5.1Virtualization & containers

A system architect inevitably faces the question of how to run multiple execution environments on limited physical hardware. The virtual-machine approach, which runs multiple whole OSes on one physical server, and the container approach, which isolates only the application execution environment on top of a single OS kernel, differ clearly in isolation strength, startup speed, and resource efficiency, and the choice must fit the requirement at hand. This section covers the difference between the two and the mechanism for running containers at scale.

5.1.1Hypervisors and server/storage virtualization

• A hypervisor is control software that runs multiple virtual machines (VMs) on physical hardware. Each VM has an entire OS, including its own independent kernel, and the hypervisor virtualizes CPU, memory, and disk to allocate them to each VM. Type 1 (bare-metal) installs directly on physical hardware without a host OS, achieving high performance and isolation (the mainstream choice for server virtualization). Type 2 (hosted) runs as an application on an existing host OS; it is easy to set up but has more overhead, making it suited to test environments.
• Storage virtualization treats multiple physical disk devices logically as a single pool, enabling capacity expansion, snapshots, and thin provisioning (dynamic allocation based on actual usage). It is used to hide changes in physical configuration from the application side and to achieve both availability (redundancy) and scalability.