SSD vs. HDD: Choosing between solid-state and hard-disk drives

Before the last hard-disk drive spins down, IT teams need to navigate an HDD-to-SSD transition. Here's a look at current options and best practices.

When it comes to venerable IT technologies, it's hard to beat the hard-disk drive (HDD).

Introduced by IBM in 1956, the Model 350 RAMAC – larger than a home refrigerator – could store up to 3.75MB of data. While modern HDDs are smaller, faster, and capable of holding multiple terabytes of data, the underlying technology has changed little over the past 60-plus years.

HDDs and tape drives ruled the desktop and data-center storage world virtually unopposed until the past decade or so, when NAND flash solid-state drives (SSD) began maturing to the point where they could not only rival or surpass HDDs in term of capacity, speed and reliability, but also on the basis of cost for certain applications.

Most experts believe that SSDs are destined to eventually become the predominant storage technology. However, making the choice between SSDs and HDDs today remains far from clear cut. "There is no straightforward answer," says Devesh Tiwari, an assistant professor of electrical and computer engineering at Northeastern University.

Tiwari advises IT leaders to consider a variety of issues before deciding on the appropriate technology for a particular storage application, including workload size and demand, latency and bandwidth needs, and storage architecture and infrastructure connectivity requirements. It's also helpful to assess basic storage factors such as elasticity, reliability and availability, while understanding that conclusions made today may not hold true in the near future as SSD technology and pricing continue to evolve. "Nothing is constant; this space is evolving rapidly," Tiwari says.

Different types of SSD drives

A traditional HDD stores data on a high-speed rotating disc, known as a platter. As the platter spins, an arm equipped with a pair of magnetic heads (one for each side of the platter) moves over the surfaces to read or write data. Bits of data are organized into concentric, circular tracks. Each track is divided into smaller areas called sectors. Most hard drives use a stack of platters, mounted on a central spindle with a small gap in-between them. A sector map created by the HDD records which sectors have been used as well as those that remain free.

Unlike an HDD, an SSD has no moving parts. Instead, data is written to and read from a substrate of interconnected flash memory chips. SSD manufacturers stack the memory chips in a grid to achieve varying densities. To prevent volatility, SSDs use floating gate transistors (FGRs) to hold the electrical charge. This technique enables an SSD to retain stored data even when it's not connected to a power source.

IT organizations can turn to several different types of SSDs, including:

  • SLC: Single-Level Cell SSDs store a single bit in each cell, an approach that aims to produce enhanced performance, endurance and accuracy. Pricier than most other flash memory options, SLC SSDs are widely used for an extensive range of mission-critical enterprise applications and storage services.
  • TLC: Less expensive than SLC is Triple-Level Cell NAND flash technology. Storing three bits per cell, TLC is typically used for applications with low performance and endurance requirements. The technology is best suited for read-intensive applications.
  • MLC: Multi-Level Cell SSDs, which store two bits per cell, are generally viewed as a consumer-grade technology. While stuffing more than one bit into a memory cell conserves space,

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