We have a whole section in our documentation's guides section on example usecases with database:
https://docs.kraft.cloud/guides/#databases
They work exactly as you suggest, they scale-to-zero when no requests are made and data can be safely stored/transactioned in persistent volumes on the platform too.
Both the twitter above and the HN user posted the link to findtime.fberge.com
Note everyone it's probably a big coincidence! These two posters most likely do not know each other. This exchange is totally legit.
Looking at https://docs.kraft.cloud/guides/mariadb/ with the Docker stuff in there I'd half assumed maybe DB instances worked differently from unikernel instances or something
Also because of this other comment:
> Because KraftCloud leverages Unikraft, a unikernel library OS project, we are single-process. Postgres is multi-process since it uses the fork() syscall. If you require Postgres, we can set it up for you as a managed service
Regarding multi-process, at present Unikraft has partial multi-process support: some multi-process apps work, others not yet -- we're hard at work completing this support.