Buying a Solid-State Drive: 20 Terms You Need to Know

Become SSD-Fluent

If you're shopping for a solid-state drive—whether as a new boot drive or as an access-speeding cache for an existing boot hard drive—you're likely tech-savvy enough to dig into the innnards of your desktop or laptop. Even so, a swarm of ever-evolving jargon buzzes around SSDs, and some of it is bewildering even to serious PC enthusiasts. Not only that, but not every spec that SSD vendors cite is necessarily meaningful when you're shopping.

It's hard to buy a bad SSD these days for general use, but first-time upgraders will need a bit of background knowledge to keep from overspending. Let us be your guide: Here's a 101-level primer to the language you need to speak SSD-savvy.

Firmware

Firmware refers to the software "instruction set" stored in an SSD in non-volatile memory. In a nutshell, it governs the operation of the drive. Firmware in an SSD context is referred to by a version number, and is flash-upgradable, usually via a manufacturer utility. The firmware is typically tied to a specific make and model of controller, so updates to the firmware for a given SSD controller chip can often be implemented across multiple manufacturers' drives, as soon as each manufacturer packages the firmware update for its drives. Firmware upgrades are typically distributed via the support section of an SSD manufacturer's website.

A firmware update can address performance issues with a given drive. Also note that a drive that has been on the market for some time may have shipped with an earlier version of a given controller's firmware early on, and a newer one later, meaning that performance or stability can vary depending on which particular sample you buy.

SSD Caching

An SSD can be installed as a boot drive, with the option to install programs and data on it (depending on the capacity of the SSD and whether the system can accommodate a secondary "data" drive). You'll see the maximum speed benefit from a given SSD if it's used in this way. But a different mode in which SSDs are used is as cache memory, usually in a system with a platter hard drive set up as the boot drive. In this kind of arrangement, the system uses the SSD to temporarily store frequently accessed data (program files, large data files, parts of the OS) for faster access from the solid-state memory than from the platter drive. This is managed automatically via the system, usually via a technology such as Intel's SRT (explained a bit later).

SSD caching was sometimes implemented in Windows ultrabooks (in which an SSD boot drive or an SSD cache arrangement is a prerequisite). On desktops, an SSD cache can be implemented using a low-capacity, conventional SATA SSD in the 2.5-inch form factor or, in some older implementations, via an mSATA SSD module. A newer version of this technique is Intel's Optane Memory technology, which we'll get to later in this story.

Serial ATA

Serial ATA, often abbreviated to SATA, has for some time been the standard bus interface for drives inside consumer and business PCs. It's employed by hard drives, SSDs, and optical drives alike. And while SSDs do come in other interfaces and designs (especially M.2; see below), the SATA SSD in its 2.5-inch form factor is the most familiar to upgraders.

A typical 2.5-inch SSD with a physical SATA interface will have both a SATA data connector (which connects, in a desktop, to one of the SATA ports on the motherboard) and a wider, blade-like "SATA-style" power connector (which connects to a SATA power lead coming from the power supply). Inside a laptop, these connectors on the drive usually engage with a hardwired connection or a very short ribbon cable with both connectors on it.

The SATA data (left) and power (right) connectors on an SSD.

The SATA interface also describes the nature of the data bus that the SSD uses, which is why some M.2 drives (which use a wholly different physical connector; more on them below) actually route their data over the SATA bus. SATA itself has speed grades, and the ones you'll see in any SSDs you're considering are SATA 2 and SATA 3, variously called "SATA II"/"SATA 3Gbps" or "SATA III"/"SATA 6Gbps," respectively. These indicate the maximum data transfer rate possible with the drive, assuming it's installed in a PC with a SATA interface supporting the same standard.

In current SATA-bus drives, SATA III/SATA 6Gbps is the standard; we mention this in the event you're shopping older, second-hand, or remaindered drives that might be 3Gbps only. To gain the maximum throughput benefit of SATA 6Gbps, a 6Gbps SSD must be connected to a 6Gbps-compatible SATA port. Connected to a SATA II port, it will work, but the maximum data transfer rate will be constrained to 3Gbps. This will only be an issue to watch for when upgrading an older PC.

mSATA

mSATA defines both a form factor and a physical interface for compact SSDs. An mSATA SSD might be used as a boot drive (in a older, compact laptop or tablet) or as an "SSD cache" (defined above), speeding up the operation of a mechanical hard drive by dynamically hosting frequently accessed files or system/program elements. It's a fading format, though.

An Intel mSATA SSD

An mSATA SSD is a bare circuit board, as opposed to the enclosed design of a 2.5-inch SSD. (It resembles, and is sometimes mistaken for, a Mini-PCI card.) It will have a blade-style data and power connector that plugs into a single mSATA slot. A subset of desktop motherboards some years back featured mSATA slots on them, to allow for the onboard installation of an mSATA SSD for caching. But mSATA has been largely supplanted by the M.2 form factor. Here in 2018, an mSATA SSD upgrade is mostly of interest to users of older laptops looking to upgrade the mSATA boot drive in their machines.

