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Understanding Solid State Drives

by Adam Lovinus

A tech writer and Raspberry Pi enthusiast from Orange County, California.

This SSD buying guide helps you pair your business requirements and budget to the correct hardware for the job. You will determine the best SSD by comparing product specifications and benchmarks.

• Read/write speeds and IOPS for an idea of the fastest SSDs.
• Understanding how mean time MTBF relates to the most reliable SSDs.
• Knowing that cheapest SSDs aren’t always the best fit—assessing and warranty and price per gigabyte.
• How SSD architecture, controller technology, and NAND type factor into speed, reliability, and compatibility.
• Installing an SSD in a laptop or desktop PC requires only the most basic technical know-how.

Why now is a great time to upgrade to SSD

After years of stubborn SSD prices, supply and demand for the flash memory market currently favors end users. Throughout 2018, manufacturers have dropped pricing for consumer 2.5-inch SSDs across the board. Even m.2 and NVMe drives, which have historically resisted price decreases, have gradually lowered in price points.

2 TB SSD prices (24 month)

256 GB SSD prices (24 month)

Source: PC Part Picker SSD trends

If you’re weighing the HDD vs SSD data storage question for expanding disk storage, think of it like this. Installing an SSD into a legacy PC or laptop boosts performance enough to add another two or three years to its lifecycle. You will find that computers with SSDs boot up faster, and applications and large files open with less waiting. You get a snappier, more pleasing user experience.

What do you need to know to choose the best SSD for your business systems? Here is what you should know if you’re planning to buy SSD storage this year.

SSD form factors and interfaces

Five years ago, internal SSDs inside PCs and laptops connected over a SATA III interface, the same connection that an internal hard disk drive would use. Nowadays, faster motherboard connections have become mainstream. Once relegated to only the highest-end workstations and servers, PCI Express SSDs now make up a significant portion of the SSD market for consumers.

Without diving too deep into the circuitry, the PCIe interface is capable of speeds several times over what SATA can do. Accounting for encoding overhead, SATA III transfers happen at roughly 600 MB/s with a solid-state drive. While this is significantly faster than HDD capabilities, it’s significantly slower PCIe, which has a practical transference rate of 985 MB/s per lane—and PCIe SSDs support 2x or 4x lanes depending on the motherboard.

The high performance of PCIe SSDs is bolstered by NVMe (non-volatile memory express), a host controller interface and storage protocol. NVMe reduces latency in reading and writing data by working in parallel with the multi-core processors inside the host computer. In doing so, NVMe SSDs eliminate a bottleneck in performance by streamlining the command set used to process the transfer. If your computer uses a multi-core processor, and most do nowadays, you will feel a substantial performance upgrade with PCIe NVMe SSDs.

2.5-inch form factor

2.5-inch solid-state drives connect via SATA interface the same as HDD storage does. 2.5-inch SSDs are designed to fit inside laptop computers and they will work inside a desktop. If you’re installing them into a mATA PC case, your drive bays are probably designed for 3.5-inch HDDs. You will want to use a 2.5” to 3.5” HDD & SSD converter to secure a snug fit for your SSD inside your PC.

M.2 form factor

The M.2 form factor specification is a newer iteration for internal solid state drives. M.2 SSDs fit into a designated M.2 slot on a computer motherboard. Depending on the motherboard and SSD, the M.2 slot on a motherboard utilizes either the SATA interface or the PCIe interface. Or either—just never both at once.  M.2 motherboard slot is used for other hardware, most commonly Wi-Fi / Bluetooth / cellular wireless networking adapters.

You want to pay attention to the lettered key notches of the M.2 slot. The key notches are identified with letters A through M, which indicate their position on the connector and the respective interfaces provided. For example a notch at position M indicates that up to 4 PCI Express lanes can be used with NVMe or a SATA storage device can be supported.

Note that there are several variants of M.2 SSDs. These vary by physical size and by the interface type they utilize.

