While there's no doubt that flash is substantially faster than magnetic cloud storage, its impact on data center design is less straightforward. Certainly, SSDs provide the exceptional speed and power required by demanding applications such as virtual desktop infrastructure and database indexing. Yet these metrics don't tell the whole story – not every workflow is Tier 0/1 and as such may not require that level of performance.

In recent years, enterprises have begun rethinking their approach to storage arrays. All-flash has a certain appeal, but it is hardly a silver bullet for the requirement of all organizations, many of which may not regularly run applications that need sub-millisecond latency and guaranteed quality of service. For companies in this boat, the most efficient approach to cloud storage systems takes into account not only SSDs, but HDDs and hybrid drives (i.e., SSHDs).

"While performance is the primary concern and more difficult challenge to address with legacy storage technology, capacity and the management of data growth still remains an important consideration of modern storage systems," explained George Crump, senior analyst at Storage Switzerland, in a blog post. "On the other hand, hybrid storage systems and especially all-flash storage platforms, have to overcome the higher cost of flash based capacity integration."

HDD storage media, though they can't match the speed of flash, typically offer more capacity, and at a lower cost per TB, too. These attributes are crucial in the context of continually rising data volumes – an April 2014 EMC/IDC report put things into perspective, claiming that all the world's digital files would fill a stack of iPad Airs spanning two-thirds the distance between the Earth and the moon. By 2020, that pile of tablets could cover more than six times the space.

What workflows benefit from SSDs and all-flash arrays?
The emergence of cloud computing services has created new expectations for what constitutes acceptable latency. Accordingly, it's natural that SSDs have become fixtures of facilities operated by cloud storage providers, who are under increasing pressure to ensure that even large volumes of data are quickly retrieved.

The strengths of SSDs are on full in workflows such as VDI. Network Computing's Jim O'Reilly pointed out that individuals are so accustomed to the instant-on behavior of smartphones and tablets that they now expect similar results from VDI, despite the challenges presented by the "boot storms" during which numerous users log in simultaneously.

Flash automatically boosts boot times compared to HDDs. Additionally, it provides superior cost/IOPS, for a workflow (VDI) in which cost/GB is not the most pressing metric. Using all-flash to address VDI ensures performance and reliability levels that cannot be obtained with legacy technology.

"A single all-flash array containing 32 terabytes provides 30 gigabytes per VDI," wrote O'Reilly. "With deduplication of the operating system and most apps, that's a good deal of space for user data. In other words, capacity and dollar per terabyte aren't issues. For now, the metrics to focus on are cost per IOPS and by extension, cost per VDI instance."

Beyond cost/GB and cost/IOPS, flash can confer broad operational benefits in areas such as power usage and rack​ space utilization. An SSD may use as little as one-tenth the power of an HDD. Plus, paying a high upfront cost for flash may be worth it if the resulting array can process transactions more rapidly than its predecessors – opex would decrease despite the rise in capex.

Why HDDs and hybrid arrays still matter
Flash is perhaps better viewed as a tool that can improve data centers than the answer to every cloud-related problem – it is ideal in some use cases and marginal in others. As Joel Hruska explained for ExtremeTech, SSDs were not meant to replace HDDs on a one-for-one basis, but rather supplement them as needed and take on those tasks that tangibly benefit from increased IOPS and speed.

There are areas in which significant gaps between SSDs and HDDs persist, such as price. Although MLC flash is becoming cheaper, by 2016, it may still be eight times as expensive as a performance HDD, with an even bigger ratio in relation to nearline HDDs. Advances in flash technology, including vertical NAND arrangements, will lead to increased densities, but even these strides likely won't eliminate the cost/GB disparity.

As the ongoing use of tape backups demonstrates, enterprises are willing to use older technology for economic reasons and to preserve continuity. DataCore's recent "The State of Software-Defined Storage" report underscored this tendency, finding slow adoption of all-flash storage arrays.

The study's coordinators argued that multiple reasons are behind the so far muted response to all-flash setups and that it is increasingly clear that "new fast hardware technology alone is not the answer," in the words of Evaluator Group analyst Randy Kerns , who contributed to the report.

More than 60 percent of the 388 IT executives who responded to DataCore's survey stated that flash constituted less than 10 percent of their overall storage capacity. These results don't completely square with those from "Managing Storage: Trends, Challenges and Options," an EMC report from 2013 that found that 56 percent of enterprises were using flash in some fashion. But perhaps the difference can be explained by the fact that most of EMC's respondents were using flash as part of hybrid cloud infrastructure rather than a standalone solution.

Ultimately, quick read/write times are a boon to many applications, but not necessarily the metric to strive for when building a storage system. The price of flash has been trending downward recently, but its trajectory is uncertain, meaning that there will be ample reason to consider alternatives for years to come. HDDs are also becoming faster and more spacious, as demonstrated by the specifications of Seagate's recently announced 6TB drive.

Flash will play an increasingly important role in the data center as its underlying technologies evolve and its economics change. However, the sheer diversity of workloads that are run in the typical data center, from intensive computing operations to cold storage, necessitates a nuanced approach to enterprise cloud storage, one incorporating the appropriate media for each task.

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