[systemd-devel] detaching existing extensions with new systemd extension attach

[serenissi]

What is the usecase for this? sysexts are extensions to /usr (and /opt). 
Services don't write to /usr usually (extensions are usually read only 
too). Anything being read is opened as a fd which survives the 
unmount-mount operation. I can't think of any reason a service would 
want to "lock" /usr mountpoint.

Further if your service need to use binaries from patch extension then 
you want to reload it somehow anyway.

Can you explain a bit more?

serene

On 10/28/24 21:30, Umut Tezduyar Lindskog wrote:
>
> Hello systemd,
>
> We are looking into systemd system extensions for quick patching our 
> immutable images while we are updating and rolling out the patched 
> immutable image.
>
> One thing that is stated in the documentation is that before attaching 
> a new extension, all previous extensions are deattached and everything 
> are attached again. We understand that this sequence is needed for 
> overlayfs stacking in case something has changed in the stack with the 
> new extension.
>
> For our use case, we would like to be able to push multiple patch 
> extensions (ex: couple of days apart) while keeping the service 
> downtime to a minimum. Technically we believe that if the extensions 
> are not overlapping there is no need to detach already attached 
> extensions. There is no built in option in systemd extensions to keep 
> the existing extensions right? Due to how overlayfs works, they 
> temporarly need to be removed and thenbrought up again, during a 
> refresh operation.
>
> While on the topic, we have investigated other file systems. One we 
> put our attention to is mergerfs, which is a fuse file system to 
> allows "mount points" to be dynamically added/removed without doing a 
> mount/unmount. Is there support in systemd extensions to let system 
> extension work with other file systems than overlayfs?
>
> Thanks
>
> Umut
>
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