I think this plugin should change it's logic/architecture.

• If we need disk size calculation feature for specific job, then we accept performance/time degradation in job.
• background thread could affect other building job (correct if i mistaken)

So i think that plugin should:
1. have ability enable/disable disk calculation feature per jobs (for example if i do chrooted build calculation will fail, btw because of thread calculator errors i got offline slave).
2. run calculation as a build step (optionally probably background thread could be kept like ("calculate later").
3. have ability to disable background thread.
4. In future this could be base for some configurable quotas for job sizes (linux specific).