Solid-state Drives

A solid-state drive (SSD) is a solid-state storage device that uses integrated circuit assemblies to store data persistently, typically using flash memory, and functioning as secondary storage in the hierarchy of computer storage. It is also sometimes called a semiconductor storage device, a solid-state device or a solid-state disk, even though SSDs lack the physical spinning disks and movable read–write heads used in hard disk drives (HDDs) and floppy disks. SSD also has rich internal parallelism for data processing.

In comparison to hard disk drives and similar electromechanical media which use moving parts, SSDs are typically more resistant to physical shock, run silently, and have higher input/output rates and lower latency. SSDs store data in semiconductor cells. cells can contain between 1 and 4 bits of data. SSD storage devices vary in their properties according to the number of bits stored in each cell, with single-bit cells ("Single Level Cells" or "SLC") being generally the most reliable, durable, fast, and expensive type, compared with 2- and 3-bit cells ("Multi-Level Cells/MLC" and "Triple-Level Cells/TLC"), and finally quad-bit cells ("QLC") being used for consumer devices that do not require such extreme properties and are the cheapest per gigabyte of the four. In addition, 3D XPoint memory (sold by Intel under the Optane brand) stores data by changing the electrical resistance of cells instead of storing electrical charges in cells, and SSDs made from RAM can be used for high speed, when data persistence after power loss is not required, or may use battery power to retain data when its usual power source is unavailable. Hybrid drives or solid-state hybrid drives (SSHDs), such as Intel's Hystor and Apple's Fusion Drive, combine features of SSDs and HDDs in the same unit using both flash memory and spinning magnetic disks in order to improve the performance of frequently-accessed data. Bcache achieves a similar effect purely in software, using combinations of dedicated regular SSDs and HDDs.

SSDs based on NAND flash will slowly leak charge over time if left for long periods without power. This causes worn-out drives (that have exceeded their endurance rating) to start losing data typically after one year (if stored at 30 °C) to two years (at 25 °C) in storage; for new drives it takes longer. Therefore, SSDs are not suitable for archival storage. 3D XPoint is a possible exception to this rule; it is a relatively new technology with unknown long-term data-retention characteristics.

SSDs can use traditional HDD interfaces and form factors, or newer interfaces and form factors that exploit specific advantages of the flash memory in SSDs. Traditional interfaces (e.g. SATA and SAS) and standard HDD form factors allow such SSDs to be used as drop-in replacements for HDDs in computers and other devices. Newer form factors such as mSATA, M.2, U.2, NF1/ M.3/ NGSFF, XFM Express ('' Crossover Flash Memory'', form factor XT2) and EDSFF (formerly known as ''Ruler SSD'') and higher speed interfaces such as NVM Express (NVMe) over PCI Express (PCIe) can further increase performance over HDD performance.

SSDs have a limited lifetime number of writes, and also slow down as they reach their full storage capacity.

Development and history

Early SSDs using RAM and similar technology

An early—if not the first—semiconductor storage device compatible with a hard drive interface (e.g. an SSD as defined) was the 1978 StorageTek STC 4305, a plug-compatible replacement for the IBM 2305 fixed head disk drive. It initially used charge-coupled devices (CCDs) for storage (later switched to DRAM), and consequently was reported to be seven times faster than the IBM product at about half the price ($400,000 for 45 MB capacity). Before the StorageTek SSD there were many DRAM and core (e.g. DATARAM BULK Core, 1976) products sold as alternatives to HDDs but they typically had memory interfaces and were not SSDs as defined.

In the late 1980s, Zitel offered a family of DRAM based SSD products under the trade name "RAMDisk", for use on systems by UNIVAC and Perkin-Elmer, among others.

Flash-based SSDs

The basis for flash-based SSDs, flash memory, was invented by Fujio Masuoka at Toshiba in 1980 and commercialized by Toshiba in 1987. SanDisk Corporation (then SanDisk) founders Eli Harari and Sanjay Mehrotra, along with Robert D. Norman, saw the potential of flash memory as an alternative to existing hard drives, and filed a patent for a flash-based SSD in 1989. The first commercial flash-based SSD was shipped by SanDisk in 1991. It was a 20 MB SSD in a PCMCIA configuration, and sold OEM for around $1,000 and was used by IBM in a ThinkPad laptop. In 1998, SanDisk introduced SSDs in 2.5-inch and 3.5-inch form factors with PATA interfaces.SanDisk Product Brochure dated October 1998

In 1995, STEC, Inc. entered the flash memory business for consumer electronic devices.

In 1995, M-Systems introduced flash-based solid-state drives as HDD replacements for the military and aerospace industries, as well as for other mission-critical applications. These applications require the SSD's ability to withstand extreme shock, vibration, and temperature ranges.

In 1999, BiTMICRO made a number of introductions and announcements about flash-based SSDs, including an 18  GBOne gigabyte (1 GB) is equal to one billion bytes (1000³ B). 3.5-inch SSD. In 2007, Fusion-io announced a PCIe-based Solid state drive with 100,000  input/output operations per second (IOPS) of performance in a single card, with capacities up to 320 GB.

At Cebit 2009, OCZ Technology demonstrated a 1  TBOne terabyte (1 TB) is equal to one trillion bytes (1000⁴ B). flash SSD using a PCI Express ×8 interface. It achieved a maximum write speed of 0.654 gigabytes per second (GB/s) and maximum read speed of 0.712 GB/s. In December 2009, Micron Technology announced an SSD using a 6 gigabits per second (Gbit/s) SATA interface.

