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CPU utilization near 10%. Many people would like to use most of the rest of this
capacity, which can be achieved by consolidating workloads.
Workloads?
Operating systems designed for use by multiple users (e.g., most UNIX deriva-
tives) have a long history of running multiple applications simultaneously. These
operating systems include sophisticated features that isolate running programs,
preventing them from interfering with one another and attempting to provide
each program with its fair share of system resources. Even these systems have
limitations, however. For example, the application might assume that only one
instance of that application will be running on the system, and it might acquire
exclusive access to a singular, non-shareable system resource, such as a lock file
with a fixed name. The first instance of such an application locks the file to ensure
that it is the only application modifying data files. A second instance of that appli-
cation would then attempt to lock that same file, but the attempt would inevitably
fail. Put simply, multiple instances of that application cannot coexist unless they
can be isolated from each other.
Even if multiple workloads can coexist, other obstacles to consolidation may be
present. Corporate security or regulatory rules might dictate that one group of
users must not be able to know anything about programs being run by a different
group of users. Either a software barrier is needed to prevent undesired observa-
tion and interaction, or those two user groups must be restricted to the use of
different systems. The different user groups might also have application require-
ments for different OS patch levels, or operate with different system availability
and maintenance windows. In general, however, UNIX-like operating systems are
good platforms for consolidation because they provide user separation and re-
source management capabilities and scale well on large platforms.
Some other operating systems—particularly those that were originally designed
to be single-user systems—cannot be used as a base for consolidated workloads
as easily. Their architecture can make coexistence of similar workloads impos-
sible and coexistence of different workloads difficult. Modifying a single-user OS
so that it can run multiple workloads concurrently can be much more difficult
than designing this capability into the system at the beginning. The use of these
platforms as single-application servers led to the industry mindset of one applica-
tion per server, even on systems that can effectively run multiple applications
simultaneously.
Another solution is needed: the ability—or apparent ability—to run mul-
tiple copies of the operating system concurrently with one workload in each
OS, as shown in Figure 1.3. To the hardware, this arrangement does not differ
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