SSD (solid-state drive) – TechTarget

An SSD, or solid-state drive, is a type of storage device used in computers. This non-volatile storage media stores persistent data on solid-state flash memory. SSDs replace traditional hard disk drives (HDDs) in computers and perform the same basic functions as a hard drive. But SSDs are significantly faster in comparison. With an SSD, the device’s operating system will boot up more rapidly, programs will load quicker and files can be saved faster.
A traditional hard drive consists of a spinning disk with a read/write head on a mechanical arm called an actuator. An HDD reads and writes data magnetically. The magnetic properties, however, can lead to mechanical breakdowns.
By comparison, an SSD has no moving parts to break or spin up or down. The two key components in an SSD are the flash controller and NAND flash memory chips. This configuration is optimized to deliver high read/write performance for sequential and random data requests.
SSDs are used anywhere that hard drives can be deployed. In consumer products, for example, they are used in personal computers (PCs), laptops, computer games, digital cameras, digital music players, smartphones, tablets and thumb drives. They are also incorporated with graphics cards. However, they are more expensive than traditional HDDs.
Businesses with a rapidly expanding need for higher input/output (I/O) have fueled the development and adoption of SSDs. Because SSDs offer lower latency than HDDs, they can efficiently handle both heavy read and random workloads. That lower latency stems from the ability of a flash SSD to read data directly and immediately from stored data.
High-performance servers, laptops, desktops or any application that needs to deliver information in real-time can benefit from solid-state drive technology. Those characteristics make enterprise SSDs suitable to offload reads from transaction-heavy databases. They can also help to alleviate boot storms with virtual desktop infrastructure, or inside a storage array to store frequently used data locally using a hybrid cloud.
An SSD reads and writes data to underlying interconnected flash memory chips made out of silicon. Manufacturers build SSDs by stacking chips in a grid to achieve different densities.
SSDs read and write data to an underlying set of interconnected flash memory chips. These chips use floating gate transistors (FGTs) to hold an electrical charge, which enables the SSD to store data even when it is not connected to a power source. Each FGT contains a single bit of data, designated either as a 1 for a charged cell or a 0 if the cell has no electrical charge.
Every block of data is accessible at a consistent speed. However, SSDs can only write to empty blocks. And although SSDs have tools to get around this, performance may still slow over time.
SSDs use three main types of memory: single-, multi- and triple-level cells. Single-level cells can hold one bit of data at a time — a one or zero. Single-level cells (SLCs) are the most expensive form of SSD, but are also the fastest and most durable. Multi-level cells (MLCs) can hold two bits of data per cell and have a larger amount of storage space in the same amount of physical space as a SLC. However, MLCs have slower write speeds. Triple-level cells (TLCs) can hold three bits of data in a cell. Although TLCs are cheaper, they also have slower write speeds and are less durable than other memory types. TLC-based SSDs deliver more flash capacity and are less expensive than an MLC or SLC, albeit with a higher likelihood for bit rot due to having eight states within the cell.
Several features characterize the design of an SSD. Because it has no moving parts, an SSD is not subject to the same mechanical failures that can occur in HDDs. SSDs are also quieter and consume less power. And because SSDs weigh less than hard drives, they are a good fit for laptop and mobile computing devices.
In addition, the SSD controller software includes predictive analytics that can alert a user in advance of a potential drive failure. Because flash memory is malleable, all-flash array vendors can manipulate the usable storage capacity using data reduction techniques.
The benefits of SSDs over HDDs include:
Downsides that come with SSDs include:
NAND and NOR circuitry differ in the type of logic gate they use. NAND devices use eight-pin serial access to data. Meanwhile, NOR flash memory is commonly used in mobile phones, supporting 1-byte random access.
Compared with NAND, NOR flash offers fast read times, but is generally a more expensive memory technology. NOR writes data in large chunks, meaning it takes longer to erase and write new data. The random-access capabilities of NOR are used for executing code, while NAND flash is intended for storage. Most smartphones support both types of flash memory, using NOR to boot up the operating system and removable NAND cards to expand the device’s storage capacity.
Types of SSDs include:
SSD manufacturers offer diverse form factors. The most common form factor is a 2.5-inch SSD that is available in multiple heights and supports SAS, SATA and NVMe protocols.
The Solid State Storage Initiative, a project of the Storage Networking Industry Association, identified the following three major SSD form factors:
Two types of RAM are used in a computer system: DRAM, which loses data when power is lost, and static RAM. NVDIMMs provide the persistent storage a computer needs to recover data. They place flash close to the motherboard, but operations are carried out in DRAM. The flash component fits into a memory bus for backup on high-performance storage.
