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May 05, 2014

Citrix XenDesktop at the Speed of Memory

Aaron Parker - Atlantis




Atlantis Computing has conducted a series of tests specified by the Citrix Ready program designed to validate storage solutions for 750 seat XenDesktop deployments.  The testing was performed with the Atlantis ILIO Diskless VDI solution, eliminating the need of physical storage for the VDI images by utilizing Atlantis ILIO  In-Memory storage technology.

In this blog post, we present the results of testing a high-performance XenDesktop 7.5 deployment, highlighting very good user experience, reducing the risk of managing storage and lowering the cost of storage to below that of a regular PC.

The test setup in the Atlantis Labs has been reviewed and validated by the Login VSI team.

Test Results Summary


The test results show very good performance for stateless desktops, eliminating the need for storage for the Windows images or the PVS Write Caches, and simplifying the XenDesktop deployment with PVS . With Atlantis ILIO, XenDesktop deployments scale linearly as the VDI deployment can grow by simply adding servers and the Atlantis ILIO software.

The highlights of the test results are:

  • User Experience – Excellent Login VSI scores with a baseline of 1,236 and a maximum VSI Average score of approximately 1800
  • Storage Performance – 425 IOPS available on average per virtual desktop, with complete linear scalability by simply adding servers and the Atlantis ILIO storage optimization software
  • Risk Reduction – Reducing the PVS Write Cache sizing risk by oversizing the size of the PVS Write Cache without using extra storage or memory capacity
  • TCO – Storage CAPEX cost of $59 per virtual desktop including both Atlantis ILIO and user profile storage, providing on average 425 IOPS per desktop
  • Simplicity – Deploy your stateless XenDesktop deployment without storage or SSDs

The Business Challenge

Virtual Desktop Infrastructure (VDI) provides a compelling way to deliver Windows desktops. VDI provides the benefits of centralized management and improved security over physical desktops while enabling users more flexibility in the way they work. VDI is not without its own challenges however, with storage performance and capacity being the primary blockers to successful VDI deployments.

Centralizing Windows desktops also centralizes the high random IO generated by those desktops and thus places pressure on the storage platform. This is different than server virtualization, because the ability of the storage system to provide large amounts of random IO for each desktop will has a direct impact on the user experience.

Organizations need to address this challenge without exceeding their VDI budgets by delivering VDI environments that can scale storage performance linearly without the risk of introducing storage or network bottlenecks.

Overview: Citrix VDI Capacity Program for Storage Partners

In Q1 2014, Citrix launched a new program designed specifically to address the storage needs of customers who are implementing XenDesktop using the VDI FlexCast approach. VDI presents multiple types of data, each with its own unique requirements, to the storage infrastructure tier. Storage in turn can respond to with these requirements using a variety of hardware and software based approaches, some of which can be combined in hybrid solutions. The variety of choices and the differences between them has led to some confusion for customers and partners. To resolve this, Citrix designed a series of storage-related tests to validate the performance, capacity and cost of storage for pooled (a.k.a stateless, non-persistent) desktops with Citrix Provisioning Server (PVS). All tests were run using Windows Server 2012 with Hyper-V.

Test Methodology

The focus of the testing approach has been to provision an appropriate amount of storage performance and capacity for an environment of 750 desktops. As an industry standard approach to testing scalability and gather metrics, Login VSI has been used to validate the environment.

Login VSI was used to simulate a standard user workload (Medium with Flash) using an image loaded with common applications including Microsoft Word, Excel, PowerPoint and Internet Explorer and Adobe Reader.

A single VDI host configuration was used to deliver a minimum of 68 Windows 7 desktops with the PVS write cache being hosted by Atlantis ILIO Diskless VDI. As explained later in this blog, the architecture of Atlantis ILIO Diskless VDI with Citrix PVS provides a high-performance In-Memory datastore for the PVS Write Cache that uses the RAM on each server. As a result it is only necessary to test a single server, which are then used as building blocks to achieve a 750 user or larger deployment that scales linearly.

Atlantis ILIO Solution Overview

Atlantis ILIO optimization software delivers a unique solution that addresses the challenges of stateless virtual desktop environments. The solution complements Citrix XenDesktop to simplify and de-risk stateless VDI deployments.

