If you’re unfamiliar, PVS (Citrix Provisioning Server) is a vDisk deployment mechanism available for use within a XenDesktop or XenApp environment that uses streaming for image delivery. Shared read-only vDisks are streamed to virtual or physical targets in which users can access random pooled or static desktop sessions. Random desktops are reset to a pristine state between logoffs while users requiring static desktops have their changes persisted within a Personal vDisk pinned to their own desktop VM. Any changes that occur within the duration of a user session are captured in a write cache. This is where the performance demanding write IOs occur and where PVS offers a great deal of flexibility as to where those writes can occur. Write cache destination options are defined via PVS vDisk access modes which can dramatically change the performance characteristics of your VDI deployment. While PVS does add a degree of complexity to the overall architecture, since its own infrastructure is required, it is worth considering since it can reduce the amount of physical computing horsepower required for your VDI desktop hosts. The following diagram illustrates the relationship of PVS to Machine Creation Services (MCS) in the larger architectural context of XenDesktop. Keep in mind also that PVS is frequently used to deploy XenApp servers as well.
PVS 7.1 supports the following write cache destination options (from Link):
- Cache on device hard drive - Write cache can exist as a file in NTFS format, located on the target-device’s hard drive. This write cache option frees up the Provisioning Server since it does not have to process write requests and does not have the finite limitation of RAM.
- Cache on device hard drive persisted (experimental phase only) - The same as Cache on device hard drive, except cache persists. At this time, this write cache method is an experimental feature only, and is only supported for NT6.1 or later (Windows 7 and Windows 2008 R2 and later).
- Cache in device RAM - Write cache can exist as a temporary file in the target device’s RAM. This provides the fastest method of disk access since memory access is always faster than disk access.
- Cache in device RAM with overflow on hard disk - When RAM is zero, the target device write cache is only written to the local disk. When RAM is not zero, the target device write cache is written to RAM first.
- Cache on a server - Write cache can exist as a temporary file on a Provisioning Server. In this configuration, all writes are handled by the Provisioning Server, which can increase disk IO and network traffic.
- Cache on server persistent - This cache option allows for the saving of changes between reboots. Using this option, after rebooting, a target device is able to retrieve changes made from previous sessions that differ from the read only vDisk image.
Results
Even when caching directly to HDD, PVS shows lower IOPS/ user numbers than MCS does on the same hardware. We decided to take things a step further by testing a number of different caching options. We ran tests on both Hyper-V and ESXi using our standard 3 user VM profiles against LoginVSI’s low, medium, high workloads. For reference, below are the standard user VM profiles we use in all Dell Wyse Datacenter enterprise solutions:Profile Name | Number of vCPUs per Virtual Desktop | Nominal RAM (GB) per Virtual Desktop | Use Case |
Standard | 1 | 2 | Task Worker |
Enhanced | 2 | 3 | Knowledge Worker |
Professional | 2 | 4 | Power User |
Hyper-visor | PVS Cache Type | Workload | Density | Avg CPU % | Avg Mem Usage GB | Avg IOPS/User | Avg Net KBps/User |
ESXi | Device HDD only | Standard | 170 | 95% | 1.2 | 5 | 109 |
ESXi | 256MB RAM + Overflow | Standard | 170 | 76% | 1.5 | 0.4 | 113 |
ESXi | 512MB RAM + Overflow | Standard | 170 | 77% | 1.5 | 0.3 | 124 |
ESXi | Device HDD only | Enhanced | 110 | 86% | 2.1 | 8 | 275 |
ESXi | 256MB RAM + Overflow | Enhanced | 110 | 72% | 2.2 | 1.2 | 284 |
ESXi | 512MB RAM + Overflow | Enhanced | 110 | 73% | 2.2 | 0.2 | 286 |
ESXi | HDD only, thin provisioned | Professional | 90 | 75% | 2.5 | 9.1 | 250 |
ESXi | HDD only thick provisioned | Professional | 90 | 79% | 2.6 | 11.7 | 272 |
ESXi | 256MB RAM + Overflow | Professional | 90 | 61% | 2.6 | 1.9 | 255 |
ESXi | 512MB RAM + Overflow | Professional | 90 | 64% | 2.7 | 0.3 | 272 |
Hyper-visor | PVS Cache Type | Workload | Density | Avg CPU % | Avg Mem Usage GB | Avg IOPS/User | Avg Net KBps/User |
Hyper-V | Device HDD only | Standard | 170 | 92% | 1.3 | 5.2 | 121 |
Hyper-V | 256MB RAM + Overflow | Standard | 170 | 78% | 1.5 | 0.3 | 104 |
Hyper-V | 512MB RAM + Overflow | Standard | 170 | 78% | 1.5 | 0.2 | 110 |
Hyper-V | Device HDD only | Enhanced | 110 | 85% | 1.7 | 9.3 | 323 |
Hyper-V | 256MB RAM + Overflow | Enhanced | 110 | 80% | 2 | 0.8 | 275 |
Hyper-V | 512MB RAM + Overflow | Enhanced | 110 | 81% | 2.1 | 0.4 | 273 |
Hyper-V | HDD only, thin provisioned | Professional | 90 | 80% | 2.2 | 12.3 | 306 |
Hyper-V | HDD only thick provisioned | Professional | 90 | 80% | 2.2 | 10.5 | 308 |
Hyper-V | 256MB RAM + Overflow | Professional | 90 | 80% | 2.5 | 2.0 | 294 |
Hyper-V | 512MB RAM + Overflow | Professional | 90 | 79% | 2.7 | 1.4 | 294 |
Implications
So what does it all mean? If you’re already a PVS customer this is a no brainer, upgrade to v7.1 and turn on “cache in device RAM with overflow to hard disk” now. Your storage subsystems will thank you. The benefits are clear in both ESXi and Hyper-V alike. If you’re deploying XenDesktop soon and debating MCS vs PVS, this is a very strong mark in the “pro” column for PVS. The fact of life in VDI is that we always run out of CPU first, but that doesn’t mean we get to ignore or undersize for IO performance as that’s important too. Enabling RAM to absorb the vast majority of user write cache IO allows us to stretch our HDD subsystems even further, since their burdens are diminished. Cut your local disk costs by 2/3 or stretch those shared arrays 2 or 3x. PVS cache in RAM + overflow allows you to design your storage around capacity requirements with less need to overprovision spindles just to meet IO demands (resulting in wasted capacity).References:
DWD Enterprise Reference Architecture
http://support.citrix.com/proddocs/topic/provisioning-7/pvs-technology-overview-write-cache-intro.html
When to Enable Intermediate Buffering for Local Hard Drive Cache
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