System virtual machine advantages
- Multiple OS environments can co-exist on the same primary hard drive, with a virtual partition that allows sharing of files generated in either the "host" operating system or "guest" virtual environment. Adjunct software installations, wireless connectivity, and remote replication, such as printing and faxing, can be generated in any of the guest or host operating systems. Regardless of the system, all files are stored on the hard drive of the host OS.
- Application provisioning, maintenance, high availability and disaster recovery are inherent in the virtual machine software selected.
- Can provide emulated hardware environments different from the host's instruction set architecture (ISA), through emulation or by using just-in-time compilation.
The main disadvantages of VMs are:
- A virtual machine is less efficient than an actual machine when it accesses the host hard drive indirectly.
- When multiple VMs are concurrently running on the hard drive of the actual host, adjunct virtual machines may exhibit a varying and/or unstable performance (speed of execution and malware protection). This depends on the data load imposed on the system by other VMs, unless the selected VM software provides temporal isolation among virtual machines.
- Malware protections for VMs are not necessarily compatible with the "host", and may require separate software.
- The desire to run multiple operating systems was the initial motivation for virtual machines, so as to allow time-sharing among several single-tasking operating systems. In some respects, a system virtual machine can be considered a generalization of the concept of virtual memory that historically preceded it. IBM's CP/CMS, the first systems to allow full virtualization, implemented time sharing by providing each user with a single-user operating system, the CMS. Unlike virtual memory, a system virtual machine entitled the user to write privileged instructions in their code. This approach had certain advantages, such as adding input/output devices not allowed by the standard system.
- As technology evolves virtual memory for purposes of virtualization, new systems of memory overcommitment may be applied to manage memory sharing among multiple virtual machines on one actual computer operating system. It may be possible to share "memory pages" that have identical contents among multiple virtual machines that run on the same physical machine, what may result in mapping them to the same physical page by a technique known as Kernel SamePage Merging. This is particularly useful for read-only pages, such as those that contain code segments; in particular, that would be the case for multiple virtual machines running the same or similar software, software libraries, web servers, middleware components, etc. The guest operating systems do not need to be compliant with the host hardware, thereby making it possible to run different operating systems on the same computer (e.g., Microsoft Windows, Linux, or previous versions of an operating system) to support future software.
- The use of virtual machines to support separate guest operating systems is popular in regard to embedded systems. A typical use would be to run a real-time operating system simultaneously with a preferred complex operating system, such as Linux or Windows. Another use would be for novel and unproven software still in the developmental stage, so it runs inside a sandbox. Virtual machines have other advantages for operating system development, and may include improved debugging access and faster reboots.
- Multiple VMs running their own guest operating system are frequently engaged for server consolidation