Citrix Hypervisor Features

[Dagny Westall]

Ahypervisor is the basic abstraction layer of software. The hypervisor performs low-level tasks such as CPU scheduling and is responsible for memory isolation for resident VMs. The hypervisor abstracts the hardware for the VMs. The hypervisor has no knowledge of networking, external storage devices, video, and so on.

You need an Intel VT or AMD-V 64-bit x86-based system with one or more CPUs to run all supported guest operating systems. For more information about Citrix Hypervisor host system requirements, see System requirements.For a complete list of Citrix Hypervisor certified hardware and systems, see the Hardware Compatibility List (HCL).

The Xen Project hypervisor is an open-source type-1 or bare-metal hypervisor. It allows many instances of an operating system or different operating systems to run in parallel on a single machine (or host). Xen hypervisor is used as the basis for many different commercial and open-source applications, such as: server virtualization, Infrastructure as a Service (IaaS), desktop virtualization, security applications, embedded, and hardware appliances.

The Control Domain, also called Domain 0, or dom0, is a secure, privileged Linux VM that runs the Citrix Hypervisor management toolstack known as XAPI. This Linux VM is based on a CentOS 7.5 distribution. Besides providing Citrix Hypervisor management functions, dom0 also runs the physical device drivers for networking, storage, and so on. The control domain can talk to the hypervisor to instruct it to start or stop guest VMs.

The Toolstack, or XAPI is the software stack that controls VM lifecycle operations, host and VM networking, VM storage, and user authentication. It also allows the management of Citrix Hypervisor resource pools.XAPI provides the publicly documented management API, which is used by all tools that manage VMs, and resource pools. For more information, see -hypervisor/docs/overview.

Full virtualization, or hardware-assisted virtualization uses virtualization extensions from the host CPU to virtualize guests. Fully virtualized guests do not require any kernel support. The guest is called a hardware virtual machine (HVM). HVM requires Intel VT or AMD-V hardware extensions for memory and privileged operations. Citrix Hypervisor uses Quick Emulator (QEMU) to emulate PC hardware, including BIOS, IDE disk controller, VGA graphic adaptor, USB controller, network adapter, and so on. To improve the performance of hardware-sensitive operations like disk or network access, HVM guests are installed with the Citrix Hypervisor tools. For more information, see PV on HVM.

PV on HVM is a mixture of paravirtualization and full hardware virtualization. The primary goal is to boost performance of HVM guests by using specially optimized Paravirtualized drivers. This mode allows you to take advantage of the x86 virtual container technologies in newer processors for improved performance. Network and storage access from these guests still operate in PV mode, using drivers built in to the kernels.

The I/O drivers contain front-end storage and network drivers, and low-level management interfaces. These drivers replace the emulated devices and provide high-speed transport between VMs and Citrix Hypervisor product family software.

XenServer VM Tools for Windows must be installed on each Windows VM for the VM to have a fully supported configuration. A VM functions without the XenServer VM Tools for Windows, but performance will be significantly hampered when the I/O drivers (PV drivers) are not installed.

You cannot use the Dynamic Memory Control (DMC) feature on Red Hat Enterprise Linux 8, Red Hat Enterprise Linux 9, Rocky Linux 8, Rocky Linux 9, or CentOS Stream 9 VMs as these operating systems do not support memory ballooning with the Xen hypervisor.

Citrix Hypervisor allows you to manage multiple servers and their connected shared storage as a single entity by using resource pools. Resource pools enable you to move and run virtual machines on different Citrix Hypervisor hosts. They also allow all servers to share a common framework for network and storage. A pool can contain up to 64 servers running the same version of Citrix Hypervisor software, at the same patch level, and with broadly compatible hardware. For more information, see Hosts and resource pools.

Citrix Hypervisor resource pool adopts a primary/secondaries architecture, implemented by XAPI. XAPI calls are forwarded from the pool master (the primary) to pool members (the secondaries). Pool members make DB RPCs against the pool master. The master host is responsible for coordination and locking resources within the pool, and processes all control operations. Member hosts talk to the master through HTTP and XMLRPC, but they can talk to each other (over the same channel) through mirror disks (storage migration)

Citrix Hypervisor storage targets are called storage repositories (SRs). A storage repository stores Virtual Disk Images (VDIs), which contains the contents of a virtual disk.SRs are flexible, with built-in support for SATA, SCSI, NVMe, and SAS drives that are locally connected, and iSCSI, NFS, SAS, SMB, and Fibre Channel remotely connected. The SR and VDI abstractions allow advanced storage features such as thin provisioning, VDI snapshots, and fast cloning to be exposed on storage targets that support them.

