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SSDs vs. HDDs: When is all-flash storage overkill?

The SSD vs. HDD analysis isn't as simple as it looks. SSD prices are falling, and capacities increasing. But HDDs are still a good deal and fine for many noncritical workloads.

In the early days of all-flash arrays, organizations used SSDs only when performance needs outweighed price considerations. But as flash prices drop and the technology improves, many organizations are replacing aging HDDs with SSDs.

Yet, HDDs are far from finished in the enterprise. They still make up the bulk of storage in corporate data centers. The case for all-flash isn't nearly as cut and dried as many SSD vendors suggest. As a result, IT decision-makers face a tough choice when comparing SSDs vs. HDDs.

The all-flash revolution

It's no surprise that IT teams turn to SSDs. All-flash arrays deliver lower latency rates and higher IOPS compared to HDDs. They take up less space and use less energy. Until recently, however, SSDs were also known for their low capacities and high cost, especially on a per-gigabyte basis. But with SSD capacities higher than ever and prices dropping, the all-flash array is now a viable storage alternative for any organization.

The biggest boon for flash has been in primary storage, particularly for workloads with high performance requirements, such as virtual desktop infrastructure (VDI) and relational database management systems.

HDDs have a tough time keeping up, especially as innovation and interest move to flash technologies. HDDs reach their maximum throughputs faster than SSDs, and they aren't nearly as reliable or durable.

When looking at SSDs vs. HDDs, the hard disk isn't at the end of its functional life, but progress has slowed to a crawl -- not so for SSDs, where new technologies are enhancing performance, improving capacity and driving down prices. They could leave HDDs in the dust.

Storage capacity planning
Nearly three-quarters of survey respondents said increasing capacity is the goal of their next storage purchase.

Increasing SSD capacities

All-flash SSDs can handle greater capacities than when SSDs first became a viable option for enterprise storage. Samsung came out with a 30 TB SSD in February, and a month later, Nimbus Data unveiled a 100 TB SSD. Compare that to HDDs, in which vendors have only just reached the 14 TB threshold, with industry projections for 20 TB drives pushed off until 2020.

SSDs' increasing capacities are the result of the emergence of new technologies. First, the single-level cell gave way to the multi-level cell, allowing 2 bits of data to be stored in each NAND cell. Then, the triple-level cell came along, allowing 3 bits per cell. Now, we have 3D NAND flash, which vertically stacks multiple layers of NAND cells, providing greater capacities than ever.

NAND flash types compared

SSD vendors are also incorporating advanced capabilities, such as compression and in-line deduplication, into their products. This has resulted in three to five times the amount of usable storage, while still delivering high IOPS and low latencies.

That said, be cautious when estimating the amount of usable storage. Much will depend on the type of data you're working with. For example, if files are already compressed, as in the case of many video files, the compression and deduplication technologies will do little good. The same is true for encrypted files, such as large database backups. Deduplication can't do much because encryption tends to remove the repeatable patterns that the deduplication process looks for, resulting in less usable storage.

Another area in which capacity planning can be tricky is overprovisioning, a practice used with SSDs to enable dynamic reallocation of bad sectors. Be sure to take into account overprovisioning when calculating usable storage.

So, SSDs still come out ahead of HDDs when it comes to capacity but perhaps not to the degree that you might expect, and this can affect TCO.

Arriving at a true TCO

When assessing SSDs vs. HDDs, don't stop at a simple cost-per-gigabyte comparison. Instead, do an all-encompassing evaluation of Capex and Opex. For example, SSDs are normally more reliable and durable than HDDs because they have no moving parts and are less susceptible to shock. And although both types of devices have limited lifespans, SSDs tend to have a lower failure rate and a more predictable point of failure.

In addition, SSDs tend to be easier to use and manage, reducing costs further. They also use less energy than HDDs, are easier to cool and take up less space, leading to further savings.

Another important consideration with SSDs vs. HDDs is performance. If all-flash storage leads to greater productivity, as in the case of VDI, the potential cost savings should be considered, although this is a difficult metric to quantify.

SSDs may be getting all the attention, but HDDs are still a good deal for the money and can handle many noncritical workloads just fine.

HDDs might require a lower initial investment, but long-term considerations can outweigh that advantage. Even so, initial costs shouldn't be ignored. Although SSDs appear to be moving toward per-gigabyte parity with HDDs, they're not there yet, especially when it comes to commodity HDDs.

SSDs built for the enterprise data center cost 25 to 40 cents per gigabyte. High-end HDDs that target the enterprise and deliver 15,000 RPMs cost 25 to 30 cents per gigabyte. Although estimates can vary and prices fluctuate, SSDs and high-end HDDs are approaching parity. That said, many in the industry believe the edge still goes to HDDs.

Commodity HDDs are a different story. They might deliver only 7,500 RPMs, but they come with a significant price advantage over SSDs, costing only a few cents per gigabyte. For many organizations, the advantages that SSDs offer aren't nearly enough to offset the rock-bottom prices of commodity HDDs, especially for storing certain types of data.

Avoiding all-flash overkill

For primary workloads in which performance is critical, SSDs -- or some hybrid variation -- are probably the best option. At this point, IT teams will likely consider HDDs for critical workloads only under special circumstances, such as a short-term business strategy or their company owns a warehouse full of usable HDDs. But if performance isn't a priority, as in the case of backups or archived data, HDDs will probably be the better value, especially if commodity disks are an option.

Keep in mind when assessing SSDs vs. HDDs that using SSDs for secondary storage can offer benefits that may not be apparent at first. For example, they can speed up backup and restore operations, support big data analytics and content streaming, and provide faster response times to unpredictable requests. But if these types of operations aren't a priority and the workloads aren't latency-sensitive, there's little reason to invest in all-flash arrays, at least not until the prices drop further.

Dennis Martin of Demartek talks about ideal workloads for SSD devices.

Before making any decisions, determine how the data will be used and accessed and how much data will be stored. Also, consider the nature of the applications that will access the data. The goal is to arrive at a true TCO that takes into account all considerations. SSDs may be getting all the attention, but HDDs are still a good deal for the money and can handle many noncritical workloads just fine.

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