Whether you allocate enough storage and IOPS to handle boot storms and other times of peak demand can make or break your VDI deployment. One way to deal with high demands is by implementing hybrid storage. In this excerpt from Chapter 2, Hybrid Storage for Virtualization of The Shortcut Guide to Balancing Storage Costs and Performance with Hybrid Storage (Realtime Publishers, 2014), author Dan Sullivan explains how flash can help solve common storage problems in virtual environments. The complete e-book is available to download for free.
Desktop virtualization is technology for separating tightly coupled OSes and applications with underlying end-user hardware. The software components of a desktop are virtualized to run on servers in a data center rather than on local hardware. There are many advantages of virtual desktops:
- Centralized management of desktop images
- Data is saved and backed up centrally
- Potentially improved security
- Access to desktops from multiple devices
- Potentially reduced maintenance costs
Along with these benefits of desktop virtualization come a number of challenges for systems administrators.
Challenges to implementing desktop virtualization
Perhaps the most significant challenge to desktop virtualization in many businesses is user acceptance. The benefits of desktop virtualization, particularly the ability to access a persistent virtual desktop from any device, can be undermined by poor performance. If virtual desktops require long periods of time to boot or application performance is sluggish, users may opt not to use the virtualized desktops that IT departments spend significant time and resources to implement.
Consider the common problem of boot storms, which occur when a large number of users start their virtual desktops at the same time. Many of us work on similar schedules, so it is not surprising that multiple users will need to start their desktops around the same time (see Figure 2.8).
One solution to the problem of boot storms is over-provisioning. Administrators could configure shared storage systems to meet the IOPS and throughput requirements of peak demand, but this approach has cost implications, as discussed earlier. As with server virtualization, the cost savings that might be realized by implementing virtual desktop environments can be quickly undermined with the cost of additional storage hardware.
One of the problems with provisioning for peak demand with hard disk-based storage is that the cost of achieving the necessary IOPS can be high. Disk-based storage systems might provide in the range of 4,000 to 6,000 IOPS, but boot storms could require 10,000s of IOPS to maintain adequate performance.
Hybrid storage is once again a cost-effective solution to this problem. Flash devices provide significantly higher IOPS than hard disk drives, so virtual desktop users can receive the overall device performance they expect, and IT departments can deliver that level of service without unnecessarily high capital expenditures.
Storage requirements for desktop virtualization
To successfully implement VDI, IT professionals have to address three key requirements: storage system performance, cost and desktop image management
VDI has similar demands as mixed-workload virtualization in some cases, and it may utilize the same shared storage as mixed-workload virtualized servers. It is essential to provide sufficient IOPS and throughput to simulate a near-equivalent experience of using a physical desktop. Users may tolerate some variation in response time -- and even some level of reduced performance-- but to ensure adoption, users should not have to compromise performance for the other benefits of a virtual desktop.
The cost factor of shared storage is important as well. Trying to achieve sufficient storage performance using only hard drive-based storage systems can drive up the cost of VDI in unexpected ways. For example, hard disk drive-only storage arrays may not be able to provide sufficient IOPS to meet peak demands when all virtual desktop users are using the same storage systems. In such cases, IT departments may turn to using multiple storage arrays and dividing virtual desktop users across multiple storage systems. This setup has obvious and significant cost implications for delivering VDI.
The methods of desktop image management will vary, to some degree, depending on how VDI is deployed. In some cases, users are assigned desktops from a pool of identical desktops. Users do not have the ability to change these desktops, so they can be assigned any desktop from the pool without altering their user experience. Other VDI implementations provide for persistent desktops. In this case, users can customize their desktops and save those changes. Copies of individual desktops are maintained for each user. Systems administrators have to plan their storage capacity according to the types of desktops (pooled or persistent) they plan to deploy.
Desktop virtualization faces many of the same challenges as mixed-workload virtual environments, as well as some particular to VDI. In both cases, hybrid storage can deliver the performance required to meet business and user requirements while maintaining favorable cost profiles when compared with hard disk–only storage systems.
Benefits of hybrid storage for server and desktop virtualization
The preceding discussion of server virtualization and desktop virtualization presents a series of challenges systems administrators must overcome to deliver cost-effective and adequate storage performance. These include: sufficient IOPS, quality of service, and balancing sufficient IOPS with capacity.
CPUs have steadily increased in performance at a rate faster than the speed of disk drives. Although the capacity of hard drives has grown steadily, the rate of growth in IOPS has not kept a similar pace. In order to provide sufficient IOPS for demanding applications, a combination of flash devices and hard disk drive technologies are offering an alternative to storage systems based solely on hard-drive technology.
Although flash devices can increase the IOPS performance and throughput of a storage device, systems administrators may still be tempted to over-provision in order to ensure adequate performance of mission-critical and business-critical applications during peak demand periods. This problem is better addressed by using hybrid storage systems that support quality of service controls. With such controls, applications can be categorized as mission-critical, business-critical, and non-critical. During times of high demand for storage resources, those resources can be allocated to the most important applications and thus reduce the need to over-provision.
In addition to having adequate performance in a virtualized environment, administrators must consider capacity. This scenario is where hard disk drive technology excels. The cost per gigabyte of hard-disk storage has decreased steadily over the past few decades. By combining flash devices with hard disk drives, hybrid storage systems can deliver a balanced storage system that provides both the performance and capacity needed for today’s enterprise applications.
Getting the most out of VDI IOPS
Deploying a hybrid storage system? Here are three ways to do it.