Atlantis ILIO Diskless VDI provides IT with a simple,  powerful and scalable platform for deploying stateless VDI  that eliminates the risks of the storage and network bottlenecks  that often derail VDI projects. As the project scales, the  solution provides consistent and reliable desktop performance  that is easier to manage and maintain than local SSDs.ilioarchitecture

Atlantis ILIO storage optimization software enables customers to deploy stateless VDI with no storage or SSDs required, while lowering the CAPEX and OPEX costs by up to 80 percent . The simplified storage and network architecture costs less to operate and is more reliable than traditional shared storage or local SSDs

Atlantis ILIO In-Memory Storage™ technology enables organizations to deploy virtual desktops using server RAM as primary storage for virtual desktops. This unique solution delivers a fantastic user experience that drives virtual desktop acceptance and adoption of VDI across your organization.

Atlantis ILIO Diskless VDI Delivering Management at Scaleiliocenter

Atlantis ILIO simplifies and automates the deployment of infrastructure to support thousands of VDI desktops, while enabling high availability. Atlantis ILIO Center Deployment Services tools have been used at some of the largest desktop virtualization deployments in the world to automate deployments, provide fast provisioning and scale up Citrix XenDesktop deployments. Atlantis ILIO Center tools help prevent configuration mistakes and eliminates the time-consuming process of integrating servers, storage and networking. Atlantis ILIO seamlessly integrates with the hypervisor’s high availability capabilities to provide automated recovery when a server or rack encounters a failure.

Atlantis ILIO Diskless VDI Key Benefits:

  • Risk Reduction – Eliminates the risks of storage and network bottlenecks, simplifies implementations with automated deployment and fast cloning
  • Scalability – Provides linear scalability without requiring costly and complex storage infrastructure
  • Cost – Reduces VDI CAPEX/OPEX by up to 80%, driving down storage and networking costs
  • Performance – Delivers better-than-PC performance with faster boot times, application launches and search
  • Flexibility – Works with any storage and any server hardware to simplify XenDesktop implementations

Test Summary

The Atlantis ILIO Diskless VDI solution was tested by Atlantis Computing using Login VSI to scale horizontally up to 750 virtual desktops provisioned by Citrix Provisioning Server (PVS). The solution easily passed the Login VSI Medium workload test, showing amazing user experience with a VSI Index Average of between 1239 ms and 1800ms. On average, 425 IOPS were available per virtual desktop, providing very fast boot times and near-instant application launch times.

As the Atlantis ILIO Diskless VDI architecture scales completely horizontally without the presence of common VDI bottlenecks such as shared storage arrays or storage networking, virtual desktop deployments can scale to any level by just adding new servers and the Atlantis ILIO Diskless VDI software.

The solution as tested is available at a storage price of $59 per desktop, and includes the following components:

  • Atlantis ILIO Diskless VDI
  • Dell PowerVault NX3200 for User Data and ILIO SnapClone storage

Storage Solution Architecture Design

Typical virtual desktop deployments use shared storage, such as SAN or NAS, to store the virtual desktop images or the PVS Write Caches.  This shared storage quickly becomes a bottleneck when scaling up a virtual desktop deployment, as the performance of the storage cannot keep up with the IO requirements from all the virtual desktops.

Atlantis ILIO Diskless VDI completely eliminates the storage requirement for the virtual desktop images (or PVS Write Cache) by using its In-Memory Storage Technology, allowing the virtual desktops to use RAM as primary storage.  This also allows the virtual desktop deployment to scale horizontally, because there are no resource bottlenecks anymore.  By adding new servers and Atlantis ILIO software, you can scale up your VDI deployment easily to the level you want to be.

This horizontal scalability also means that the user experience of a virtual desktop installed in a single server environment is exactly the same as the one for a 750 seat virtual desktop deployment.  As such, our test environment only contained one physical server for the 68 virtual desktops. The Citrix test environment contained 11 servers for 750 desktop (or 68 virtual desktops per physical server).  We actually provisioned and tested with 80 virtual desktops on the host to fill out the available RAM.

pvs per vmSample Write Cache drive seen from within the virtual desktop

The system specification for the server we used are:

  • SuperMicro X8DTU-6+
  • 2 x Intel Xeon CPU X5670 @ 2.93 GHz (12 physical cores)
  • Memory – 144 GB
  • Disk Used for SnapClone : 270 GB SATA

The Citrix Ready VDI Capacity program required that the solution provides enough storage for the PVS Write Cache as well as the user data.  The Atlantis ILIO solution does not require physical storage for the PVS Write Cache, instead storing them on the In-Memory datastore, so we can eliminate the storage performance that was reserved for them.

With ILIO, physical storage is not required for the PVS write cache, the datastore is held in RAM. To allow for maintenance of the hypervisor, Atlantis ILIO has a feature called SnapClone. SnapClone saves a snapshot of the In-Memory datastore to persistent storage so it can recover the virtual desktops after a power outage, or reboot of the hypervisor. which would wipe out the In-Memory datastore. SnapClone is not used to provide active storage performance for the virtual desktops, only to recover the datastore for operational tasks. SnapClone storage can be provided by existing SAN storage or local disk.