M.2

Formerly known as NGFF (Next Generation Form Factor), M.2 solid-state drives are, like their mSATA predecessors, small circuit boards studded with flash-memory and controller chips instead of slab-shaped devices containing those chips. The latter give laptop and desktop makers speedier storage interchangeable with 2.5-inch hard drives, but mSATA and M.2 permit much smaller and skinnier designs overall.

Different sizes of Apacer M.2 SSDs

M.2 SSDs come in a variety of stick-of-gum sizes, typically 80mm, 60mm, or 42mm long by 22mm wide, with NAND chips on one or both sides. An important thing to note: An M.2 SSD, depending on the model, will be designed for use on either the SATA or (faster) PCI Express bus. Many of today's affordable laptops use SATA M.2 SSDs as the boot drive, while premium models might opt for PCI Express parts. The real-world performance difference isn't colossal, but you'll want to pay attention to what's what for compatibility's sake.

Most late-model desktop motherboards have M.2 slots nowadays, as well. You'll have to do your homework to find whether such a slot is designed for SATA- or PCI Express-bus M.2 drives. (Some support both, some just one. See our roundup, The Best M.2 Solid-State Drives.)

Write Cycles

A longevity measure for SSDs, this spec (also called "program-erase cycles") is more useful as a comparative attribute than as an absolute. It refers to the number of times a given memory cell on an SSD is likely to endure being erased and rewritten. (Typically, when a cell wears out, the drive decommissions it and activates another cell, if available, that's kept in reserve via "overprovisioning.")

In practical fact, most SSDs end up being obsolete in terms of capacity sooner than their write limits are likely to be reached. You'll tend to see higher write-cycle specs, however, for premium SSDs and drives destined for use in server or data center environments. These tend to be based on SLC, as opposed to MLC or TLC memory. (More on those terms later.)

TRIM Support

One important aspect of how an SSD works: Before you write to the drive, the SSD needs to erase any memory cells full of data before it can overwrite them with new data, if those destination cells aren't already empty. This becomes more of an issue once a drive starts to fill, and already-used cells are the only ones available for writes. If you're doing this "maintenance work" at the same time as you're trying to perform a data write, it can slow down performance.

Supported in Windows 7 and later, the TRIM command takes care of this chore in advance, looking ahead and pre-wiping available cells containing data to be deleted so they're ready for writing when the time comes. Your SSD's software utilities, as well as freeware like Crystal DiskInfo, can tell you if TRIM is activated.

RAPID Mode

RAPID Mode is a proprietary Samsung name for its SSD RAM-drive technology. It was included starting with its SSD 840 EVO line of drives out of the box, and implemented via free download for some older Samsung SSDs. It stands for "Realtime Accelerated Processing of I/O Data," and it works under Windows 7 and later versions.

RAPID Mode control panel in Samsung's Magician utility software.

In it, a portion of your main system memory, which allows for faster access than even the flash memory on your SSD, is managed via a special driver to speed up data transfers. It does this by caching frequently accessed user data and application files. It can make benchmark performance extra snappy, but know that there's a potential downside to RAPID Mode: Any power loss that occurs means that any data in the volatile RAM cache will be lost. (Remember: System memory needs to remain powered to retain its contents; the NAND chips in an SSD do not.)

NAND Flash

NAND flash is the generic term for the silicon chips that comprise the actual storage on the SSD. (The "NAND" refers, at a technical level, to the type of logic gates used in the underlying memory structure.) In essence, an SSD of whatever stripe is a circuit board with NAND chips embedded, managed by a controller (defined later in this story). This kind of memory is non-volatile, meaning that it does not require constant power to maintain the data stored on it.

The maker of the NAND on an SSD may or may not correspond to the actual brand of SSD. (For example, Samsung SSDs predictably will contain Samsung NAND, since the company also manufactures memory.) For the most part, the specific maker of the NAND is not a factor in an SSD purchase, though the kind of NAND (SLC, MLC, or TLC, defined below) might be, depending on how you will use your SSD.

SLC, MLC, and TLC NAND

These three memory types are the primary kinds of NAND chips seen in modern SSDs. The most common in the early days of consumer SSDs were MLC (multi-level cell) and SLC (single-level cell). MLC was generally the cheaper of the two. The "multi-level" of MLC refers to the ability of each MLC memory cell, in most cases, to host four states and thus two bits per cell due to its architecture. (SLC memory cells can exist in only two states, 1 and 0, and thus store one bit per cell.)

