A solid-state drive (SSD) is a nonvolatile storage device that stores persistent data on solid-state flash memory. Solid-state drives actually aren't hard drives in the traditional sense of the term, as there are no moving parts involved. A traditional hard disk drive (HDD) consists of a spinning disk with a read/write head on a mechanical arm called an actuator. An SSD, on the other hand, has an array of semiconductor memory organized as a disk drive, using integrated circuits (ICs) rather than magnetic or optical storage media. An SSD may also be referred to as a solid-state disk.
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Development and adoption of SSDs has been driven by a rapidly expanding need for higher input/output (I/O) performance. SSDs have much lower random access and read access latency than HDDs, making them ideal for both heavy read and random workloads. That lower latency is due to the ability of flash SSD to read data directly and immediately from a specific flash SSD cell location. High-performance servers, laptops, desktops or any application that needs to deliver information in real-time or near real-time can benefit from solid-state drive technology. Those characteristics make enterprise SSDs suitable to offload reads from transaction-heavy databases, to alleviate boot storms with virtual desktop infrastructure, or inside a storage array to stage hot data locally for off-site storage in a hybrid cloud scenario.
SSD form factors
- SSDs that come in traditional HDD form factors and fit into the same slots.
- Solid-state cards that use standard card form factors, such as Peripheral Component Interconnect Express (PCIe), and reside on a printed circuit board.
- Solid-state modules that reside in a Dual In-line Memory Module (DIMM) or small outline dual in-line memory module (SO-DIMM), and may use a standard HDD interface such as Serial Advanced Technology Attachment (SATA).
The following video discusses the wide range of solid-state storage form factors available, as well as some benefits and drawbacks of each form factor.
SSD vs. HDD pros and cons
SSD performance is considered to be much faster than the highest performance electromechanical disk drives. Seek time and latency are also substantially reduced, and end users typically enjoy much faster boot times. In general, SSDs are more durable and much quieter than HDDs, with no moving parts to break or spin up or down. SSDs employ wear leveling to increase drive lifespan. 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.
In addition, SSDs have a set life expectancy, with a finite number of write cycles before performance becomes erratic. This is not really a disadvantage per se, as HDDs also degrade and eventually fail over time.
The following video outlines some of the ideal workloads for SSD devices.
SSD history, emergence in enterprise storage
The earliest solid-state drives generally were designed for consumer devices. The debut of the Apple iPod in 2005 marked the first notable flash-based device to broadly penetrate the consumer market.
EMC is credited with being the first vendor to include SSDs in enterprise storage hardware when it added the technology to its Symmetrix disk arrays in 2008, spawning the advent of hybrid arrays that combine flash drives with traditional spinning disk. Initially, enterprise SSDs in hybrid arrays were relegated to caching read data in flash due to their higher cost and lower endurance versus HDDs.
The earliest commercially designed SSDs were made with enterprise MLC, which has enhanced write cycles compared to consumer-grade MLC. Newer enterprise SSDs are being marketed that use TLC NAND flash. SSDs made with 3D NAND represent the next evolution. IBM, Samsung and Toshiba produce and market SSDs with 3D NAND, in which flash memory cells are stacked atop one another in vertical layers.
Enterprise adoption of flash is on the rise as a result of improvements in solid-state wear performance and falling flash prices. Experts contend SSDs are starting to supplant traditional disk in some use cases, although flash drives and HDDs are expected to coexist in many enterprises for the foreseeable future. For example, SSDs are geared for high-performance storage, but less so for long-term archiving and backup, which typically uses fixed disk.
The following video offers advice on the most important things to consider before installing an SSD in your organization.
