All-flash storage arrays have received a lot of hype in recent years, with many enterprises drawn in by the idea that shifting all operations to SSDs will instantly produce better speeds and operational efficiencies. However, the rush toward all-flash may be subsiding as demand ramps up for hybrid infrastructure that combines SSDs and HDDs.

Hybrid storage arrays are increasingly central to data center operations
By utilizing both flash and magnetic storage, organizations may be able to cut costs in the long run. For example, in a hybrid array, superfluous SSD power isn't expended on workflows such as cloud storage, which involve seldomly accessed data and don't get much marginal benefit from the flash performance boost.

Since flash is still significantly more expensive per gigabyte than disk space, data center operators now usually look to deploy it strategically, in targeted areas. The hybrid array gives them a way to use SSDs as accelerants for storage pools while putting HDDs to work for other tasks, ultimately saving money on hardware as well as precious read/write NAND flash cycles.

In the executive summary of InformationWeek's 2014 State of Storage Report, Kurt Marko explained that opportunistic use of flash (as part of hybrid storage arrays) would enable enterprises to net economic and operational benefits. Moreover, the move toward hybrid is taking place as cloud computing services become a critical tier within the storage hierarchy, making it increasingly important to efficiently manage large quantities of data.

Hybrid storage's fundamental advantage is that it applies just enough flash power to ensure that demanding workloads such as VDI and database indexing are properly executed. It doesn't wear out SSDs by treating them as a one-size-fits-all solution to all data center tasks.

"Much like a hybrid car where the electric motor handles demanding start/stop driving much more efficiently than a gasoline engine but only over a limited range, hybrid arrays provide just enough flash memory for the workloads that need them," Marko explained in a piece for Network Computing. "Hybrid designs allow independent scaling storage capacity and I/O throughput and hence yield a much lower total cost for workloads needing more than about 700 IOPs per TB of storage."

On a more technical level, operators still have to deal with finding a good ratio of flash to disk. Improvements in software are making it easier to dynamically move workloads between SSDs and HDDs in a fraction of a second, but data centers haven't reached the point at which large pools of different drives can be consistently assigned to cloud workloads in response to real-time conditions. For now, though, there are exciting advances in deduplication, which can dramatically reduce the storage footprint.

Why hybrid arrays may be a better choice than all-flash
Many organizations think that they want all-flash arrays, due to marketing promises. A recent survey of 100 IT managers, conducted by X-IO, found that roughly three-quarters of respondents had a positive conception of all-flash and 76 percent thought that these pure setups were faster than hybrid alternatives

Flash is certainly faster in many scenarios, and it goes a long way in reducing the latency for random reads. But this doesn't mean that it's always the right option for each organization's needs, or that day-to-day conditions in the data center don't change flash in unexpected ways.

For example, flash arrays may require more caching and cooling infrastructure than their hybrid counterparts do, canceling out the nominal advantage that SSDs have in power consumption. At the same time, top-grade hybrid arrays may offer better performance than pure flash for sequential write cycles, even if they trail them in random operations.

Three-quarters of X-IO's respondents cited cost as the main impediment to adopting all-flash arrays, but their other responses suggest that workflows don't require such a setup in the first place. Most of them had an application peak around 54,000 IOPS, with only 10 percent topping 100,000 IOPS. This level of activity wouldn't necessarily justify an all-flash array, something that 90 percent of respondents admitted to.

One of the ongoing issues in cloud hardware procurement is finding devices that are up to the task at hand. More than half of the surveyed managers were concerned about the use of consumer-grade flash in enterprise solutions. Marko made a similar argument, pointing out that the MLC devices used in high-capacity SSDs have limited endurance, an aspect that exacerbates issues such as asymmetric read/write performance and increasing slowness as blocks fill up.

Similarly, the perception gap between all-flash and hybrid arrays may be due to overuse of low​-end, relatively unreliable SATA drives rather than the performance and capacity HDDs needed in the data center.The latter move may be an attempt to drive down costs, but there are better ways.

Organizations can capitalize on business opportunities and keep operations running smoothly by using HDDs with SSD cache or SSHDs. In other words, they can use flash as one piece in the puzzle, ensuring that they don't waste money on extraneous power, get set back by outages or fall into an overly rapid replacement cycle.

"IT budgets are currently under tight restrictions, IT managers need to implement a storage solution which provides the right amount of performance required whilst remaining cost effective," X-IO's executive Gavin McLaughlin wrote in Datacenter Dynamics. "In time, storage architects and buyers will realize flash is a tool rather than a solution."

Vendor growth rates underscores advantages of hybrid storage arrays
The growing practicality of hybrid arrays can be seen in the amount of potential business activity surrounding firms that specialize in hybrid storage and software-defined solutions. Writing for ComputerWeekly, 451 Research's Simon Robinson noted that while the all-flash market may be played out as a few large vendors such as NetApp continue to push in-house technologies, a number of startups were seeing rapid growth by offering hybrid products.

Like Marko, Robinson mentioned Nimble Storage as a key player in the hybrid space. Robinon also pointed out the accelerating growth of other vendors such as Tenti and Nexenta, a Seagate Cloud Builder Alliance partner, which could draw the attention of competitors and investors.

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