You're mixing up IBM z (mainframes, z/OS) with IBM i (AS/400). The latter has a single level storage system where disks and memory are part of single address s

You're mixing up IBM z (mainframes, z/OS) with IBM i (AS/400). The latter has a single level storage system where disks and memory are part of single address space, and a variant of DB2 database is part of core OS thus making it common to use SQL for many things people wouldn't expect.