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SSD Explained: Everything You Need To Know about Solid-State Drives

You might have heard of solid-state drive (SSD). This post will explain this type of storage and its variant (i.e., SATA SSD vs. NVMe SSD) and how they are different from the traditional hard drive (HDD).

And even if you’re familiar with them all, this will still be a good read. Let’s start with the basic similarities and differences when it comes to SSD vs. HDD.

Dong’s note: I originally published this post on February 22, 2018, and updated it on June 3, 2020, with additional relevant information.

A Bunch of SSDs
Solid-state drives come in many different shapes and sizes.

HDD vs. SSD: Similarities

From a user’s perspective, a solid-state drive and a hard drive behave the same: they store information and make it available when you need it.

And that’s because SSDs initially are based on HDDs by sharing the same interface called serial AT attachment (SATA). This allows an SSD to replace an HDD without extra hardware or software requirements seamlessly.

The SATA interface comes in two prominent designs: 3.5-inch (for desktop computers) and 2.5-inch (for laptops). (You’ll find SSDs only in the 2.5-inch form, but they work in all applications where a hard drive fits.)

SATA is currently in its third revision (SATA 3), with a top speed of 6 gigabits per second (Gbps) or 750 megabytes per second (MB/s).

Future of SATA

There will not be a newer and faster version of SATA in the future. That’s because SSDs have been increasingly popular and more affordable, and they have designs and interfaces that deliver significantly higher speeds.

The truth is, the SATA connection standard is often the bottleneck for SSDs’ performance. As a result, the growing popularity of SSDs will eventually render SATA obsolete.

HDD vs. SSD: Differences

First and foremost, SSDs are significantly faster than HDDs. It’s safe to say that even the fastest hard drive is slower than the slowest SSD.

On average, you can expect a SATA SSD to have at least twice the sequential (copy) speed of a SATA hard drive. SSDs’ strong point is the random access performance, which contributes more to the general performance.

That’s because, on the inside, an SSD and an HDD are totally different.

HDD 3 5 vs 2 5 vs SSD
A desktop (3.5-inch) HDD, a laptop (2.5-inch) HDD, and a SATA SSD. Note how they all share the same SATA connector. These three drives can work in the place of one another, connection-wise.

A hard drive’s basics

Open up a hard drive; you’ll find a few platters stacked on top of one another like a spindle of compact discs. Each disc has a thin layer of metal on top that can be magnetized by the read/write head — which hovers on top of each platter — into binary patterns to store information.

This process is the same as writing on a portion — called sector — of the platter for the first time or any subsequent times (overwriting). In either case, the drive magnetizes the sector directly into the patterns required at the time of writing, regardless of their state.

A hard drive’s reading/writing speed depends mainly on how fast it can spin its platters. Most consumer-grade HDDs spin at either 5,400 rounds per minute or 7,200 RPM. They are mechanical machines.

A close-up of a laptop HDD next to a SATA SSD. Both are intact.

An SSD’s basics

An SSD has no moving parts. It’s an integrated circuit that uses less energy yet can provide much faster access to the information it stores than an HDD.

It also can come in much smaller physical sizes and doesn’t necessarily need to conform to a standard design. Still, most do anyway for ease-of-use, though many also come in a proprietary form that fits only in a particular application.

HDD vs SSD Opened
A close-up on the same two drives above opened up. Note the plates on the HDD (left) and the circuit on the SSD (right). The HDD is now toasted, but the SSD still works fine.

All you need to know about solid-state drives

The world of SSDs can be quite complicated due to the nature of the technology. However, the first thing you should know about them is that you can only write so much to an SSD before you can’t anymore.

Finite P/E cycles

That’s because there’s no “overwriting” with SSDs.

Except when the SSD is brand-new, writing to an SSD always means erasing existing information from memory cells and programming them with new information.

For this reason, writing to an SSD is often referred to as a program/erase cycles (or P/E) cycle, similar to writing on a whiteboard with a Sharpie — you need to clean the board before you can draw something new on it.

And just like a whiteboard that won’t last forever, an SSD also has a finite amount of P/E cycles. You can only program a memory cell so many times before it wears out and becomes unreliable.

And programing information on an SSD is a complicated and inefficient process by nature.

Blocks and pages

An SSD organizes its memory cells in pages and blocks, with one block containing many pages. Here’s the problem: An SSD writes page by page but erases block by block.

As you can imagine, if you want to write more pages to a half-used block, the SSD will first need to copy the good pages (those with valid information) to a different place, erase the entire block, then write back the good pages together with the new pages to that block.

As a result, an SSD has to write more than the amount of information you need to store most of the time, which brings us to some interesting terms.

SSD-specifics terms

The writing process on an SSD is so complex that it breeds half a dozen lingoes. To understand this type of storage well, you need to have them all in your vocabulary.

Write amplification

As mentioned above, the terms refer to the phenomenon that an SSD has to write more than the amount of information the user needs to write. The higher the write amplification, the shorter the life span of a drive.

Garbage collection

A process where an SSD needs to reallocate pages of a block before erasing the entire block so it can write on that block, as described above.

The more efficient this process is, the faster a drive can write.