You also differentiate M.2 SSDs by the interface and bus they use to connect. When you’re upgrading make sure to know which your system uses by checking the specifications of your motherboard.

• M.2 PCI-express NVMe SSDs – The NVMe protocol is a higher-performing architecture that connects over the PCI express bus in the PC motherboard. Older versions of M.2 PCIe SSDs connected over the PCIe 2.0 x 2 interface. Most current model M.2 PCIe SSD connect over PCIe x 4.
• M.2 SATA SSDs – M.2 drives that use a SATA interfaces perform on par with 2.5-inch and mSATA drives. M.2 NVMe drives offer the fastest performance at a premium price point.
• M.2 SSD AIC – AIC (short for add-in cards) are designed for older motherboards without an M.2 slot. One advantage that many M.2 SSD AICs have is a heatsink inside the build. Excessive heat isn’t a huge problem for most PC users, unless you are using your computer as a server.
• Mini-SATA (mSATA) – The mSATA specification is designed for SSDs inside small laptops and tablets. It’s in large part obsoleted by M.2 drives, but if you have a legacy Ultrabook this is the form factor that fits inside your laptop.

NAND Type

The type of NAND used in a SSD matters—a whole lot, in fact. But what is NAND? NAND is a type of non-volatile flash memory, meaning it does not require power to retain or store data. Devices such as digital cameras, USB flash drives, smartphones, and SSDs utilize NAND flash memory for storage. NAND falls into several types: single-level cell (SLC), multi-level cell (MLC), enterprise MLC (eMLC), triple-level cell (TLC), Redundant Array of Independent NAND (RAIN), and the new 3D vertical NAND (3D V-NAND).

SLC NAND

A type of high-performance NAND flash memory that costs more than other types of flash memory to manufacture. SSDs with NAND memory chips never gained mass appeal due to high per-GB prices, and are found mainly in enterprise-grade SSDs. Also, SLC memory chips feature better write/rewrite endurance than MLC, meaning data can be written and rewritten before performance degrades. Few mainstream SSDs utilize SLC memory chips.

• Pros: Faster performance, better write endurance
• Cons: Higher price

MLC NAND

For the last few years, MLC was the go-to choice for storage manufacturers to use in solid state drives. While slightly slower than SLC memory, MLC could be produced at much lower cost and therefore was the primary type of NAND flash memory used in SSDs.

• Pros: Lower price
• Cons: Slower performance

eMLC NAND

A type of MLC NAND aimed towards light enterprise use or high-end mainstream use. Features higher write/rewrite endurance than MLC, but lower than SLC. A lower cost alternative to SLC.

• Pros: Lower cost than SLC, faster performance than MLC
• Cons: Higher price than MLC, lower endurance than SLC

TLC NAND

A type of MLC designed for use in budget-oriented SSDs. TLC flash memory features lower write/rewrite endurance than MLC. With a low per-GB cost, TLC SSDs make a strong case for value.

• Pros: Lower prices than MLC
• Cons: Performance slightly slower than MLC, lowest write endurance

QLC  NAND

Quad-Layer Cell is the latest NAND architecture. Offers 33 percent more bit density over TLC NAND.

• Pros: Stores more data on less material, lower SSD prices
• Cons: Less reliable than previous architectures

3D V-NAND

The most common MLC technology found in SSDs. Instead of having flash memory cells stacked horizontally, V-NAND technology stacks memory cells vertically. To use an analogy, imagine a neighborhood. Traditional MLC SSDs represent a suburb with many single- or two-story houses. V-NAND is a neighborhood of high-rise apartment buildings. For the buyer, V-NAND allows for high SSD storage capacities without a dramatic increase in price.

• Pros: Mid-range storage capacities, lower prices
• Cons: Performance on par with TLC SSDs and slightly slower than MLC

(Click for Full Size)

Memory Controller

NAND flash memory cells do not exist in a vacuum inside a SSD. Every SSD features a controller chip that manages data on the memory cells and communicates the other components on the computer, such as the motherboard and other data storage devices. Memory controllers handle many prominent features found in SSDs, such as wear-leveling, reading data, writing data, data provisioning, and more. Because of that, the type of memory controller used can impact drive performance, reliability, endurance, and other extraneous features.