SLC in general is stabler over longer periods but also more expensive. MLC's higher densities make it cheaper to manufacture (you get more chips out of a given wafer), but error compensation in the firmware is necessary to keep it in check. MLC also tends to be rated for fewer read/write cycles than SLC. A variant of MLC, enterprise MLC (eMLC), uses technologies that forestall cell wear and thus data loss, and premium-price drives based on these "stabler" drives are marketed for business or high-access environments.

Then there's TLC. It emerged as an up-and-coming memory type first via Samsung in its 840 Series SSDs, with other NAND makers also jumping on board. Standing for "triple-level cell," TLC can host eight states and three bits per cell. The even greater density pushes cost down, but TLC requires even more error-correcting overhead, and the increased complexity and varying voltages per cell mean likely faster wear per cell, all else being equal. TLC, however, has proliferated in consumer SSDs that won't be subjected to mission-critical, enterprise workloads.

The next evolution, 3D NAND, is evident in the many 3D TLC-based consumer SSDs now on the market; with these, the architecture sees the memory cells "stacked" in 3D space instead of simply laid out in a planar fashion. The technical specifics are irrelevant to most consumer buyers, but the advent of 3D TLC has strengthened competition among the major SSD players.

Controller

The silicon chip that acts as "traffic cop" for the SSD, the controller is typically the biggest differentiator among SSDs if you get down in the technical weeds. Some manufacturers of SSDs have acquired controller makers over the years and incorporated those technologies into homegrown controllers (for example, Indilinx and OCZ, before OCZ was acquired by Toshiba), while others make use of widely used controllers from companies such as Marvell and Phison. Drives with the same onboard controller and of the same capacity tend to perform similarly, though different firmware versions and other factors can introduce variation.

Drive Z-Height

With a typical 2.5-inch SSD, the "z-height" refers to the thickness of the drive. For a while, 2.5-inch SSDs came in two common z-heights, 7mm and 9.5mm, though 7mm now prevails. This doesn't matter much for drives being installed in a desktop PC, which can accommodate drives of either height with ease, but for a laptop install, the z-height can be crucial.

Though many thin laptops now use M.2 SSDs or soldered-down storage, older models using a 2.5-inch SSD or a hard drive may require a 7mm or 9.5mm z-height drive to fit, depending on the design. Some SSD makers will include a "spacer" (usually, a frame of plastic) with their 7mm models to help them fit securely in a laptop drive bay meant for a 9.5mm-thick drive without wobbling around.

Migration Software

As a category, this is software that may or may not come packaged with an SSD to assist in copying a source drive to an SSD. (The most likely scenario in which it will be used is if you intend to install the SSD as a boot drive.) It's not possible to simply copy a bootable hard drive to an SSD, bit by bit, within Windows, and have the SSD be bootable. Because this operation needs to happen outside of Windows, special software is required.

That said, the lack of migration software does not have to be a deal-killer; freeware like EaseUS's Disk Copy can take its place. Some SSDs will supplement the migration software with a SATA-to-USB cable (for transferring the contents of a laptop drive over USB); when that's included, the SSD is often marketed as a "laptop upgrade kit."

Overprovisioning

Because memory cells fail over time as they are written and erased over and over, an SSD's effective capacity can drop gradually as memory cells fall out of the running. Some makers of SSDs, to forestall this, provide more memory than advertised, or "overprovision" the drive, in essence reserving some for a rainy day. Overprovisioning also can explain the slight variances in published capacities for drives of the same rough class (say, 240GB versus 250GB versus 256GB).

You won't be able to see this extra memory in the advertised capacity of the drive, or in normal use; the drive firmware may invisibly bring some of these cells online as others die. But it's a sign that the SSD maker is factoring in gradual data-cell mortality. A secondary consideration: Overprovisioning means that the SSD can write to a wider range of cells, which proportionally reduces wear across the whole array.

Sequential and 4K Reads and Writes

The most common SSD benchmarking software programs, including the AS-SSD and Crystal DiskMark utilities that we use in our tests, typically test two kinds of data transfers: sequential reads/writes, and random (usually "4K") reads/writes. Sequential reads and writes involve large files; testing in this fashion gives an idea of speeds when transferring large amounts of data. The term is a vestige of such operations on conventional hard drives, in which large files would often have most of their parts in a row, in physical proximity, on the actual drive platter.

Random reads and writes, on the other hand, access small (usually 4K in size) blocks of data, simulating the device saving and reading much smaller bits of data scattered across the drive. All of these measures are reported in megabytes per second (MBps or MB/second), higher being better. Note that when SSD vendors report claimed read and write speeds, they're usually sequential numbers, both because most data accesses on a client PC tend to be sequential, and because these numbers look the biggest. Some software and SSD makers report this kind of data in IOPS (input/output operations per second).

