Linux Virtualization Concepts

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External

Internal

Generic Virtualization Concepts

Virtualization Concepts

Virtualization Solutions from Red Hat

RHEL 7 includes a hypervisor and a number of virtualization tools, which allows running guest operating systems, so it can function as a virtualization platform. However, the solution supports a limited number of guests per host and a limited range of guest types. Red Hat Virtualization is an enterprise virtualization solution based on the KVM technology, offering more features than Red Hat Enterprise Linux. Red Hat OpenStack Platform support OpenStack clouds.

KVM (Kernel-based Virtual Machine)

KVM is a hypervisor that only runs on systems with hardware supporting virtualization extensions. It is built into the standard RHEL 7 kernel, and it can run Linux, Windows, Solaris and BSD guests. KVM is integrated with the Quick Emulator (QEMU). KVM is managed with the libvirt API. The virtual machines are executed as multi-threaded Linux processes controlled by tools built in top of libvirt. KVM supports overcommitting, kernel same-page merging (KSM), disk I/O throttling, automatic NUMA balancing and virtual CPU hot add.

Xen

Xen can do full virtualization on systems that support virtualization extensions, but can also work as hypervisor on the machines that don't.

Quick Emulator (QEMU)

QEMU Guest Agent

The QEMU guest agent runs on the guest operating system and makes it possible for the host machine to issue commands to the guest operating system.

RHEL 7 Virtualization Administration Guide - QEMU guest agent

libvirt

libvirt is a package providing a hypervisor-independent C-based API that can be used to manage KVM as well as Xen. libvirt's clients are the virtualization management tools. The management tools do not need to be on the same physical machine as the machines on which the hosts are running.

libvirt provides access to management of virtual machines on a host. The API can be used to provision, create, modify, monitor, control, migrate and stop virtual machines. Resources such as CPUs, memory, storage, networking and non-uniform memory access (NUMA) partitions can be listed with libvirt. libvirt delegates management operations to the hypervisor, so only operations supported by the hypervisor can be performed with libvirt.

There is an associated libvirtd service. where? host? guests?.

virsh, the primary command-line virtualization management tool, is baed on libvirt.

libvirt Network Support

Also see Networking and KVM Virtualization below.

Network Filtering Rules

virtio

virtio is a package that provides KVM hypervisor-specific code, and exposes paravirtualized devices to the guest operating system. virtio is a layer sitting between the hypervisor and the guest. All virtio devices have two parts: the host device and the guest driver. The paravirtualized device drivers allow the guest operating system to access the corresponding physical device installed on the host system, and they must be installed on the guest operating system. Examples of paravirtualized devices: the paravirtualized network device virtio-net, the paravirtualized block device (virtio-blk), which is a high-performance virtual storage device supported by the hypervisor, the paravirtualized controller device (virtio-scsi), and others (clock, virtio-serial, virtio-balloon, virtio-rng, QXL graphic card)

RHEL 7 Virtualization Administration Guide - Paravirtualized virtio Drivers

Also see paravirtualization and paravirtualized devices.

KVM and Physically Shared Devices

Also see physically shared devices.

Storage and Virtualization

Virtualization Host Storage

Storage Pool

RHEL 7 Virtualization Administration Guide - Storage Pools

A storage pool is a file, directory or storage device managed by libvirt to provide storage to virtual machines. Storage pools are divided into storage volumes that store virtual machine images or are attached to virtual machines as additional storage. Multiple guests can share the same storage pool. A local storage pool is directly attached to the host server, as a local directory, directly attached disk, physical partition and logical volume management (LVM) volume. Local storage pools do not support live migration. A network (shared) storage pool implies storage shared over the network using standard protocols.

Storage Volume

A storage pool is divided into storage volumes, which are abstractions of physical partitions/LVM logical volumes/etc. Storage volumes are presented to virtual machines as local storage devices, regardless of the underlying hardware.


Guest-Side Storage

The storage for virtual machines is abstracted from the physical storage attached to the host. The storage is attached to the virtual machine using paravirtualized or emulated block device drivers, deployed within the virtual machine. Commonly used storage devices:

Virtual Machine Image Formats

The virtual machine image file can only be stored on a host file system, such as xfs, ext4 or NFS.

Disk image formats include:

  • raw - which contain the content of the disk with no additional metadata. Raw files can be pre-allocated or sparse. Sparse files allocate host disk space on demand, and are therefore form of thin provisioning. Pre-allocated files are fully provisioned but have higher performance than sparse files. Raw files are desirable when disk I/O performance is critical and transferring the image file over a network is rarely necessary.
  • qcow2 - image files offer backing files, snapshot, compression and encryption. They can be used to instantiate virtual machines from template images. qcow2 files are typically more efficient to transfer over the network, because only sectors written by the virtual machine are allocated in the image.

Networking and KVM Virtualization

A virtual machine guest that connects to an external network uses the software network components of the physical host, which are managed by libvirt's virtual network configuration. The host acts as a virtual network switch. By default, all guests on a single host are connected to the same virtual network, named "default". Guests have default direct IP connectivity to all other guests and to the host. However, libvirt network filtering and guest operating system iptables rules apply. Guests have external outbound access via NAT, subject to the host system's firewall rules. From the point of view of the guest operating system, a virtual network connection is the same as a normal physical network connection.

The guest network interfaces can be set to one of the following modes:

  • isolated mode - the network won't allow any traffic beyond the virtualization host.
  • routed mode - the network will route traffic between the guest and external hosts without performing any NAT. This enables incoming connections but requires extra routing table entries for the systems on the external network.
  • bridged mode - the guests are connected to a bridge device that is also connected directly to a physical ethernet device connected to the local ethernet. This makes the quests directly visible on the physical network, and thus enables incoming connections, but does not require any extra routing table entries.

Security and Virtualization

KVM virtual machines use SELinux and sVirt to enforce security.

sVirt

sVirt is a technology included in RHEL 7 to integrate SELinux and virtualization. It applies Mandatory Access Control (MAC) to improve security when using virtual machines.

Steal Time

"Steal time" is the percentage of time a virtual CPU waits for real CPU while the hypervisor is servicing another virtual processor.

A high value means the physical CPU is overcommitted and more physical CPU should be allocated to the environment - or the VM should be moved on a different physical server. The steal time is reported by vmstat st.

RHEL 7 Virtualization Resource Limits

RHEL capabilities and limits
RHEL KVM virtualization capabilities and limits
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