Appendix B. Red Hat Virtualization system architecture

Appendix B. Red Hat Virtualization system architecture

A functional Red Hat Virtualization system is multi-layered and is driven by the privileged Red Hat Virtualization component. Red Hat Virtualization can host multiple guest operating systems. Each guest operating system runs in its own domain, Red Hat Virtualization schedules virtual CPUs within the virtual machines to make the best use of the available physical CPUs. Each guest operating systems handles its own applications. These guest operating systems schedule each application accordingly.

You can deploy Red Hat Virtualization in one of two choices: full virtualization or para-virtualization. Full virtualization provides total abstraction of the underlying physical system and creates a new virtual system in which the guest operating systems can run. No modifications are needed in the guest OS or application (the guest OS or application is not aware of the virtualized environment and runs normally). Para-virtualization requires user modification of the guest operating systems that run on the virtual machines (these guest operating systems are aware that they are running on a virtual machine) and provide near-native performance. You can deploy both para-virtualization and full virtualization across your virtualization infrastructure.

The first domain, known as domain0 (dom0), is automatically created when you boot the system. Domain0 is the privileged guest and it possesses management capabilities which can create new domains and manage their virtual devices. Domain0 handles the physical hardware, such as network cards and hard disk controllers. Domain0 also handles administrative tasks such as suspending, resuming, or migrating guest domains to other virtual machines.

The hypervisor (Red Hat's Virtual Machine Monitor) is a virtualization platform that allows multiple operating systems to run on a single host simultaneously within a full virtualization environment. A guest is an operating system (OS) that runs on a virtual machine in addition to the host or main OS.

With Red Hat Virtualization, each guests memory comes from a slice of the host's physical memory. For para-virtualized guests, you can set both the initial memory and the maximum size of the virtual machine. You can add (or remove) physical memory to the virtual machine at runtime without exceeding the maximum size you specify. This process is called ballooning.

You can configure each guest with a number of virtual cpus (called vcpus). The Virtual Machine Manager schedules the vcpus according to the workload on the physical CPUs.

You can grant a guest any number of virtual disks. The guest sees these as either hard disks or (for full virtual guests) as CD-ROM drives. Each virtual disk is served to the guest from a block device or from a regular file on the host. The device on the host contains the entire full disk image for the guest, and usually includes partition tables, multiple partitions, and potentially LVM physical volumes.

Virtual networking interfaces runs on the guest. Other interfaces can run on the guest like virtual ethernet Internet cards (VNICs). These network interfaces are configured with a persistent virtual media access control (MAC) address. The default installation of a new guest installs the VNIC with a MAC address selected at random from a reserved pool of over 16 million addresses, so it is unlikely that any two guests will receive the same MAC address. Complex sites with a large number of guests can allocate MAC addresses manually to ensure that they remain unique on the network.

Each guest has a virtual text console that connects to the host. You can redirect guest logins and console output to the text console.

You can configure any guest to use a virtual graphical console that corresponds to the normal video console on the physical host. You can do this for full virtual and para-virtualized guests. It employs the features of the standard graphic adapter like boot messaging, graphical booting, multiple virtual terminals, and can launch the x window system. You can also use the graphical keyboard to configure the virtual keyboard and mouse.

Guests can be identified in any of three identities: domain name (domain-name), identity (domain-id), or UUID. The domain-name is a text string that corresponds to a guest configuration file. The domain-name is used to launch the guests, and when the guest runs the same name is used to identify and control it. The domain-id is a unique, non-persistent number that gets assigned to an active domain and is used to identify and control it. The UUID is a persistent, unique identifier that is controlled from the guest's configuration file and ensures that the guest is identified over time by system management tools. It is visible to the guest when it runs. A new UUID is automatically assigned to each guest by the system tools when the guest first installs.


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