Each Citrix Hypervisor host can use multiple SRs and different SR types simultaneously. These SRs can be shared between hosts or dedicated to particular hosts. Shared storage is pooled between multiple hosts within a defined resource pool. A shared SR must be network-accessible to each host in the pool. All hosts in a single resource pool must have at least one shared SR. Shared storage cannot be shared between multiple pools.

While Xen Hypervisor works at the core level, there are Citrix Hypervisor specific add-ons related hypervisor-agnostic applications and services available to make the virtualization experience complete.

A windows GUI client for VM management, implemented based on the management API. XenCenter provides a rich user experience to manage multiple Citrix Hypervisor hosts,resource pools, and the entire virtual infrastructure associated with them.

An appliance that balances your pool by relocating virtual machines onto the best possible servers for their workload in a resource pool. For more information, see Workload balancing (/en-us/citrix-hypervisor/wlb.html).

A virtual appliance that enables users to convert existing VMware virtual machines into Citrix Hypervisor virtual machines, with comparable networking and storage connectivity. For more information, see XenServer Conversion manager.

A Virtual Desktop Infrastructure (VDI) product specialized to Windows desktops. Citrix Virtual Desktops uses XAPI to manage Citrix Hypervisor in a multi-host pool configuration. For more information, see Citrix Virtual Apps and Desktops.

Citrix Hypervisor offers two servicing options, the Current Release (CR) and the Long Term Service Release (LTSR). Citrix released regular CRs from XenServer 7.2 through Citrix Hypervisor 8.1, and you can dive further into the CR milestones. If you are on the CR track, Citrix Hypervisor 8.2 is your next upgrade because it is both a CR and an LTSR.

The Citrix Hypervisor LTSR has an extended lifecycle, which means the LTSR will reach End of Life (EoL) five years after the launch date. Organizations are also eligible to add extended support for up to five years after the release has reached EoL. The LTSR will reach End of Extended Support (EoES) at least 10 years after the release date, meaning organizations can stick with this release for up to 10 years if they want.

Citrix recommends customers maintain their LTSR sites with the latest Cumulative Update (CU). CUs are offered on a regular basis and include security patches and updates for the release. To maximize stability and limit code change, few, if any, additional features are added in a CU. Organizations must be on an active Customer Success Services (CSS) program to be eligible for the CUs and extended support lifecycle of the LTSR. For our Citrix Hypervisor Express customers who are using the free version of Citrix Hypervisor, they will no longer receive hot fixes (HF) once the first CU is released.

The LTSR version of Citrix Hypervisor follows the same guidelines as the Citrix Virtual Apps and Desktops LTSR. This is great news for customers implementing Citrix Virtual Apps and Desktops 1912 LTSR because their support lifecycles are on very similar timelines.

We did add one new feature to further enhance the admin experience and the stability of the LTSR. With the release of Citrix Hypervisor 8.2 LTSR, we are introducing the ability for admins to easily replace the Citrix Hypervisor Host TLS certificate. Putting this capability in the hands of the admins, rather than having the certificates be used by default, enables pool admins to ensure the authenticity of the hypervisor in TLS connections.

If you are maintaining a CR environment and are currently on Citrix Hypervisor 8.0 or 8.1, you simply need to perform an update within XenCenter. This is easy with the XenCenter update wizard, which will guide you through the process.

For organizations on the LTSR track, on an earlier version of XenServer, or performing a fresh install, you will be performing an upgrade. You can also perform an upgrade in the XenCenter upgrade wizard, which will guide you through the steps needed.

Get the latest in Citrix Hypervisor features, with extended support and maintenance, by downloading the Citrix Hypervisor 8.2 LTSR today! You can find additional information on both the Citrix Hypervisor LTSR and the Citrix Virtual Apps and Desktops LTSR on the LTSR FAQ page.

Monica Griesemer is a Product Marketing Manager for Citrix DaaS and Citrix Virtual Apps and Desktops. As an advocate for virtualization, she works across product features to bridge individuals with technology. Find her on Twitter at @Monica_Griz.

3a8082e126