Determining the best memory controller can be difficult, especially as they generally perform well. However, it is wise to check forums or do a general web search for the memory controller used in a SSD you may be interested in purchasing. By doing this research, you may uncover reliability issues, necessary firmware updates, known compatibility issues, or more. For example, cursory research into SandForce flash memory controllers reveal that the first generation suffered from compatibility issues with the Intel® Haswell platform and some users of the SF-2000 series reported freezing and blue screens of death.

Drive Performance: IOPS vs latency

Consumers shopping for SSDs and hard drives frequently pay close attention to SSD throughput— commonly presented as maximum read/write—as the key factor in determining drive performance. While true, read/write rates do affect the speed of writing and reading files to and from the drive—they do not matter significantly. Read/write speeds matter primarily when transferring a large amount of data on or off the drive.

In most use cases for business, I/O per second, or IOPS, is the metric that best measures SSD performance. IOPS counts the random pings to the drive, and gauges performance you feel when booting up a computer and opening applications. Again, we will not deep dive into the physics here. IOPS indicates how often a SSD can perform a data transfer every second to fetch data randomly stored on a disk. For office applications and production software, IOPS serves as a better metric for drive performance than throughput. It translates into how often and quickly data can be accessed in a multiple user setting.

SSD storage capacity

How large should your SSD be? How much data do you want to store? Your ideal SSD capacity depends largely on your usage scenario. In the current market, SSDs generally range from 256 GB to 2 TB. As drives get more spacious, the cost per GB generally is less. Historically, a popular configuration for desktop computers is to have a smaller SSD (250 GB) to store the operating system and main productivity applications. The SSD is used in tandem with a larger HDD that stores work files and media. At this point, SSD storage prices have fallen low enough to where all-SSD storage is a sensible upgrade for most use cases.

SSD reliability and lifecycle

The common reliability rating for SSDs is mean time between failures, and it’s sort of a tricky concept to grasp. Wikipedia defines it like this: MTBF is the predicted elapsed time between inhere failures of a mechanical or electronic system during normal system operation. Now we’ll get into what this actually means.

You will find that MTBF ratings are in the millions of hours. If the MTBF is 1.5 million hours, this doesn’t mean that your SSD will literally last 1.5 million hours, which is more than 170 years. Instead, MTBF is a measure of likelihood of failure in the context of a large sample size of drives.

Say the MTBF rating is 1.2 million, and that drive is used eight hours a day. In a sample size of 1,000 drives you will find that, on average, one drive will fail every 150 days or so.

Let’s do the math:

• 1,000 drives @ 8 hours a day = 8,000 operational hours.
• 8000 operational hours @ 150 days = 1.2 million total operational hours.

Write cycles, also called program and erase or P/E cycles, are another important metric touching on SSD reliability. SSDs are able to endure a finite number of write cycles. When you write, erase, and overwrite data to the metal NAND of an SSD, the process deteriorates the oxide layer that holds electrons in the memory cell.

Different types of NAND architectures are more resilient than others.

Source: https://searchstorage.techtarget.com/definition/write-cycle

Crucial SSDs

	Form Factor	Interface	Capacity	NAND	Max Read MB/s	Max Write	Read IOPS	Write IOPS
P1 NVMe	M.2	PCIe	500 GB	QLC NAND	1,900	950	90k	220k
			1 TB		2,000	1,700	170k	240k
			2 TB		2,000	1,750	250k	250k
MX500	2.5-inch	SATA	250 GB	3D NAND	560	510	95k	90k
			500 GB
	M.2		1 TB
			2 TB