MTBF

For "mean time between failures," this is another spec that, if it's meaningful at all when shopping, is only useful for comparison among drives from the same maker. It's a measure of the expected rate of failures in a population of drives, and not as the projected absolute lifetime of any given drive in hours. (MTBF is often cited as a measure for other kinds of computer hardware, too, such as platter disk drives, but it's only useful as a measure within hardware of its own type.)

A JEDEC standard outlines the testing of SSDs for longevity under reads and writes, but it's not always clear if a given SSD vendor is using the same metrics and workloads as another to test for longevity. As a result, MTBFs are really only relevant for buyers if you're looking at drives within the same manufacturers' families.

Wear Leveling

Wear leveling is an internal management technique used by solid state drives' firmware, to maximize the viability of all memory on the drive. In it, write and erase operations are spread across the entire drive, instead of concentrated on the same block of cells over and over, even if the drive is not filled to capacity. Because all cells have a finite write/rewrite life, doing so "wears" the cells across the drive evenly.

PCI Express AIB SSD

As we noted earlier, a number of M.2 SSDs use the PCI Express, as opposed to SATA, bus interface. But you can also find solid-state drives that are designed with a physical PCI Express interface to fit into a desktop's PCI Express expansion slots, as actual cards. These "add-in-board" (AIB) SSDs install like a video card. They'll use both the PCI Express data bus and a PCI Express slot.

Some of these PCIe cards have flash and controller silicon on board; others, such as Kingston HyperX Predator PCIe SSD, are essentially M.2 drives mounted on adapter cards, for motherboards that lack M.2 slots.

Smart Response Technology (SRT)

SRT is an Intel technology that lets you install a low-capacity solid-state drive as a high-speed cache for a standard platter hard drive. It debuted some years back with Intel's Z68 chipset, and to implement it, you'll need a compatible Intel-based PC, along with any SSD and hard drive. With SRT active, the system gradually "learns" which files and system elements you use the most, caching those to the SSD for faster access. In that way, you can get the advantage of the inexpensive high capacity of a conventional hard drive along with some of the access speed of an SSD.

A schematic of Intel's SRT.

Implementing SRT makes sense if you already have a hard drive in place as a boot drive and don't want to go to the trouble of making an SSD your boot drive. However, over time, boot SSDs at capacities of 256GB and larger have gotten so cheap that there's less incentive to do SRT for cost reasons, nowadays; those capacities are big enough as boot and program drives for most buyers. And depending on how your system is configured, you may need to reinstall Windows on your hard drive, in any case, to configure things properly for SRT.

SATA Express

The first SATA Express-capable motherboards began to appear for PC desktops with the May 2014 wave of boards based on the Intel Z97 and H97 chipsets. Alas, though, the promised SATA Express SSDs that were to use these ports never arrived.

Two SATA Express ports (at far right and far left) bracketing four regular SATA ports.

SATA Express is implemented via a dedicated connector on the motherboard that resembles an internal SATA port, but keyed differently. In essence, it employs the same principle as a PCIe SSD, in that the SSD makes use of PCI Express lanes for greater bandwidth. However, M.2 drives won this battle, and SATA Express is now obsolete. However, we mention it in the event you have a desktop PC from a few years ago that has one or more of these ports. No, alas, you won't find an SSD for it.

Extra Credit: Two Bonus Terms

NVMe

Non-Volatile Memory Express is an open standard backed by more than five dozen companies for accessing solid-state drives over the PCI Express bus. (All NVMe drives are PCIe drives, but not all PCIe SSDs are NVMe-compatible components.) It's essentially a transfer protocol that replaces the AHCI protocol used by SATA drives. AHCI was originally designed for platter-based hard drives, while NVMe was designed from the ground up for flash-based storage.

Designed both to take advantage of SSDs' low latency and internal parallelism, and to eliminate the need for device-specific drivers, NVMe allows for substantially faster transfer rates than SATA/AHCI, making it the acronym to look for if you want the speediest SSD available. Note that an older system may not be able to boot from an NVMe drive.

Optane

Optane is an Intel trademark for the 3D Xpoint (pronounced "cross point") memory it co-developed with Micron, which is non-volatile--like NAND flash, it retains data when the power's switched off--but faster than NAND, and almost as fast as DRAM. It debuted in April 2017 in small 16GB and 32GB caching modules (confusingly called "Optane Memory") for desktops with SATA hard drives. Placed between the processor and the slow hard drive, Optane Memory served as a system accelerator, boosting responsiveness and cutting program load times.

In December 2017, Optane made the jump to full-fledged 280GB and 480GB SSDs, the Intel 900P series, available in 2.5-inch or PCIe AIB form factors. These drives draw more power and (at this writing) cost about twice as much per gigabyte as NVMe SSDs, but they're lightning-fast temptations for desktop enthusiasts with up-to-date Intel CPUs and Windows 10.