They do have backing files - after all, unlogged tables can end up many times the size of memory. We need to reserve space for them, so there's a very high like

They do have backing files - after all, unlogged tables can end up many times the size of memory. We need to reserve space for them, so there's a very high likelihood we could evict the buffers / shut down. But they're just written out due to buffer pressure or shutdown checkpoints (+things like slowing the tablespace).