Data Center Tech Blog

At SanDisk®, we have been researching and testing the most advantageous impacts of SSDs in virtualized environments. In the weeks approaching VMworld 2014, I will be sharing more of our testing results and deployment suggestions, including VMware Virtual SAN technical considerations, a white paper we recently published, vSphere’s ‘Swap to Host Cache’ which I recently wrote about on this blog, and other scenarios where SSDs can benefit VDI and virtualized environment workloads.

These blog posts will be followed by detailed white papers to provide helpful guidelines for how to deploy SSDs to achieve greatest performance benefits and cost efficiencies in virtualized environments.

VMware VDI Boot Storm Experiment Using SanDisk SSDs

In this blog post I will demonstrate the benefits of including SanDisk’s ULLtraDIMM SSD, a memory channel storage block device, to significantly reduce desktop boot time and enable the availability of desktops to end users dramatically faster than in an HDD environment. This not only saves end-users frustrating time waiting for their desktops to load, but also delivers improved “desktop boot time” service level agreements that can be defined.

Before discussing our testing results and suggested configurations, I would like to thank Banit Agrawal, Sr. Performance Engineer, from VMware for providing the boot storm measurement scripts which helped us carrying out our testing successfully.

Desktop Boot Storm

VDI consolidates many physical desktops (virtual machines) into one single server. But the benefits of this consolidation and its other features can only be realized if the end user experience is the same or better than their physical desktop experience. What I mean by this is that end users need to have everything from the application response time, to security, ease of use etc. be the same as (or even better) than a physical desktop experience.

The problem for the VDI administrator is that there are critical challenges that need to be addressed before a desktop can be made ready for end user qualification or use. Solving these challenges is critical as they define the overall success of VDI deployment in an organization.
One such challenge is what is termed VDI ‘Boot storm’, which is what happens for example on Monday morning when all employees power on their desktops at the same time, and access the same shared resources simultaneously.

What Happens Behind the Scenes


To better understand the technical challenges (and bottlenecks) of VDI boot storm let us explore how a desktop becomes ready for the end user both VDI and physical environments.

In the case of a physical environment, the user has a dedicated storage drive in their desktop/laptop computer. When the desktop is powered on and boots up, it needs to load the operating system to the memory from the storage drive. This operation is very read intensive, and depending on the performance capability of the storage, the loading time will vary. Typically if an SSD drive is used, the boot time is much faster compared to HDD drive (learn more here).

With each user’s hardware resources fully independent and no resource sharing, this is easy to carry out. And with similar configuration across the workforce, desktops will be ready for use at almost the same time.

However, in VDI environments, where the resources are pooled (often to achieve better efficiencies, for data security requirements, or when remote access is needed), the situation becomes far more complicated. Upon booting, each VDI desktop or virtual machine (VM) consumes a portion of compute and storage from a single server, shared by additional virtual machines that are running in the same host. Though each end user will have a logically dedicated storage drive, the underlying storage hardware is shared among all the VDI desktops running in the server.

And this is where things get complicated – when all these VMs are powered on simultaneously (let’s say on Monday morning) and a very high amount storage IO demand is created from each VDI desktop for the read-intensive booting process. This creates a tremendous amount of IOPS in storage causing IO bottleneck and severely impacting the boot time of each VDI desktop. This impact is termed as “Boot Storm” and is a big challenge for VDI administrators to manage.

If in a physical environment there are, let’s say, 50 users, there will also be 50 dedicated storage drives in each individual machine, whereas for VDI environments, the entire process needs to be contained within just a few drives.

For this reason, it is very critical that the right storage sizing is done to address the VDI boot storm needs and to simplify VDI administrator’s work and ensure minimal end user’s wait time.

Testing Overview

We carried out the following experiment by powering on 50 desktops simultaneously using VMware’s View Planner, VMware’s proprietary VDI workload generator tool (View Planner 3.0.1), and measured individual and total time taken to boot the desktops until ready for use.
We executed this same test using HDDs and ULLtraDIMM and observed the time difference between these two platforms.

To carry out the test we created VMware Horizon View Floating pool desktops in which a Windows 7 desktop master template is created and a desktop pool is created using this master. VMware “Host Cache” feature for pool was disabled in this testing. We initially created the pool in the VMware Datastore backed by HDD drive, and then, using the same master template desktop pool we tested this with the ULLtraDIMM-backed VMware Datastore.

SanDisk ULLtraDIMM

The ULLtraDIMM™ SSD connects flash storage to the memory channel via standard DIMM slots, in order to close the gap between storage devices and system memory. ULLtraDIMM SSDs use SanDisk’s proprietary Guardian Technology™ Platform to meet the endurance needs of write-intensive and mixed-use application workloads. ULLtraDIMM™ SSD is also certified as VMware memory channel storage device.

Testing Results

The graph below shows the impact on boot storm for 50 VM:

VDI boot time ssd vs. hdd ulltadimm

Fig 1: Total Time Taken for 50 VM Boot Simultaneously

Test Bed Configuration for Boot Storm Testing

Storage Configuration:

Server Configuration:
• 2 socket 8 core with HT enabled
• 128 GB of RAM

VDI boot time ULLtraDIMM SSD vs. HDD

Fig 2: Individual VM Boot Time for 50 VM

As you can see from the graph that boot time for each desktop is around 15x faster when the VDI environment is configured to run on ULLtraDIMM. Each VDI desktops running on ULLtraDIMM took less than a minute to booth, where HDD’s required over 10 minutes! This is a significant difference that not only helps the VDI administrator to overcome the technical challenges of VDI Boot Storm, but the user experience sees a very dramatic improvement that is similar to physical desktop environment. It can contribute better workforce productivity and can reduce frustration of lagging infrastructure.


Configuring VMware VDI on ULLtraDIMM is incredibly advantageous in addressing VDI Boot Storm. As you can see from our testing results, VDI desktops can be made available much earlier than in an HDD environment, significantly improving desktop boot time SLAs.

Additionally, in case of ULLtraDIMM, it can be seen that all the desktops are booted up almost at the same time, consistently. This guarantees defining an appropriate SLA and delivers the same physical desktop experience across all users.

From a business perspective, end user VDI desktops will be available much earlier and almost every user will have their VDI desktops ready at same point of time, improving employee productivity.

To learn more about SanDisk solutions for virtualization and VDI workloads visit our website. If you have any questions, you can reach me at, or join the conversation on Twitter with @SanDiskDataCtr

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