Hybrid Solid State Disk Solutions

High performance and capacity at a lower cost than pure SSDs

Solid state disks (SSDs) are a popular replacement for standard magnetic hard disk drives. I've used several SSDs in my various computers over the past few years and can vouch for the extreme performance increase, not to mention the reduced heat and noise and excellent power savings. The only drawback is the price of such devices. For use as local storage, the price of SSDs has come down greatly over the past few years while the raw capacity of magnetic drives has gone up. This leads to a difficult decision between a slower but larger-capacity magnetic drive and a faster but smaller-capacity SSD. In many cases, the extra capacity of magnetic drives is hard to ignore, despite the extreme speeds that are possible only with SSDs

SSDs quickly moved from being used only in local storage to being used in network storage. Products such as Texas Memory Systems' RamSan offer great performance in a small package, but, again, the main drawback is price. Many IT pros are left asking themselves if the price-performance trade-off is worth it. After all, not every workload needs the performance that SSDs provide. For example, it makes more sense to store archived files that are accessed rarely or not at all on less expensive hard disk drives than SSDs. But what if you need the speed that SSDs provide but only at certain times of the year (e.g., during tax season)? Or what if you have an application or set of applications that needs the best performance possible, but the cost of an SSD solution is out of your budget?

Fortunately, many vendors jumped on the bandwagon of combining SSDs and hard disk drives, including:

These vendors' hybrid SSD solutions endeavor to balance the best of both worlds: the lower price and larger capacity of hard disk drives with the higher performance and reliability offered by SSDs.

As with everything else in IT, however, nothing is that simple. After speaking with several vendors and doing some research, I learned that not every hybrid SSD solution is created equal. These products are really all about the software that controls them. In this article, I'll focus on two vendors-XIO Storage and NexGen Storage-that focus on providing the best possible performance through their customized software yet differ in how they physically build their hybrid SSD solutions.

XIO Storage

"We do things definitely different, but our lineage goes back for years," Steve Sicola, CTO of XIO Storage told me when speaking about Hyper ISE, the company's hybrid SSD solution. Hyper ISE is a storage array that combines multiple "data packs" consisting of 10 hard disk drives and 10 SSDs in one sealed unit, as Figure 1 shows. Data is moved between the adaptive DRAM cache, the hard disk drives, and the SSDs.

 Figure 1: XIO Storage Hyper ISE
Figure 1: XIO Storage Hyper ISE

Keith Hageman, storage technology evangelist at XIO Storage, expanded on the solution a bit more. "The hottest data gets moved to the SSDs. Cooler data remains on the hard drives. From the server and OS standpoint, it looks like the same LUN."

Hyper ISE takes care of the movement of the data automatically. Every five seconds, the Hyper ISE software checks to see whether the "hottest" 120MB block of data is present on the SSDs. If it isn't, the software moves the hottest data from the hard disk drives to the SSDs.

"We write [the data] to the adaptive DRAM cache first, then the hard drives, and then move it to the SSDs," Sicola said. Because all the data is written to the hard disk drives first, there's a bit of a performance hit when the array is first installed. "By taking the initial writes of the data to the hard drives, we don't have the best performance at startup," Hageman explained. Over time, however, the software moves the 120MB block of hot data to the SSDs, where the best performance exists. Furthermore, the Hyper ISE software is intelligent. It has been "taught" what various workloads (e.g., Microsoft SQL Server workloads, Microsoft Exchange Server workloads) "look like" so that when a data stream comes into the array, the software knows how to best optimize the data placement on disk.

"We've been actively working with the Microsoft SQL Server 2012 team," Hageman told me. "In 2010, the best [SQL Server] performance was on a Hitachi solution, utilizing almost 500 15,000rpm hard disk drives across multiple racks. They delivered 5,400 transactions per second on a 7TB data set at a cost of over $2 million. In 2011, our product handled 11,500 transactions per second in only 3U of rack space with 40 drives at a cost of only $150,000."