An SSD can dedicate a part of its storage (typically 10%) specifically for this process to make garbage collection more efficient.

This is like having an extra room in your house to store stuff temporarily when you need to do a major cleanup. As a result, many SSDs don’t have full capacity. For example, 256GB or 512GB drives only deliver 240GB or 480GB of real storage space, respectively.

TRIM command

This is an actual command of an operating system and not an acronym.

The command notifies the SSD when a page of old data is no longer valid — the garbage collection will skip it during the reallocation. When enabled, TRIM helps reduce Write amplification a great deal.

Endurance and wear-leveling

Endurance is the amount of data that you can write to an SSD before it becomes unreliable.

To increase usability, SSDs use wear-leveling, algorithms that make an SSD use up all of its memory chips, cell by cell before the first cell is erased and written on again.

Consequently, the entire drive “wears” evenly. For this, SSDs with larger capacities generally have higher endurance than smaller ones.

SSD Types
From bottom to top: A 2.5-inch SATA SSD, an mSATA SSD, a B-key m.2 SSD, and an NVMe SSD (inside a PCIe adapter card).

Of all the items above, endurance is likely the most important since it decides the value of a drive. Let’s find out more about it.

SSD endurance

SSD endurance is generally presented in two ways: Terabytes Written (TBW) or drive writes per day (DWPD).

Terabytes written

TBW is the total amount of data you can write to an SSD over its life span before it becomes unreliable. The higher the TBW value, the better the endurance, the longer the drive will last.

It’s important to note that TBW can be misleading since larger capacities generally mean a higher TBW rating. For example, if you stack a 1TB low-endurance SSD against a 250GB high-endurance SSD, chances are the former will have a higher TBW value.

For this reason, there’s another more consistent measurement for endurance.

Drive writes per day

DWPD is the percentage of a full drive writes per day over the SSD’s warranty period, which tends to be between three and five years.

For example, if a 250GB SSD has 1 DWPD rating and a warranty of three years, that means you can expect to write up to 250GB to it per day and every day for three years. If the same drive has the DWPD value of .5, you can write 125GB to it per day, so on and so forth.

The higher the DWPD value means, the better the endurance but keep in mind that DWPD needs to be weighed against the warranty period. For example, a drive with .5 DWPD over three years has lower endurance than one with .4 DWPD over five years.

The endurance anxiety

Since the write on SSDs is finite, we tend to worry a lot about their longevity. In reality, though, there’s no need to worry at all.

Even though you can’t write to an SSD forever, with normal usage, it’d take you many years to deplete even a low-capacity SSD’s P/E cycles. Most of us don’t write more than 10GB per day to a drive, and many days we don’t write anything at all.

I consider myself a heavy user who moves data around a lot, yet I’ve never been able to wear out an SSD, including my very first, a 256GB Samsung 830, I bought back in 2011. It’s still working now.

That said, if you still want to extend the life span of an SSD even more, then reduce any unnecessary amount of writing you do to it. Chances are, though, you don’t need to worry about this at all.

Most storage vendors make both types of SSDs. Here we have the WD Blue vs. the WD Black as representatives of SATA SSDs vs. NVMe SSDs.

SSD types: NVMe SSD vs. SATA SSD

There have been many standard designs (form factors) of SSDs over the years. That’s not to mention most Apple computers use proprietary SSDs — the pool of SSD shapes and sizes so large that hardly anyone can remember them all.

The classification of SSDs is also confusing because the form factors (designs, physical shapes), the interfaces (how a drive connects to a host), and the speed standards overlap between different types.

For example, SATA is both an interface as well as a speed standard. And then, you have standard SATA and mSATA form factors. mSATA itself is also another interface variant of SATA.

That said, strictly from the interface point of view, you only need to know two popular types: SATA and M.2. I’ll explain the form factors and their speeds.


The SATA standard has been in use for the past few decades and is currently at the third revision — SATA 3 — which has the cap speed of 6Gbps (or 750MB/s). In real-world usage, the fastest SATA SSD, after overheads, has the top sustained copy speed of around 550MB/s.

SATA SSDs come in two main designs:

Standard SATA: Standard SATA SSDs share the same design as a laptop (2.5-inch) hard drives though most are slightly thinner than 7mm, compared to 9.5mm of the HDD. Examples of SATA drives are the entry-level Toshiba OCZ TR200, the mainstream Crucial MX500, or the high-end Samsung 860 Pro.

mSATA SSD: Short for mini-SATA, mSATA is a variant of standard SATA with a much smaller form factor and uses a different interface (called mSATA) to connect to a host. Many laptops use mSATA drives, and you can also use them as regular SATA drives via adapters.


M.2 is the latest interface that has the highest number of design variants via different lengths and “module keys.”

Toshiba RC100 SSD
The Toshiba RC100 is one of a very few SSDs that use the 2242 design and, therefore, are tiny.

Length: Typically, an M.2 drive is always 22mm wide. Its length, however, varies from 42mm to 110mm. Most M.2 SSDs, however, use the 2280 (22mm wide and 80mm long) design.