To mirror a typical real-world environment, we have added the storage capacity requirements for SnapClone images to the solution requirements. The following Dell PowerVault storage solution was selected to provide the storage for the user data as well as the SnapClone images:

  • PowerVault NX3200, Intel Xeon E5-2609v2, 2.5GHz, 8GB RAM
  • 4TB 7.2K RPM SATA 3Gbps 3.5in Hot-plug Hard Drive
  • Intel Ethernet X540 DP 10Gb BT + I350 1Gb  BT DP Network Daughter Card
  • iDRAC7 Express
  • Two Hot-plug Power Supply (1+0), 750W
  • 3Yr Basic Hardware Warranty Repair: 5×10 HW-Only, 5×10 NBD Onsite

The quote via the Dell website came out to $8386.53. Most customers would typically receive a discount on their Dell purchase, so in this case we’ve conservatively applied a 20% discount to the list price, resulting in a cost of $6709.22. For the physical storage then, this equates to a cost of $8.95 per desktop.

dell powervault 
Dell PowerVault NX3200

Atlantis ILIO was configured with 50GB of reserved memory.  It was configured with 2 virtual CPUs, and the storage for the ILIO operating system was provided by a 270 GB local SATA disk.

Atlantis ILIO Diskless VDI was configured to export an iSCSI LUN to the Microsoft Windows Server machine, and the Microsoft iSCSI initiator was used to connect to the LUN.  PVS was configured to store the write caches on the Atlantis ILIO disk.

We increased the size of the Write Cache to 10GB per virtual desktop to significantly decrease the risk associated with Write Cache Sizing.  Because of the Atlantis ILIO storage optimization, any unused PVS Write Cache capacity requires no additional storage or memory capacity.

The Windows virtual desktops were configured per the following table:

Windows Virtual Desktop Specification  
Operating System Windows 7, 32 bit
Storage Specification 40 GB base image, 10 GB PVS Write Cache
Memory 1.5 GB (Dynamic memory enabled)
Processors 1vCPU


Storage Solution Test Data

Storage Capacity Savings

The memory utilization of the ILIO Virtual Machine was measured after 3 Login VSI runs, resulting in a memory consumption of 9,076 MB (this includes some memory for the ILIO system itself).  The per-VM memory consumption was 113MB.

The following screenshot shows the 894 GB of Write Cache for the virtual desktops as seen by the host operating system (on the I: drive); however, in reality the actual amount of consumed space can be seen by looking at the memory utilization on the ILIO appliance.

hypervstoragesummaryWrite Cache consumed space as seen by the host operating system

The second screenshot of the ILIO command line shows the memory utilization after 3 Login VSI runs (see 9076 under the Used column):

iliomemfreeMemory statistics on the ILIO appliance showing the 9076 MB of actual usage after deduplication

This represents a storage capacity savings of 99% (892GB of Write Cache vs 9,076 MB of ILIO memory consumption).

Storage Performance Testing

Storage performance was tested using Iometer, with a configuration to represent a typical desktop workload, as outlined in a previous blog post. The result of the Iometer test shows there are 28,949 IOPS available per server.  With the test density of 68 virtual desktops per host this comes out to 425 IOPS per desktop available on average, resulting in very fast desktop boot and login times, as well as near-instant application launches. The average latency during the Iometer test was 2.2 ms.

Total IOPS seen with an Iometer test


IOmeter-Latency-Read-Write-MixIometer test results

Typical Login VSI testing with the Medium workload generates less than 13 IOPS per VM for the duration of the test, meaning that you theoretically could run more than 2000 Login VSI sessions on a single host with Atlantis ILIO.

User Experience

The user experience was tested with Login VSI 3.7 and showed fantastic results.  At all times during the complete Login VSI test run, the response times stayed below 2000ms, with the maximum VSI Index Average reaching about 1,800 and a VSI Baseline of 1,239. This demonstrates that with Atlantis ILIO Diskless  providing optimised storage user experience will be consistently great regardless of the load on the server. Not achieving the VSImax value represents the fact that there was additional capacity above the 76 test users run concurrently during the test.

loginvsigreenLogin VSI results (click for larger view)

As stated above, an Atlantis ILIO Diskless VDI deployment does not have bottlenecks like shared storage, so the user experience (and Login VSI scores) do not degrade at all when scaling up your virtual desktop deployment.

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