What exactly is an SSD, and how does it function?

Hard drives have undergone continuous improvements since the 1950s, but they’ve always been prone to damage and data loss. That changed with the creation of the solid-state drive (SSD). So, what is an SSD exactly?

Unlike traditional hard drives, which use magnetic media, an SSD relies on semiconductor chips to store data. These chips are non-volatile, meaning they keep your data intact even when there’s no power. But how do they manage that? Let’s dive into SSD technology and its inner workings.

A Brief History of Storage Technology

In 1956, IBM introduced the first-ever hard disk drive (HDD) with the RAMAC 305 system. The drive featured 50 platters, each 24 inches (61 cm) in diameter, and could store only 5 megabytes of data, occupying more space than two refrigerators. The price tag? A whopping $50,000, which would be equivalent to around $570,000 in 2024.

Since then, hard drives have shrunk in size, increased in capacity, and become more affordable. Take, for instance, the Seagate Momentus laptop hard drive, which is just 2.5 inches (6.4 cm) in size, offering 750 gigabytes of storage for under $100. However, despite modern advancements in protection, hard drives, like the Momentus, remain susceptible to failure due to their mechanical components. A simple drop could cause the read-write heads to come into contact with the spinning platters, often resulting in significant data loss.

What Exactly Is a Solid-State Drive?

In computing terms, memory and storage are distinct. Random-access memory (RAM), or simply memory, temporarily holds the program a computer is running, along with any active data. Like human short-term memory, RAM is fleeting and requires power to function. In contrast, storage is where all your digital data — apps, files, photos, and music — are saved. Unlike RAM, storage retains this data even when the power is off.

Both RAM and storage are measured by how many bytes they can hold. Modern computers typically come with 4, 6, or 8 gigabytes of RAM, while storage can hold nearly 100 times that amount — for instance, a typical laptop hard drive might have a capacity of 500 gigabytes.

Here's where it gets a bit tricky. Some storage devices feature something called flash memory, a term that causes some confusion by blurring the lines between RAM and storage. Devices equipped with flash memory can hold a lot of data, and they do so whether the power is on or off.

However, unlike traditional hard drives, which contain spinning platters and turntable-like arms with read-write heads, flash-memory devices have no moving parts. They’re made up of transistors and other elements you'd find in a computer chip. This design gives them the label ‘solid state,’ referring to devices that utilize the properties of semiconductors.

The Rise of NAND Flash Memory

Flash memory comes in two types: NOR and NAND. Both feature cells — transistors — arranged in a grid, but the way the cells are connected differs. In NOR flash, the cells are connected in parallel, whereas in NAND flash, they’re connected in series. The cells in NOR flash are more complex and larger because they have more wires, while NAND cells need fewer wires and can be packed more densely on a chip.

As a result, NAND flash memory is more affordable and offers much faster read and write speeds. This makes NAND flash the preferred storage technology and explains why it's the dominant type of memory in solid-state drives. In contrast, NOR flash is better suited for low-density, high-speed, read-only tasks, such as in code storage.

How SSDs Are Replacing Traditional Hard Drives

With this understanding, we can now define a solid-state drive more accurately: it’s a device that utilizes NAND flash to provide non-volatile, rewritable memory. In computers, SSDs serve as storage devices, replacing the older hard disk drives.

In fact, SSD manufacturers often design these drives to resemble HDDs in shape and size so that the two technologies can be used interchangeably. However, that’s where the similarity ends. If you were to open up an SSD, you wouldn’t find any spinning platters or moving actuator arms. Let’s take a look inside next.

How Solid-state Drives Store and Retrieve Data

Externally, solid-state drives resemble traditional hard disk drives. They are rectangular, encased in a brushed-metal casing, and designed to conform to industry-standard sizes for hard drives — typically 2.5 and inches (6.4 and 8.9 centimeters).

However, beneath the metallic shell, there is a collection of chips mounted on a circuit board, with no magnetic or optical storage media. While these components could be more compact, SSD manufacturers add extra ‘housing’ to ensure the components fit into the drive slots of laptops and desktop PCs.

In contrast to the minimalistic design of a solid-state drive, the inner workings of a hard drive are a fascinating spectacle of motion, noise, and action. Circular platters, mounted on a spindle, can rotate at speeds of 7,200 revolutions per minute. An actuator arm, branching out into multiple read-write heads, darts across the platters in rapid, almost invisible movements. This arm connects to an actuator block, which stores the commands for directing the read-write heads.

As the instructions are called upon — sometimes as often as 50 times per second — the arm rotates at one end and moves the heads together over the platters. Once the head reaches its designated position, an electromagnet creates a magnetic field that aligns data-storing domains on the track below. These domains can either be aligned to represent a 1 or a 0. As these orientations change, they form patterns corresponding to individual pieces of digital data.

Understanding Data Storage with NAND

The NAND flash storage of a solid-state drive manages data in an entirely different way. Recall that NAND flash organizes its transistors in a grid of rows and columns. If a series of transistors allows current to flow, it is assigned a value of 1. If current is blocked, it registers as 0. Initially, all transistors are set to 1, but when a save operation occurs, some transistors have current blocked and are switched to 0. This is due to the transistor arrangement. At the intersection of each column and row, two transistors form a cell.

One of these transistors is called a control gate, while the other is the floating gate. When current reaches the control gate, electrons move to the floating gate, creating a positive charge that disrupts the current flow. By applying specific voltages to these transistors, a unique combination of 1s and 0s is generated, creating a data pattern.

