The Free On-line Dictionary of Computing (19 January 2023):

OS/VS2
Single Virtual Storage
SVS

    An operating system from IBM, released
   with the IBM S/370 and originally known as OS/MVT.  OS/VS2
   R1 was known as SVS (Single Virtual Storage) as it had a
   single 16 MB virtual address space.  OS/VS2 R2 was known as
   MVS - Multiple Virtual Storage.

   (1999-01-13)