Design Considerations for Virtualizing Citrix Provisioning Services

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  Consulting White Paper | Citrix Provisioning Services   Design Considerations for VirtualizingCitrix Provisioning Services   Page 2 Introduction  Today, IT architects strive to virtualize most server workloads in the datacenter. When designing a virtual desktop solution with Citrix XenDesktop or Citrix XenApp, there are conflicting schools of thought with regard to implementing Citrix Provisioning Services as virtual servers. As architectsbegin to standardize on a virtualized platform, the imminent question that always appears to arise is ‘Should Citrix Provisioning Services be installed on a Physical or Virtual Server?’ In the past, it wasalways considered a best practice to have Citrix Provisioning Services installed on a physical server.Now with advances in virtualization technology, there are several options available that make virtualizing Provisioning Services a reality in the enterprise.Citrix Consulting Solutions has been involved in multiple scenarios where Provisioning Services wassuccessfully virtualized within XenDesktop and XenApp environments on all three majorhypervisors (Citrix XenServer, Microsoft Hyper-V, and VMware vSphere). The most notablereference of these successful implementations was accomplished in partnership with Cisco and isdocumented within the Cisco Validated Design documents.In each circumstance, the Provisioning  Services virtual server was designed to ensure that it was adequately able to handle the assigned workload and ensure that the virtual Provisioning Services server was not the major restricting factor within the environment. 1  Based on the experience of the Citrix Consulting Solutions team, this document provides detaileddesign considerations for virtualizing Provisioning Services such as:    Ensure that the hypervisor host is able to distribute processing power across multiple CPUs.     A 10Gbps network is the most conducive environment for virtualizing Provisioning Servicesand the respective Provisioning Services network traffic.    If a 10Gbps network is not available, consider link aggregation at the hypervisor level toprovide more available bandwidth for the virtual Provisioning Services machine.    Consider utilizing SR-IOV or Pass-Through to minimize the virtualization overheadassociated with network intensive virtual machines, such as Provisioning Services.     Always configure Provisioning Services in a high-availability configuration with multiple virtual machines distributed across different hypervisor hosts.     Virtualize Provisioning Services on an x64 version of Windows to take advantages of  Windows System Cache.In summary, Provisioning Services workloads can and have been successfully virtualized in bothXenDesktop and XenApp deployments if the design considerations outlined in this document arethoroughly evaluated. 1 The amount of physical memory available within the physical hypervisor host was the most restrictive factor inscaling the environment.   Page 3 Considerations for Virtualizing Provisioning Services  The decision to implement Provisioning Services on a virtual machine is a decision that needs to bemade based on several considerations with the balance of administration ease, cost savings, and enduser performance all in mind. The following sections outline those key design considerationsassociated with deploying Provisioning Services on a virtual server: Hypervisor  For wide spread adoption, Provisioning Services is hypervisor agnostic, able to be virtualized onXenServer, vSphere, and Hyper-V. Virtualizing the Provisioning Services server can provideadministrative and cost benefits such as the ability to share resources with other virtual machineson the same infrastructure, create a simplified business continuity plan and quickly implementadditional Provisioning Services servers to address demand. But along with the cost andadministrative benefits associated with virtualizing Provisioning Services, IT administrators mustconsider the overhead that virtualization adds into the environment. The architecture of each of the major hypervisors is different, but the fact still remains that virtualization adds an additionallayer of overheard that can affect a Provisioning Services virtual machine performance. Eachhypervisor needs to handle the process of routing the network traffic requests between the virtualmachine and the physical network adapters on the hypervisor server. Figure 1: Hypervisor Architecture  A traditional hypervisor has a virtualization layer that acts as a broker between the virtualmachines and physical hardware devices, such as the network adapter. If a virtual machine needsto send a packet across the network within the hypervisor architecture, the virtualization layerinterprets the request from the driver on the virtual machine, processes the request against thedriver for the physical resources and transfers the request or packet to the physical network adapter. In instances where there is a high demand for network resources within the virtualmachine, such as true with Provisioning Services, the hypervisor overhead may become moreapparent as the amount of instances where the virtualization layer is required to broker requestsbetween virtual to physical resources increases. The demand of processing these types of requests is typically reflected in more intense CPU cycles, therefore reflecting a spike in CPU   Page 4 utilization. The following sections will provide more design considerations to better address thisconcern. Networking  As previously mentioned, Provisioning Services can tax networking resources during peak loadsuch as simultaneous boot-up or logins; therefore, Provisioning Services will greatly benefit fromincreased bandwidth and direct access to the network card. For this reason, it is important tounderstand what existing options are available within a virtual environment to help reduce thehypervisor overhead and increase throughput. The network demand for machines managed by Provisioning Services is greatest during themachine boot-up process. In order to stream the boot-up of a Windows 7 image, Provisioning Services needs to send approximately 210 MB across the network. Individually this data volumeis not significant. However, in the event that a large number of desktops are simultaneously booted (e.g. a restart after a datacenter maintenance window) the network can becometemporarily saturated. For example, multiply 210 MB for a single Windows 7 boot by thenumber of desktops that could be simultaneously starting in a XenDesktop environment to beginto understand the potential network demand. If the Provisioning Services write cache isconfigured for server-side versus the more common client- side or ‘cache on device’s HD’ option,then the network traffic is further increased. If the network in the datacenter does not support10Gbps Ethernet, architects may choose to aggregate multiple 1Gbps network adapters toprovide more through-put. Link aggregation must be configured at the hypervisor level to beleveraged by a virtual Provisioning Services machine. Within each hypervisor, the following methods are supported for NIC bonding:    Citrix XenServer: XenServer provides Source Load Balancing (SLB) in an active/activemode, but only supports load-balancing of virtual machine traffic across the physicalNICs.( ). XenServer will load balance multiple VMs across multiple NICs, but a single VMs traffic is not split across twophysical NICs (i.e. A single VM cannot get the combined data throughput of both NICs).    Microsoft Hyper-V: Microsoft does not officially support link aggregation or NICteaming within Hyper-  V since it’s a third party technology  , but Microsoft doesrecommend working directly with that hardware vendor to determine interoperability  with Hyper-V.( )      VMware ESX: VMware ESX 4.x supports 802.3ad and LACP in static configurations.LACP has both a static and dynamic mode, but only the static mode is supported in ESX.( )  Alternatively, a virtual Provisioning Services machine running within a blade environment can beconfigured so that it only provisions virtual machines within its own individual blade enclosure.
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