In 2016, Seagate demonstrated 10 GB/s sequential read and write speeds from a 16-lane PCIe 3.0 SSD, and a 60 TB SSD in a 3.5-inch form factor. Samsung also launched to market a 15.36 TB SSD with a price tag of US$10,000 using a SAS interface, using a 2.5-inch form factor but with the thickness of 3.5-inch drives. This was the first time a commercially available SSD had more capacity than the largest currently available HDD.

In 2018, both Samsung and Toshiba launched 30.72 TB SSDs using the same 2.5-inch form factor but with 3.5-inch drive thickness using a SAS interface. Nimbus Data announced and reportedly shipped 100 TB drives using a SATA interface, a capacity HDDs are not expected to reach until 2025. Samsung introduced an M.2 NVMe SSD with read speeds of 3.5 GB/s and write speeds of 3.3 GB/s. A new version of the 100 TB SSD was launched in 2020 at a price of US$40,000, with the 50 TB version costing US$12,500.

In 2019, Gigabyte Technology demonstrated an 8 TB 16-lane PCIe 4.0 SSD with 15.0 GB/s sequential read and 15.2 GB/s sequential write speeds at Computex 2019. It included a fan, as new, high speed SSDs run at high temperatures. Also in 2019, NVMe M.2 SSDs using the PCIe 4.0 interface were launched. These SSDs have read speeds of up to 5.0 GB/s and write speeds of up to 4.4 GB/s. Due to their high speed operation, these SSDs use large heatsinks and, without sufficient cooling airflow, will typically thermally throttle down after roughly 15 minutes of continuous operation at full speed. Samsung also introduced SSDs capable of 8 GB/s sequential read and write speeds and 1.5 million IOPS, capable of moving data from damaged chips to undamaged chips, to allow the SSD to continue working normally, albeit at a lower capacity.

Enterprise flash drives

Enterprise flash drives (EFDs) are designed for applications requiring high I/O performance (IOPS), reliability, energy efficiency and, more recently, consistent performance. In most cases, an EFD is an SSD with a higher set of specifications, compared with SSDs that would typically be used in notebook computers. The term was first used by EMC in January 2008, to identify SSD manufacturers who would provide products meeting these higher standards. There are no standards bodies who control the definition of EFDs, so any SSD manufacturer may claim to produce EFDs when in fact the product may not meet any particular requirements.

An example is the Intel DC S3700 series of drives introduced in the fourth quarter of 2012, which focuses on achieving consistent performance, an area that had not received much attention but which Intel claimed was important for the enterprise market; In particular, Intel claims that, at a steady state, the S3700 drives would not vary their IOPS by more than 10–15%, and that 99.9% of all 4 KB random I/Os are serviced in less than 500 µs.

Another example is the Toshiba PX02SS enterprise SSD series announced in 2016, optimized for use in server and storage platforms requiring high endurance from write-intensive applications such as write caching, I/O acceleration, and online transaction processing (OLTP). The PX02SS series uses 12 Gbit/s SAS interface, featuring MLC NAND flash memory and achieving random write speeds of up to 42,000 IOPS, random read speeds of up to 130,000 IOPS, and endurance rating of 30 drive writes per day (DWPD).

SSDs based on 3D XPoint have higher IOPS (up to 2.5 million) but lower sequential read/write speeds than their NAND-flash counterparts.

Drives using other persistent memory technologies

In 2017, the first products with 3D XPoint memory were released under Intel's Optane brand; 3D Xpoint is entirely different from NAND flash and stores data using different principles.

Architecture and function

The key components of an SSD are the controller and the memory to store the data. The primary memory component in an SSD was traditionally DRAM volatile memory, but since 2009, it is more commonly NAND flash non-volatile memory.

Controller

Every SSD includes a controller that incorporates the electronics that bridge the NAND memory components to the host computer. The controller is an embedded processor that executes firmware-level code and is one of the most important factors of SSD performance. Some of the functions performed by the controller include:
* Bad block mapping
* Read and write caching
* Encryption
* Crypto-shredding
* Error detection and correction via error-correcting code (ECC) such as BCH code
* Garbage collection
* Read scrubbing and read disturb management
* Wear leveling

The performance of an SSD can scale with the number of parallel NAND flash chips used in the device. A single NAND chip is relatively slow, due to the narrow (8/16 bit) asynchronous I/O interface, and additional high latency of basic I/O operations (typical for SLC NAND, ~25 μs to fetch a 4  KiB page from the array to the I/O buffer on a read, ~250 μs to commit a 4 KiB page from the IO buffer to the array on a write, ~2 ms to erase a 256 KiB block). When multiple NAND devices operate in parallel inside an SSD, the bandwidth scales, and the high latencies can be hidden, as long as enough outstanding operations are pending and the load is evenly distributed between devices.

Micron and Intel initially made faster SSDs by implementing data striping (similar to RAID 0) and interleaving in their architecture. This enabled the creation of SSDs with 250 MB/s effective read/write speeds with the SATA 3 Gbit/s interface in 2009. Two years later, SandForce continued to leverage this parallel flash connectivity, releasing consumer-grade SATA 6 Gbit/s SSD controllers which supported 500 MB/s read/write speeds. SandForce controllers compress the data before sending it to the flash memory. This process may result in less writing and higher logical throughput, depending on the compressibility of the data.

Wear leveling

If a particular block is programmed and erased repeatedly without writing to any other blocks, that block will wear out before all the other blocks—thereby prematurely ending the life of the SSD. For this reason, SSD controllers use a technique called wear leveling