system volumes on scaleway can't easily be modified - instead one has to create
a new image with the desired system volume size. This is way out of scope of
terraform - see https://community.online.net/t/expanding-lssd/907/2 for steps on
how to build a new image.
the `scaleway_server` `volume` attribute should only be used if you want to
attach additional volumes to a server which will share the lifetime of the
server, e.g. they will be destroyed once the server is shut down.
To have volumes which outlive the attached server one should use
`scaleway_volume` and `scaleway_volume_attachement` instead.
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