Module keys: This determines how an M.2 device connects to a host (like a computer motherboard). There are A-key, E-key, B-key, and M-key. The first two are used mostly in Wi-Fi, Bluetooth, and cellular cards; SSDs only use B-key and M-key.

Module keyMeasurementsBus speed/interfacesUsage
B3042 (30mm wide + 42mm long), 
2230, 2242, 2260, 2280, 22110
PCIe x 2 (up to 16Gbps),
SATA (up to 6Gbps),
 PCIe x2 SSDs
M2242, 2260, 2280, 22110PCIe x4 (up to 32Gb/s), 
SATA (up to 6Gbps)
 PCIe x4 (NVMe) SSDs
Variants of M.2 SSDs

All variants of M.2 SSD have even smaller physical sizes than mSATA. So small that people tend to call M.2 devices “sticks” or “cards.”

M.2 SSD: B-key vs. M-key (SATA M.2 vs. NVMe M.2)

The M.2 keys are it gets confusing. M.2 SSDs are always super compact but not always necessarily fast.

NVMe M.2 SSDs always uses the M-key format.
NVMe M.2 SSDs always use the M-key format. (Click to enlarge).

Many M.2 drives use the SATA speed standard for compatibility reasons and therefore are no different from SATA SSDs in speed. Here’s the break-down:

  • Original versions of M.2 SSDs use the B-key. They tend to use SATA speed standards and therefore are not faster than regular SATA SSDs. Some B-key M.2 drives use the 2x PCIe speed standard and are slightly faster. These drives only fit in the B-key socket on a host (like a motherboard).
  • Some M.2 SSDs use B & M keys to fit in both B-key and M-key sockets. These drives also tend to use SATA or x2 PCIe speed standards.
  • The latest M.2 SSDs use the M-key. These drives use x4 PCIe (or faster) speed standards and are the fastest SSDs on the market. They are known as NVMe drives — more below.

Generally, if you want the fastest performance, go with SSDs that use the M-key M.2 connector.

M.2 SSD speed

M.2 SSDs have a few (and growing) speed variations.

Speeds of SATA-based M.2 

SATA M.2 drives (like the Samsung 860 EVO) have the same speed as regular SATA SSDs (6Gbps).

Speeds of PCIe-based M.2

M.2 drives that use PCI Express (PCIe) lanes to communicate with a host computer are labeled as NVMe (Non-Volatile Memory Express) M.2 drives.

(By the way, NVMe comes in a few flavors, including NVMe 1.3, NVMe 1.3c, and more. These decide mostly how the information is organized on the drive via the blocks and pages mentioned above. They have little to do with speeds and nothing to do with the physical design.)

As a result, they have much faster speed than SATA SSDs and will get even faster in the future with new generations of PCIe, as shown in the table below.

Specifically, currently, PCIe Gen 3 has a top speed of 8Gbps (985MB/s) per lane. That said, a PCIe Gen 3 x2 (two lanes) M.2 drive has a cap speed of up to 16Gbps. Similarly, a PCIe Gen 3 x4 SSD has a ceiling speed of up to 32Gbps.

Commercially AvailableRate per lane
x16 Speed
120032 Gbps250 MB/s0.5 GB/s1.0 GB/s2 GB/s4.0 GB/s
220074 Gbps500 MB/s1 GB/s2.0 GB/s4 GB/s8.0 GB/s
320108 Gbps984.6 MB/s1.97 GB/s3.94 GB/s7.88 GB/s15.8 GB/s
4202016 Gbps1969 MB/s3.94 GB/s7.88 GB/s15.75 GB/s31.5 GB/s
5TBD32 Gbps3938 MB/s7.88 GB/s15.75 GB/s31.51 GB/s63 GB/s
PCI Express PCIe in brief
Note: 1 Gigabyte per second (GB/s) = 1000 Megabyte per second (MB/s) | 1 Gigabit per second (Gbps) = 125 MB/s

In my experience, a good NVMe SSD (like the Samsung 970 EVO) can deliver sustained copy speeds of over 2,000MB/s.

Future NVMe SSDs will be even faster, thanks to the faster lane speed of next-generation PCIe. The Samsung 980 PRO, the first PCIe 4.0 drive, can deliver almost double the speed of the 970 EVO.

By the way, like the case of SATA, PCIe SSDs generally work across different generations at the speed of the slower party.

The takeaway

Solid-state drives, especially NVMe SSDs, are the way of the future. Most modern motherboards and laptops now tend to include an NVMe M.2 socket. You can upgrade most desktops to an NVMe drive by using a PCIe adapter.

But the hard drive won’t go away, either. It still provides a low-cost storage solution where speed is not a big issue.

In a computer, it’s a great idea to use an SSD as the primary (boot) storage and a hard drive as a secondary backup storage space. You can almost always do that with a desktop computer — more and more laptops also have designated spaces for both.

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2 thoughts on “SSD Explained: Everything You Need To Know about Solid-State Drives”

  1. Dong, Good luck on your new ventures. Over the CNET years, I am a fan of yours. I will keep reading and learning from what you publish. Spread your wing a little, it will bring you new success!

    • Thanks, Bob. I appreciate your continued support. Please spread the words so folks know I’m still alive and kicking. 🙂


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