Three ways to deploy a hybrid storage system

Hybrid storage lets you keep costs reasonable but still reap the rewards that come with flash. There are three ways you can implement your hybrid approach: use solid-state drives or a shared flash appliance, or install PCIe flash devices on servers.

With the right architecture and a combination of flash and disk storage, you can use the performance benefits of flash and the cost advantages of disks to improve your overall VDI storage performance.

Network and disk latency can adversely affect VDI storage performance, but flash storage offers high performance. It comes at a substantial cost over disk-based storage systems, however.

The key to flash device performance is the solid-state technology, which has no moving parts. Instead of accessing data with read/write heads, solid-state drives (SSDs) read data from flash storage in much the same way that RAM provides data. The latency associated with rotating disk platters and moving read/write heads is absent in flash devices.

Three ways to build a hybrid storage system

The first thing to know about hybrid storage is that there are several different architectures you can deploy. Each has its own performance characteristics and suits different requirements.

Perhaps the simplest method for creating a hybrid storage system is to use solid-state drives alongside disk drives in your storage system. The flash device is encased in a disk drive-like enclosure so it attaches to a storage system the way a hard disk drive does. An SSD also implements the same protocols as hard disk drives, so they are essentially interchangeable with disk drives.

Some advantages of using SSDs include compatibility with existing storage infrastructure and easy installation. You will get faster read and write performance from an SSD than a hard disk drive, but you will not realize the full potential performance gain of a flash device; hard disk drive protocols are not designed for flash storage technology, so they must be mapped to flash device operations.

The second way to deploy a hybrid storage system is as a shared flash appliance between servers and disk-based storage systems. The flash appliance operates as a persistent cache to improve the speed of read and write operations. For example, a VDI application such as a word processing program or a spreadsheet can write a block of data to the flash device and receive an acknowledgement faster than a comparable operation on a hard disk. The application can continue on with other tasks while the flash appliance writes the newly received data to the disk storage system.

This approach confers a couple of advantages. First, because the flash appliance sits between servers and the storage system, multiple servers can enjoy the benefits of the flash appliance. Also, such flash storage can be installed on a PCI Express (PCIe) bus instead of a hard disk drive enclosure. PCIe delivers faster speeds and avoids the overhead involved in emulating hard disk drive protocols.

A third approach requires installing PCIe flash devices directly on servers. This is an especially high-performance option because it eliminates network latency between a CPU and persistent storage. It also gets rid of overhead associated with hard disk drive protocols, as well as latency from the rotation of disk platters and movement of read/write head.

VDI users expect desktop or laptop-like performance from their virtual desktops. Increasing the number and speed of your CPUs and adding more memory can help, but write-intensive operations can still introduce apparent lags in system performance. Other types of peak-demand situations, such as boot storms, can put a heavy read load on VDI environments, too. Alas, while flash-based arrays can ameliorate the lag involved, they can't eliminate such delays altogether.

In an ideal world, you might deploy a large flash-only storage system. But in many situations, that is impractical from a cost standpoint. Nevertheless, deploying flash devices on the PCIe bus of VDI servers can offer substantial performance gains. A shared appliance approach can also improve performance over hard disk drives and SSD deployments.

Ultimately, the cost of deploying a hybrid storage system has to be balanced with the benefits of lower latencies, faster application response time and, in some cases, the ability to scale existing VDI infrastructure to support more users.

About the authors:
Dan Sullivan, M.Sc., is an author, systems architect and consultant with more than 20 years of IT experience. He has worked in industries including financial services, manufacturing, pharmaceuticals, software development, government, retail, gas and oil production, power generation, life sciences, and education. Dan is a series editor and author with Realtime Publishers, a leading provider of expert, third-party content for the IT industry. He has written sixteen books as well as numerous articles and custom white papers.

Ed Tittel is a 30-year-plus veteran of the computing industry, who's worked as a programmer, technical manager, classroom instructor, network consultant and technical evangelist for companies that include Burroughs, Schlumberger, Novell, IBM/Tivoli and NetQoS. He has written and blogged for numerous publications, including Tom's Hardware, and is the author of more than 140 computing books with a special emphasis on information security, Web markup languages and development tools, and Windows operating systems.

Next Steps

Hybrid storage arrays vs. all-flash arrays

Choosing VDI storage

Improving VDI performance with SSD

This was first published in August 2014

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