The emergence of all-flash storage arrays
Nimbus Data, Pure Storage, Texas Memory Systems and Violin Memory were among the startups that helped pioneer the adoption of all-flash arrays that rely on solid-state drive storage to replace hard disk. The success of all-flash startups prompted established vendors to begin selling retrofitted all-flash versions of their traditional disk-based arrays. IBM is considered to be the first major storage vendor to bring out a dedicated all-flash array platform, called FlashSystem, based on technology it picked up through its acquisition of Texas Memory Systems in 2012. Also in 2012, EMC acquired XtremIO and now ships an all-flash system based on the XtremIO technology. Dell and Hewlett Packard Enterprise (HPE) sell all-flash versions of their respective Compellent and 3PAR arrays, but neither vendor has yet designed a system based solely around solid-state storage. NetApp added an all-flash array by acquiring SolidFire in 2015.
As of 2016, Dell, Pure, HPE, Kaminario and SolidFire have announced plans to ship all-flash systems that incorporate SSDs made with TLC NAND nonvolatile drives. TLC-based SSDs deliver more flash capacity and are cheaper than MLC or SLC, albeit with a higher likelihood for bit rot due to having eight states within the cell.
SSD nonvolatile memory types
NAND flash contrasts with NOR flash memory, which is commonly used in mobile phones. NAND and NOR circuitry differ in the type of logic gate used, with NAND devices using eight-pin serial access to data. NOR devices support 1 byte random access.
SSDs usually are built with single-level cell (SLC) or multilevel cell (MLC) NAND flash memory. SLC drives store 1 bit of data per cell of flash media. MLC-based SSDs double the drive capacity by writing data in two segments. Newer SSDs, known as triple-level cell NAND flash (TLC), are being marketed that store 3 bits of data per flash cell. TLC is less expensive than SLC or MLC, which makes it an attractive option for manufacturers of consumer-based flash devices.
Solid-state PCIe SSDs, NVMe flash devices
Solid-state flash drives have traditionally been designed to use the SATA interface to connect storage to networked servers, using host bus adapters and other components. A newer iteration of server-based flash storage involves SSDs designed for installation in PCIe slots in servers. Each PCIe-enabled SSD communicates directly with a server motherboard using a dedicated point-to-point connection, essentially eliminating resource contention and reducing latency.
Another type of server-based storage deployment involves inserting flash memory in motherboard DIMM slots. Also known as in-memory storage, a flash-based DIMM does not need to traverse the PCIe controller or contend with other cards, thus lowering latency even further versus PCIe flash cards. Diablo Technologies is considered to be the leading flash DIMM card supplier through an OEM partnership with SanDisk.
SSD vendors also are developing PCIe devices around the emerging nonvolatile memory express (NVMe) protocol, a set of specifications designed to operate at the host controller level. NVMe specifications ostensibly aim to increase throughput of PCIe devices by streamlining the I/O stack, removing latency associated with SAS- and SATA-based SSDs.
Hybrid DRAM-flash storage
Advances in SSD manufacturing and other enhancements position the technology to play a greater role in nonvolatile storage. However, new memory channel configurations are emerging that combine flash and server DRAM. Micron and Intel are developing a product for persistent storage -- branded 3D XPoint -- that claims to be as fast as DRAM yet is priced between DRAM and NAND. SanDisk and HPE are partnering on a similar project to develop ReRam, also known as resistive RAM, which is said to be similar to 3D NAND flash but cheaper to manufacture than DRAM. These hybrid flash storage devices arose in response to DRAM approaching its theoretical scaling limit.
The SSD market is dominated by a handful of large manufacturers, including Intel, Micron, SK Hynix, Samsung, SanDisk, Seagate, Toshiba and Western Digital Corp. Micron, Samsung, Seagate and Toshiba produce and sell NAND flash chipsets to SSD vendors, while also marketing branded SSDs based on their own flash chips.
Historically, pricing for SSDs has been much higher than that of conventional hard drives. Due to improvements in manufacturing technology and expanded chip capacity, SSD prices have dropped, allowing consumers and enterprise-level customers to re-evaluate SSDs as viable alternatives to conventional storage.