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Citrix -Provisioning Services Architecture

In a XenServer-only implementation, virtual disk files for each virtual machine are stored in a storage location. Organizations with a significant number of virtual machines incur an increase in storage cost, as well as maintenance costs.
Provisioning Services complements XenServer by providing simplified virtual machine management and reduced storage requirements. Provisioning Services allows an engineer to create one standard virtual machine image that is stored and updated. This standard image is streamed to server or desktop target devices. Resource mappings stored on a Provisioning Services server identify which virtual machine images are assigned to each target device. Workloads can easily be repurposed by updating a vDisk mapping and rebooting the target device.

Provisioning Services vDisk changes can be stored in a cache file. Storing changes for each target device that shares a common virtual machine image requires far less storage space than individual virtual machine files in an environment that utilizes server or desktop provisioning.

Traditional Disk Architecture

In a traditional disk architecture, input, such as key strokes or mouse clicks, is passed to the server and is stored on the server RAM. The processor requests instructions from the hard disk controller; these are then passed to the hard disk. The hard disk finds the requested data then passes it to the hard disk controller which then passes it back to the RAM. The RAM sends the data and instructions to the processor where it is processed into output that is sent to the user in the form of screen updates

Provisioning Services Disk Architecture
In a Provisioning Services architecture, input, such as keystrokes or mouse clicks, is passed to the target device and is stored in the server RAM. Unlike a traditional disk architecture, the request for instructions from the hard drive is sent to the NIC on the target device. The request is then sent to the hard disk controller, which in this case is the Provisioning Services server. The Provisioning Services server then locates the appropriate hard disk for that target device from a storage device and forwards the data request. The correct data is found on the hard disk, sent back to the Provisioning Services server, and then sent back to the NIC on the target device. That data is sent back to the RAM, then processed into output in the form of screen updates

Provisioning Services Farm Layout
Although a Provisioning Services farm layout can vary based on individual organization requirements, a farm always consists of a grouping of Provisioning Services servers, target devices and vDisks that are connected to the same database and license server. Provisioning Services servers maintain constant database connectivity to stream vDisks to target devices; therefore the database should be located in physical proximity to the Provisioning Services servers to minimize latency and ensure optimal target device performance.
The database can be placed on a LAN or WAN as long as all Provisioning Services servers can access it. However, streaming vDisks to target devices requires a robust data link between all Provisioning Services servers and target devices in a farm, such as a LAN, to ensure optimal performance.

Traffic Isolation
When possible, vDisk streaming traffic should be isolated from normal production network traffic, such as Internet browsing, printing and file sharing. It is best to use multiple NICs—a single NIC for PXE traffic and bonded NICs for streaming the vDisks to target devices. This separation provides more consistent performance to the streamed operating systems and also prevents conflicts between the streaming traffic and the production network traffic.

Configuring NICs to Separate Stream Service from LAN Traffic
An engineer can use the following procedure to connect separate NICs to a PXE network and a LAN.
  1. Verify that the network interfaces are on isolated subnets so that the PXE and LAN traffic are routed separately and do not conflict with each other.
  2. Run the Provisioning Services Configuration Wizard to bind the Stream Service to the network interface that will be used for vDisk streaming.
  3. Specify the network interface that will be used for the PXE services from the Provisioning Services server
NIC Teaming
Network I/O on the Provisioning Services server can be a limiting factor in server scalability. Teaming two NICs for throughput provides the server with a maximum of 2Gb of network I/O, which increases the network performance and helps alleviate a potential bottleneck.
Additionally, teaming the NICs eliminates a single point of failure, which occurs when only one NIC is available.
The following NIC teaming considerations should be taken into account:
  • Provisioning Services supports Broadcom and Intel NIC teaming drivers.
  • A target device will only failover to Provisioning Services servers that are in the same subnet as the PXE boot server.
  • Load balancing is not supported in NIC teaming implementations.
This must be taken into account for large scale Provisioning Services implementations.
For a virtualized Provisioning Services server, it is recommended to create a NIC bond at the host server level to mitigate the load balancing limitation and decrease the complexity of the Provisioning Services configuration because the NIC teaming requirements do not have to be met on the virtual machine. 

Write Cache Location
A vDisk write cache can be stored in any of the following locations:
  • Target device RAM
  • Target device local storage
  • Target device shared storage
  • Provisioning Services local storage
  • Provisioning Services shared storage
The decision for where to place the target device write cache will impact server and target device performance, server scalability and overall cost. The following descriptions provide benefits and considerations for each write cache location that can help engineers determine the most appropriate location for it in the environment
Target Device RAM
Selecting this write cache location sets aside a portion of the RAM on the target device for the write cache.
Fastest type of write cache
  • RAM is diverted from workload use
  • The cost is greater than using storage
  • Determining the amount of RAM required for the write cache is difficult, yet critical to the stability of the environment
  • Target device fails when the allocated write cache space reaches capacity

Target Device Local Storage
Selecting this write cache location dedicates a portion of the target device local storage for the write cache. The local storage can be either a physical or virtual disk drive.
  • Does not require additional resources if physical target devices already have local disks installed and unused
  • Provides fast response times because the read/write to/from the write cache is performed locally
  • Determining the size of the write cache is critical to prevent server failure
  • Local storage typically provides more than enough space for the write cache, minimizing risk of underestimating disk requirements
  • Live migration is not possible on virtual target devices because the storage is not shared across virtual infrastructure servers
  • Not as fast as target device RAM cache
Target Device Shared Storage
Selecting this write cache location places the write cache on a shared storage device attached to the target device. This write cache type is usually only valid in environments that use virtual target devices, such as Citrix XenServer. The storage is assigned to each virtual machine from a shared storage repository.
  • Response times are faster
  • Storage costs are significantly cheaper than RAM
  • Live migration is possible because the target device cache storage is accessible from multiple virtual machines
  • Slower than target device RAM or local disk cache
  • Setup and configuration of a shared storage solution is required if one is not already in place
Provisioning Services Local Storage
Selecting this write cache location stores the write cache on the physical disks on the Provisioning Services server.
  • Simplest option to set up
  • No additional resources or configuration within the environment required
  • Disk space is inexpensive
  • Performance is the slowest due to requests to/from the write cache traversing the network between the target device and Provisioning Services server.
  • Provisioning Services server scalability is reduced because Stream service must also service the write cache requests.
  • Provisioning Services HA is not possible because the write cache storage is not accessible by other Provisioning Services servers.
  • Provisioning Services server will fail if the local storage space is exceeded.
Provisioning Services Shared Storage
Selecting this write cache location places the write cache on a shared storage solution that is connected to the Provisioning Services server.
  • Provisioning Services HA is possible because all Provisioning Services servers attached to shared storage can access the write cache
  • Shared storage devices typically hold a large amount of data, which mitigates storage size concerns
  • Performance is reduced because requests to/from the write cache must traverse the network between the target device, Provisioning Services server and the shared storage.
  • Provisioning Services server scalability is reduced because Stream service must also service the write cache requests.
  • Setup and configuration of a shared storage solution is required, if one is not already in place.

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