Solid-state storage, built on a foundation of NAND flash memory technology, has radically altered the look of enterprise and personal storage infrastructure in little more than a decade. Flash shook up a stodgy storage market that was beginning to feel the burden of technology developed back during Eisenhower's first term. Since solid state came on the scene, stacking platters to create more capacity and short stroking to pump up performance now seem like desperate attempts to squeeze just a bit more out of spinning disk gear.
Some flash memory advantages and disadvantages were evident from the beginning, and many of the cons have been addressed as the technology evolved. The first SSDs were far faster than any HDD available, but they had limited capacities, had sticker shock-inducing price tags and labored under the constraints of legacy data pipelines. That situation didn't last long, as NAND flash technologies quickly advanced and the industry overcame many of flash memory's initial shortcomings.
The evolution of solid-state storage is an ongoing process with nearly as many developments today as in the early days. Enhancements to SSDs continue to emerge at a pace that's generally not seen with typically slow-moving computing technology.
But advancements in flash storage technology shouldn't suggest that other storage media has been languishing or left behind. Hard disks still play a key role in enterprise and cloud storage environments, and hard disk manufacturers continue to improve on HDD capacities and performance. Based on shingled magnetic recording technology, Western Digital recently rolled out a hard drive with a 26 TB capacity. Similarly, Seagate offers 22 TB hard drives but promises to more than double that capacity in the next couple of years.
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Storage technologies that aren't solid state or magnetic are also on the horizon -- most notably storage media based on DNA -- and could offer data capacities that would overshadow the capabilities of current products.
That said, solid state still holds many advantages over currently available storage media alternatives. Still, there are some important factors to consider when deciding between solid-state or spinning-disk storage -- or even a combination of the two.
Advantages of flash memory storage
As its name implies, flash storage is fast -- incredibly fast -- when compared to traditional magnetic media. Depending on the criteria used to measure performance -- throughput, IOPS or latency -- solid state tops hard drives by multiples of at least two or three times.
While flash's throughput, which is typically measured in megabytes, can easily top that of HDDs, other SSD performance stats are far more impressive. For example, depending on the data block size, a speedy HDD might clock up to 200 IOPS, while an SSD's IOPS might be measured in the tens of thousands. Improvements to solid-state hardware and software continue to widen the gap because hard disk performance has advanced slower. So, if speed's your need, flash is the clear choice.
Some of flash storage's physical characteristics also give it an edge over hard drives. Flash drives are likely to be more durable than hard drives, as they don't have moving parts like HDDs with stacks of spinning platters and read/write heads traversing those disks. Without motors, actuators and heavy platters, solid-state storage units are lighter than HDDs, use far less electricity and, therefore, produce less heat than hard drives.
SSDs are often packaged to fit drive bays intended for hard drives, but that's just for convenience when using standard interfaces initially intended for HDDs. Newer flash form factors can package the same, or more, capacity in much smaller units, so less space is needed to accommodate petabytes of storage in data center racks.
The PCIe bus and the NVMe interface and protocol provide higher performance conduits for SSD data. This technology reduces the reliance on hard disk-era SATA and iSCSI connection standards and enables more practical form factors, such as M.2 and Enterprise and Data Center Standard Form Factor.
Despite all the moving parts, hard drive technologies have continually improved over their nearly 70-year life span to achieve impressive levels of dependability and durability. Still, flash memory beats or exceeds HDD reliability and endurance, and with no moving parts to jar, flash is far more reliable for use in mobile devices.
Disadvantages of flash memory storage
It's fast, it's small and light, and it barely sips from the power supply. It's hard to imagine any shortcomings when assessing flash memory advantages and disadvantages. For sure, there aren't many downsides, but one significant issue is cost. On a capacity basis, flash storage still costs about five to eight times the price of a comparable-capacity HDD, given the recent price reductions for both types of media.
That multiple has come down significantly over the years, but clearly, all the advantages of flash memory storage come at a price premium, even if it is steadily narrowing. In certain implementations, the cost difference may nearly disappear, particularly if you're using flash drives to replace a bunch of short-stroked, low-capacity 10K rpm hard drives in a performance-oriented array. A handful of SSDs may be able to produce the same or better performance without the waste and expense that short stroking entails.