NAND flash memory comes in two variations depending on how many 1s and 0s each cell can hold. Single-level cell (SLC) NAND stores just one bit — either a 1 or a 0 — in each cell. Multi-level cell (MLC) NAND can store two bits per cell. While MLC flash provides higher storage capacity, it degrades more rapidly (yes, it wears out — more on that in a few pages). However, MLC is more affordable per gigabyte than SLC, making it the preferred choice in nearly all consumer SSDs.

The cost of flash memory, and consequently solid-state drives, has historically been a major barrier to their widespread use. However, in recent years, prices have dropped significantly. Alongside this, improvements in NAND flash technology have taken the strengths of the tech and enhanced them even further. Next, we'll explore the many benefits of solid-state drives.

Sold on SSDs: Advantages of Solid-state Drives

You've just bought a high-end laptop equipped with a 500-gigabyte hard drive, and it's performing flawlessly. All your photos, videos, music library, unfinished novels, and countless apps are neatly stored on the drive's platters. So, why even consider replacing the HDD with a solid-state drive? After all, Dad always said, "If it ain't broke, don't fix it."

Maybe Dad didn't have any hard drives. The reality is that HDDs can and do fail, often much more often than their technical specifications suggest. For example, hard drive manufacturers measure reliability through a metric known as mean time between failures, or MTBF. A typical consumer hard drive may have an MTBF rating of 500,000 hours, which means, in a sample of drives tested, one failure would occur every 500,000 hours of testing. That’s one failure every 57 years, which sounds pretty reassuring, right?

Unfortunately, MTBF figures can be misleading. They’re based on a statistical evaluation using a small sample and short testing periods. In practice, you'd also need to factor in a typical HDD’s warranty and lifespan (usually three to five years), along with the MTBF score. Since SSDs have no moving parts, they offer improved reliability. Some SSDs have an MTBF of up to 2.5 million hours, which likely translates to a longer lifespan for the device.

An even bigger advantage of solid-state drives over HDDs is their performance. With no spinning platters or moving heads, SSDs can access any piece of data just as quickly as another, regardless of proximity. This speed boost shows up in every important CPU task, from booting up system software to opening files to reading and writing data.

Comparing SSD Performance Against Traditional HDDs

The following points provide a comparison of SSDs and HDDs based on several key performance metrics:

Boot-up time (Windows 7): 22 seconds (SSD), 40 seconds (HDD)

Data read-write speed: 510-550 megabytes per second (SSD), 50-150 megabytes per second (HDD)

Excel file open speed: 4 seconds (SSD), 14 seconds (HDD)

All of this makes a noticeable difference. Even a casual user will experience a significant boost in performance with an SSD-equipped computer. However, power users will really appreciate the speed. Game designers, animators, and others working with large output files were among the first to adopt SSDs, benefiting from the time saved in reading and writing these large files. Today, gamers, photographers, and anyone involved in graphics or video editing will feel the performance enhancement provided by a solid-state drive.

SSDs are much more energy-efficient than traditional hard drives, which helps extend battery life and keep temperatures lower. They also operate silently, without the whirring and clicking sounds typical of HDDs. This can make a real difference, especially if you often travel with your computer on your lap. Even if you mainly use your laptop in one place, having a quieter and cooler machine can noticeably improve your workspace comfort.

Of course, no technology is without its drawbacks, and SSDs are no exception. In the next section, we'll explore the downsides of NAND flash and why a hybrid approach using both types of technology might be the optimal solution.

Not Sold on SSDs: Disadvantages of Solid-state Drives

Switching from an HDD to an SSD may seem like a no-brainer, but it's important to understand the potential drawbacks of SSDs before making the change. For one, cost is still a concern. While prices have come down, NAND flash memory remains pricey. For instance, a PNY Prevail SSD with 240GB of storage can cost around $280, or $1.17 per gigabyte. On the other hand, a Western Digital Scorpio Blue HDD offers 250GB of storage for roughly $65, which works out to just $0.26 per gigabyte.

Another issue to consider is the lifespan of SSDs. NAND flash used in these drives has a limited number of write cycles. Why? Because SSDs can't write a single bit of data without erasing and rewriting large blocks at once. Each time a cell is erased, a small amount of charge remains in the floating-gate transistor, altering its resistance. As the resistance increases, more current is needed to flip the gate. Eventually, the gate becomes unable to flip, making the cell unusable. This decay doesn't affect reading, as reading only requires checking voltages, not changing them. As a result, NAND flash can degrade into a read-only state.

To combat the degradation of NAND flash, many manufacturers use a technique called wear-leveling. This method spreads out the data writes across all blocks, ensuring that the flash memory wears down evenly. However, even with wear-leveling, SSDs will eventually degrade. Single-level cell (SLC) NAND typically supports around 50,000 program/erase cycles, while multi-level cell (MLC) NAND, which is commonly used in consumer products, typically lasts for about 5,000 cycles.

As a result, many tech enthusiasts and data centers opt for a hybrid approach, combining both SSDs and HDDs. A common setup involves using an SSD for the laptop’s operating system and applications, while an HDD serves as external storage for music, photos, and other files. This configuration offers the best of both worlds: the rapid, random data access of an SSD combined with the cost-effective, high capacity of an HDD. If this approach sounds appealing, it's time to begin your search for a suitable SSD.

How the Cloud is Revolutionizing Storage Space

With cloud computing now taking over both personal and professional storage needs, offering speedy data recovery and transfer for all types of consumer devices, SSDs are starting to lose ground. Top SSD manufacturers such as Samsung, Seagate, SanDisk, Intel, Kingston, and others continue to release high-quality SSD storage products, but the trend is shifting towards cloud solutions.

Luckily, there will always be those who prefer traditional hard drives and SSD storage to safeguard their valuable data. However, as digital transformation accelerates, the market for these physical storage devices is expected to become more niche.