Both SSDs and RAM incorporate solid-state chips, but the two memory types function differently within a computer system.
Two newer form factors worth noting are M.2 and U.2 SSDs. An M.2 SSD varies in length — typically from 42 millimeters (mm) to 110 mm — and attaches directly to a motherboard. It communicates via NVMe or SATA. The small size of an M.2 limits the surface area for heat dissipation which, over time, will reduce its performance and stability. In enterprise storage, M.2 SSDs often are used as a boot device. In consumer devices, such as notebook computers, an M.2 SSD provides capacity expansion.
A U.2 SSD describes a 2.5-inch PCIe SSD. These small form-factor devices were formerly known as SFF-8639. The U.2 interface enables high-speed NVMe-based PCIe SSDs to be inserted in a computer’s circuit board, without the need to power down the server and storage.
The SSD market is dominated by a handful of large manufacturers, including:
These manufacturers produce and sell NAND flash chipsets to solid-state drive vendors. They also market branded SSDs based on their own flash chips. Factors to consider when shopping for SSDs include:
Historically, SSDs cost more than conventional hard drives. But due to improvements in manufacturing technology and expanded chip capacity, SSD prices had been dropping, enabling consumers and enterprise customers to view SSDs as a viable alternative to conventional storage. However, prices are increasing due to chip shortages and a general volatile market — more recently in 2020 and 2021, due to COVID-19-related supply chain issues. Fluctuating demand for flash chips has kept pricing for SSDs variable, but the price for an SSD remains higher than an HDD.
SSDs are considered much faster than the highest performing HDDs. Latency is also substantially reduced, and users typically experience much faster boot times.
Several factors influence the lifespan of SSDs and HDDs, including heat, humidity and the effect of metals oxidizing inside the drives. Data on both types of media will degrade over time, with HDDs generally supporting a higher number of drive writes per day. Industry experts recommend storing unused or idle SSDs at low temperatures to extend their life.
The moving parts of HDDs increase the chance of failures. To compensate, HDD manufacturers have added shock sensors to protect drives and other components inside PCs. This type of sensor detects if the machine is about to fall and takes steps to shut down the HDD and related critical hardware.
Read performance of an HDD can suffer when data is split into different sectors on the disk. To repair the disk, a technique known as defragmentation is used. SSDs do not store data magnetically, so the read performance remains steady, regardless of where the data is stored on the drive.
SSDs have a set life expectancy, with a finite number of write cycles before performance becomes erratic. To compensate, SSDs employ wear leveling, a process that extends the life of an SSD. Wear leveling is typically managed by the flash controller, which uses an algorithm to arrange data so write/erase cycles are distributed evenly among all the blocks in the device. Another technique, SSD overprovisioning, can help minimize the impact of garbage collection write amplification.
An embedded MultiMediaCard (eMMC) provides the onboard flash storage in a computer. It is installed directly on the computer motherboard. The architecture includes NAND flash memory and a controller designed as an integrated circuit. EMMC storage is typically found in cellphones, less expensive laptops and IoT applications.
An eMMC device delivers performance roughly equivalent to that of an SSD. But they differ in capacity, as a standard eMMC typically ranges from 1 GB to 512 GB, and SSD sizes can range from 128 GB to multiple terabytes. This makes eMMCs best suited for handling smaller file sizes.
In portable devices, an eMMC serves as primary storage or as an adjunct to removable SD and microSD multimedia cards. Although this is the historical use of eMMC devices, they are increasingly deployed in sensors inside connected internet of things devices.
Although not as widely used as a standard solid-state drive, an alternative is a hybrid hard drive (HHD). HHDs bridge the gap between flash and fixed-disk magnetic storage and are used to upgrade laptops, both for capacity and performance.
HHDs have a conventional disk architecture that adds approximately 8 GB of NAND flash as a buffer for disk-based workloads.
As such, an HHD is best suited for computers with a limited number of applications. The cost of a hybrid hard drive is slightly less than an HDD.
The earliest solid-state drives generally were designed for consumer devices. This changed in 1991 when SanDisk released the first commercial flash-based SSD. Commercially designed SSDs were made with enterprise multi-level cell flash technology, which enhanced write cycles.
Other notable dates include:
Learn more about four causes of SSD failure and best practices for dealing with them in this article.
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