Partap <mpartap () gmx ! net>
Date:       2020-01-22 17:59:20
Message-ID: a55559ad-3e45-106f-0a50-6735f18f0382 () gmx ! net
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Salut,
for our live debian USB stick distro for students ( \
https://github.com/fsfw-dresden/usb-live-linux ), we want to minimize on flash writes \
by putting several paths into a tmpfs overlay. So there is a pre-overlay@.service \
with

> [Unit]
> Description=Prepare writable overlay on %f
> DefaultDependencies=no
> RequiresMountsFor=/run
> 
> [Service]
> Type=oneshot
> ExecStart=/bin/mkdir -p '/run%f'
> ExecStart=/bin/mount -t tmpfs -o noatime tmpfs '/run%f'
> ExecStart=/bin/mkdir -p '/run%f/work' '/run%f/rw'

and f.e. a var-lib-apt-lists.mount with
> [Unit]
> Description=tmpfs overlay of %f
> Conflicts=umount.target
> Wants=pre-overlay@%p.service
> After=pre-overlay@%p.service
> 
> [Mount]
> What=overlay
> Type=overlay
> Where=%f
> Options=lowerdir=%f,upperdir=/run%f/rw,workdir=/run%f/work
> 
> [Install]
> WantedBy=local-fs.target

Currently, the file is just cloned for multiple paths, which is working but not very \
elegant. I tried changing it into an instanced unit file tmpfs-overlay@.mount, which \
did not work ("Unit type mount cannot be templated" in journal only, nothing from \
systemd-analyze verify or the systemd.mount man page).. Neither did calling the file \
tmpfs-overlay.mount.unit and symlinking to it as var-lib-apt-lists.mount, \
var-cache-apt.mount etc.

So what would be a more elegant way to not have multiple copies of the very same \
file?

Best Regards,
#marcel
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