Information Technology Reference
In-Depth Information
basic non-hierarchical file system (each user had virtual disks called “minidisks,”
implemented by cylinder extents on physical disks).
This design—a simple OS leveraging services from the hypervisor—permitted
the system to have a small memory footprint and a short instruction-path length
well suited for providing short response times on the computers of the day. The
idea of a lightweight virtualization environment can be seen in unrelated later
systems such as Solaris Containers that shift “heavy lifting” to the hosting envi-
ronment. This design pattern was also deployed for non-interactive users: CMS
guests running disconnected from a terminal were used to implement “service
virtual machines”—a precursor to UNIX daemons—for scheduled or event-driven
work. Soon, a significant client base used CP/67 virtual machines and interactive
computing.
It also became clear that the extremely complex (for the times) OS/360 would
be shipped too late to satisfy many customers' needs, and would require more
memory than was available on many 360 models. As a result, the 360 used several
incompatible operating systems (DOS/360, OS/360) with different programming
conventions. Several institutions began to use CP/67 to run multiple-guest OS
instances on the same physical machine, so the same machine could be shared by
different workloads.
Even though deployment of virtual machines remained limited, technology and
business drivers for virtual machines had clearly arrived by the early 1970s. At
this point, the virtual machines concept had already taken on its defining role—
running multiple OS instances on the same physical server.
Virtual Machines Become More Widely Adopted
Virtual memory in the IBM 360 line was available only on the 360/67, which was
considered a special-purpose system. When IBM's System/370 was announced,
it notably lacked virtual memory (typically called “virtual storage” in IBM docu-
mentation, distinguishing it from “real storage”—that is, RAM), but that capabil-
ity was introduced in 1972. CP/67 was ported to the new hardware architecture
and renamed VM/370 (and CMS was retitled “Conversational Monitor System”).
Many customers found virtual machines essential for operating multiple work-
loads or converting to new operating systems. There was no alternative, short of
dedicated test systems, to run a new operating system in parallel with the existing
production environment. VM reduced the risk of converting to a new operating
system by permitting more comprehensive testing. Customers could ensure that a
new operating system environment was reliable before installing it on bare metal.
Because VM permitted OS testing to occur during normal daytime hours in-
stead of on weekends or during the third shift at three in the morning, it was not
Search WWH ::
Custom Search