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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 pros and cons of flash memory were evident from the beginning. The first SSDs were far faster than any hard disk drive (HDD) available, but they had limited capacities and sticker-shock-inducing price tags. That situation didn't last long, as NAND flash technologies quickly evolved and the industry overcame many of flash memory's initial shortcomings. However, there still 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 -- very 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 ranging from two or three times to differences measured in orders of magnitude.
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 a solid-state drive's IOPS might be measured in the tens of thousands. 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 any moving parts compared to highly mechanical HDDs that have 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 and 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.
Despite all the moving parts, hard drive technologies have continually improved over their nearly 70-year lifespan 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 the pros and cons of flash memory. For sure, there aren't many disadvantages, but one significant issue is its cost. On a capacity basis, flash storage costs five to eight times the price of a comparable-capacity HDD.
That multiple has come down significantly over the years, but clearly, all the advantages of flash memory storage come at a price. 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, their advantages will loom even larger.
Another knock against flash memory storage is its inability to hold up under heavy write loads. Similar to hard drives, repeated use will eventually degrade 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. This issue has been addressed in a variety of ways with techniques that reduce the number of processes needed to reclaim cells for new writes.
Another endurance issue is linked to the architecture of NAND flash. The earliest solid-state drives 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 with three bits per cell and quad-level cell (QLC) with four bits per cell are the norm. These newer generations of NAND enable greater capacities and lower prices per GB, 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.
Best of both worlds
SSDs' falling prices and rising capacities have made them ubiquitous 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. And 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 -- and inexpensive -- hard disks.