[adzm]

To be technical, MSSQL uses its own bespoke scheduling, and will preempt the thread for io. All io is nonblocking. The physical thread can vary for this reason. PGSQL really does use synchronous io and a single thread though. The former is probably more scalable but the latter has been serving PGSQL fine, too.

[tychver]

I think bitmap heap scans have had concurrent IO for quite a while now? There's the effective_io_concurrency setting for it.