Samsung SSDs

SAMSUNG SSDs	Form	Interface	Capacity	NAND	Max Read MB/s	Max Write MB/s	Read IOPS	Write IOPS
860 PRO	2.5-inch	SATA	256 GB	3D MLC V-NAND	560	530	100k	90k
			512 GB
			1 TB
			2 TB
			4 TB
860 EVO	2.5-inch	SATA	250 GB	3D TLC V-NAND	550	520	98k	90k
			500 GB
			1 TB
			2 TB
			4 TB
970 PRO NVMe	M.2	PCIe	2 TB	3D MLC V-NAND	3500	2700	370k	500k
			4 TB
			512 GB
			1 TB
970 EVO NVMe	M.2	PCIe	250 GB	3D TLC	3400	2500	200k	350k
			500 GB				370k	450k
			1 TB				500k	450k
			2 TB				500k	500k
860 EVO M.2	M.2	SATA	250 GB	3D MLC V-NAND	550	520	98k	90k
			500 GB
			1 TB
			2 TB

Intel SSDs

	Form	Interface	Capacity	NAND	Max Read MB/s	Max Write MB/s	Read IOPS	Write IOPS
Pro 5400s	2.5-inch	SATA	180 GB	3D TLC	560	475	71k	85k
			240 GB
			360 GB
			480 GB
			1 TB
Pro 7600p NVMe	M.2	PCIe	128 GB	3D TLC	3230	1625	340k	275k
			256 GB
			512 GB
			1 TB
			2 TB
Pro 6000p	M.2	PCIe	128 GB	3D TLC	1800	560	155k	128k
			256 GB
			512 GB
			1 TB
545s	2.5-inch	SATA	512 GB	3D TLC	550	500	75k	80k
	M.2
660P NVMe	M.2	PCIe	512 GB	QLC 3D NAND	1800	1800	2200k	2200k
			1 TB
			2 TB

Kingston SSDs

	Form	Interface	Capacity	NAND	Max Read MB/s	Max Write MB/s	Read IOPS	Write IOPS
UV500	2.5-inch	SATA	120 GB	3D TLC	520	320	79k	18k
			240 GB		520	520	79k	25k
			480 GB		520	520	79k	35k
			960 GB		520	520	79k	45k
			1.92 TB		520	520	79k	50k
	M.2 2280	SATA	120 GB	3D TLC	520	320	79k	18k
			240 GB		520	520	79k	25k
			480 GB		520	520	79k	35k
			960 GB		520	520	79k	45k
			1.92 TB		520	520	79k	50k
A400	2.5-inch	SATA	120 GB	Planar TLC	500	320	not listed
			240 GB		500	350
			480 GB		500	450
			960 GB		500	450
A1000 NVMe	M.2 2280	PCIe	240 GB	3D TLC	1500	800	100k	80k
			480 GB		1500	900	100k	90k
			960 GB		1500	1000	100k	100k

Western Digital SSDs

	Form	Interface	Capacity	NAND	Max Read MB/s	Max Write MB/s	Read IOPS	Write IOPS
WD Blue	2.5-inch	SATA	250 GB	3D NAND	550	525	95k	81k
			500 GB		560	530	95k	84k
	M.2 2280		1 TB		560	530	95k	84k
			2 TB		560	530	95k	84k
WD Green	2.5 inch	SATA	120 GB	SLC	545	525	28k	22k
	M.2 2280		240 GB
			480 GB
WD Black NVMe	M.2 2280	PCIe	250 GB	3D NAND	3400	1600	220k	170k
			500 GB		3400	2500	410k	330k
			1 TB		3400	2800	500k	400k

SanDisk SSDs

	Form	Interface	Capacity	NAND	Max Read MB/s	Max Write MB/s	Read IOPS	Write IOPS
Extreme Pro NVMe	M.2 2280	PCIe	500 GB	3D NAND	3400	2400	410k	330k
			1 TB		3400	2800	500k	400k
Ultra 3D	2.5-inch	SATA	250 GB	3D NAND	525	550	95k	81k
			500 GB		530	560	95k	84k
			1 TB		530	560	95k	84k
			2 TB		530	560	95k	84k

by Adam Lovinus

A tech writer and Raspberry Pi enthusiast from Orange County, California.