NexGen Storage

Although most of the vendors that incorporate SSDs into their solutions are well-known and established in the storage industry (e.g., Dell EqualLogic, NetApp, Fujitsu), there are some fresh new "faces," such as NexGen Storage. When I spoke with Chris McCall, vice president of marketing for NexGen Storage, he mentioned that NexGen and its n5 Storage System, which Figure 2 shows, "just came out of stealth mode" in November 2011.

Figure 2: NexGen Storage n5 Storage System
Figure 2: NexGen Storage n5 Storage System 

"Our two founders came from Left-Hand Networks," McCall explained. "They have lots of knowledge about storage, iSCSI, and so on, and we wanted to take a fresh look at storage system architecture. The customer question was always: How many applications can I put on your SAN? Can I add 10 more applications? What will it do to the performance? It was a constant battle of trying to help the customer."

The answers to these questions are especially important in smaller organizations that need the higher performance that a hybrid SSD storage solution offers but don't have a dedicated team of storage experts who focus just on the storage solution. Many IT professionals in the small-to-midsized business (SMB) sector wear several hats. The n5 Storage System attempts to alleviate some of this management overhead.

"Just tell us how fast you want to go and the system will automatically tier it," said McCall. The NexGen product works off of a Quality of Service (QoS) number set by the administrator in terms of I/O operations per second (IOPS). If, for example, you know that the LUN dedicated to your SQL Server database files needs 20,000 IOPS, you set the NexGen product to provide that. The QoS settings are guaranteed minimums. So, for example, if you need 10,000 IOPS for a file server but the array has 20,000 IOPS available, the n5 Storage System would allow the file server to take advantage of the available 20,000 IOPS.

Another interesting aspect of NexGen's QoS approach is the ability to adjust the QoS on a schedule. For example, you can schedule a higher QoS for a two-hour window when a large report needs to be run. The n5 Storage System also optimizes performance based on the application using it. In other words, it "knows" what SQL Server data access looks like versus Exchange data access and adjusts the QoS accordingly.

Unlike Hyper ISE, the n5 Storage System writes to the SSD first and then moves data to the hard disk drives. The n5 Storage System utilizes up to four Fusion-io SSDs connected directly to the PCIe bus across a maximum of two active-active controllers. A 3U, 32TB n5 Storage System starts at $88,000. The n5 Storage System utilizes 7,200rpm Serial Attached SCSI (SAS) hard disk drives. The system supports up to three additional disk shelves if needed.

Conversely, the Hyper ISE utilizes 10,000rpm hard disk drives that are fixed within the data pack and aren't user accessible. Only the data pack itself is able to be removed and only if the system is powered off. I asked the XIO team how an administrator could replace a drive that failed.

"We have multiple patented processes to repair a drive in place," Sicola explained. "We have access to the drive manufacturing code directly from our drive vendor." Hageman continued: "If a drive really does get in trouble, we can re-manufacture the drive in the data pack."

Like Hyper ISE, the n5 Storage System offers additional features. For example, both products offer:

  • Detailed reporting that shows how an application workload would behave if put on the hybrid SSD solution without actually doing so
  • Management and monitoring software to ensure that the hybrid SSD solution is performing optimally

Different Approaches but Same Goal

Despite having different approaches, both XIO Storage and NexGen Storage are focused on the same goal: maximizing application performance automatically. And both vendors specifically told me that their software is what enables them to achieve this goal.

So, if you're in the market for a hybrid SSD storage solution, it's important to look past the raw numbers that storage vendors will put in front of you. Looking beyond the numbers to the intelligence that the vendors have built in to their products through their software could mean the difference between a storage solution that's highly optimized for your particular workloads and one that is just faster than the solution it replaced.

Although the cost of hybrid SSD solutions is still high, the price has come down considerably over the past few years. They're now at a price point that's attractive to even SMBs that need the most bang for their buck. Prices for these solutions start well under $200,000, and they offer performance up to 10 times that of traditional magnetic drive arrays, so I encourage you to take a hard look at hybrid SSD storage solutions for your next project.

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