The lab-vacuum journal namespace holds five one-megabyte files: four archives and the active one. Remove archives down to a 500K limit and see how much the journal takes.
A limit smaller than the active file
The lab-vacuum journal namespace takes 5.0M: four archives and the active file, a megabyte each. Remove archives with a 500K limit and check how much the journal takes.
First, predict
What will --disk-usage show after --vacuum-size=500K?
Now start the exercise, run the command and compare your prediction with the output.
--vacuum-size removes archives only. The 1M active file stays, so --disk-usage does not drop below its size even if the limit is smaller. You can get rid of its records too by first turning it into an archive with --rotate — but journald immediately starts a new active file of the same size.