It's important to note that NAND flash prices have steadily declined and are likely to continue to do so. They may never be as cheap as spinning disk, but as the price delta narrows, flash's advantages will loom even larger. Recent events, including the COVID-19 pandemic, supply chain issues and greater reliance on cloud storage services, have resulted in growing inventories of both flash and magnetic media, with flash prices in particular dipping significantly. For many organizations, the perfect storm that has resulted in cheap media may make this the right time to build out on-premises storage resources.
Even given their higher prices, solid-state devices may prove to be more economical in certain settings, such as larger data centers. Factoring in the associated costs of managing and maintaining a storage installation, such as power requirements, cooling and floor space, the price delta between flash and hard disk storage can disappear completely even if the solid-state devices initially cost more. Solid state's much lower power consumption, cool running and ability to cram petabytes of capacity into a single rack could add up to overall savings with flash or at least parity with hard disk implementations.
Another knock against flash memory storage is its inability to hold up under heavy write loads. Similar to hard drives, repeated use eventually degrades SSDs. With HDDs, writing new data over deleted data is a simpler process because the drive doesn't delete the old data; it just makes space available for new writes. With NAND flash drives, the old data must be removed first before the new bits can be dropped in the cells in a process called program/erase. That slows down writes and increases wear and tear. Manufacturers have addressed the issue in a variety of ways with techniques that reduce the number of processes needed to reclaim cells for new writes.
Another flash endurance issue is linked to the architecture of NAND. The earliest SSDs stored a single bit of data in each flash cell. That resulted in speedy, long-lasting and expensive flash storage. Since then, the number of bits per cell has risen steadily, and today, triple-level cell and quad-level cell (QLC) with 3 and 4 bits per cell, respectively, are the norm.
These newer generations of NAND enable greater capacities and lower prices per gigabyte, but they also slow operations and increase the chances that a bit in one cell may affect data in an adjoining cell. Again, controller technologies have been developed to address these issues, and flash memory endurance is continually improving.
Earlier flash memory drives offered far less capacity than hard disks, so they were relegated to handling specific high-performance functions that didn't require a lot of capacity. But, with flash technologies, such as QLC and 3D NAND, where cells on NAND chips are stacked in tiers on top of each other, capacities for SSDs have risen steadily and are now competitive with HDDs.
The outlook is for flash chips to become even more densely populated, and 4 bits per cell is about to be eclipsed with penta-level cell flash. Five bits get cozy in a single cell to yield SSDs that have considerably higher capacities. And the maximum number of layers in 3D NAND flash continues to grow.
Just as hard disks have bulked up in recent years, SSD capacities continue to rise with several vendors currently offering 30 TB SSDs. Proprietary designs can top that mark, such as Pure Storage's DirectFlash Module with its 48 TB capacity.
In addition, as NVMe over PCIe 4.0 becomes more ubiquitous, SSD IOPS will see a significant boost.
Specialized applications may tilt scales to hard drives
While the media to store data continues to evolve, how we use those resources and our expectations are also changing.
Cloud storage is still largely the domain of hard-disk storage, with tape used for archival applications. Certainly, most cloud storage service providers also offer solid-state storage, but the advantages of flash may be compromised by slow telecom channels. Of course, if the applications accessing the data also reside at the same cloud service, then flash's benefits will be possible.
Given the varied nature of available storage media and resources, a hybrid approach is likely a best bet for most companies, where on-premises, cloud and remote or edge storage can be managed as a single entity. Storage systems -- regardless of the media they use -- should have intelligent migration software to manage data and ensure that it resides in the proper place and on the most suitable media types.
In some instances, the role of storage systems might overshadow the media employed, such as with computational or "smart" storage. The applications that the storage is required to support may even make the case for multiple media types, especially for data-intensive apps that require both vast capacities and ample processing performance to plow through mountains of data.
Best of both worlds
SSDs' falling prices and rising capacities have made them pervasive in everything from enterprise arrays to servers to laptop PCs. But hard disks still have a place, particularly for secondary storage applications, such as backup and archiving.
While all-flash arrays have carved out a chunk of the enterprise market, there are still plenty of applications that can benefit from a hybrid environment where high-performance solid-state storage is paired with high-capacity, inexpensive hard disks. And tape still has plenty of life left in it, with an effective offline and archival repository with portability and low cost that neither HDD nor SDD can beat.