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.[5] SSDs store data in semiconductor cells. As of 2019,[update] 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 (GB) 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.[6] Hybrid drives or solid-state hybrid drives (SSHDs), such as Intel's Hystor[7] 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.[8][9][10] Bcache achieves a similar effect purely in software, using combinations of dedicated regular SSDs and HDDs.
The basis for flash-based SSDs, flash memory, was invented by Fujio Masuoka at Toshiba in 1980[38] and commercialized by Toshiba in 1987.[39][40] 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.[41] The first commercial flash-based SSD was shipped by SanDisk in 1991.[38] 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.[42] In 1998, SanDisk introduced SSDs in 2.5-inch and 3.5-inch form factors with PATA interfaces.[43]
Project: Linux on solid state flash laptop
In 1995, M-Systems introduced flash-based solid-state drives[45] 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.[46]
In 1999, BiTMICRO made a number of introductions and announcements about flash-based SSDs, including an 18 GB[47] 3.5-inch SSD.[48] 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.[49]
Drives known as hybrid drives or solid-state hybrid drives (SSHDs) use a hybrid of spinning disks and flash memory.[114][115] Some SSDs use magnetoresistive random-access memory (MRAM) for storing data.[116][117]
The size and shape of any device are largely driven by the size and shape of the components used to make that device. Traditional HDDs and optical drives are designed around the rotating platter(s) or optical disc along with the spindle motor inside. Since an SSD is made up of various interconnected integrated circuits (ICs) and an interface connector, its shape is no longer limited to the shape of rotating media drives. Some solid-state storage solutions come in a larger chassis that may even be a rack-mount form factor with numerous SSDs inside. They would all connect to a common bus inside the chassis and connect outside the box with a single connector.[6]
Most of the advantages of solid-state drives over traditional hard drives are due to their ability to access data completely electronically instead of electromechanically, resulting in superior transfer speeds and mechanical ruggedness.[148] On the other hand, hard disk drives offer significantly higher capacity for their price.[5][149]
As with HDDs, there is a tradeoff between cost and performance of different SSDs. Single-level cell (SLC) SSDs, while significantly more expensive than multi-level (MLC) SSDs, offer a significant speed advantage. At the same time, DRAM-based solid-state storage is currently considered the fastest and most costly, with average response times of 10 microseconds instead of the average 100 microseconds of other SSDs. Enterprise flash devices (EFDs) are designed to handle the demands of tier-1 application with performance and response times similar to less-expensive SSDs.[154]
SSDs were originally designed for use in a computer system. The first units were intended to replace or augment hard disk drives, so the operating system recognized them as a hard drive. Originally, solid state drives were even shaped and mounted in the computer like hard drives. Later SSDs became smaller and more compact, eventually developing their own unique form factors such as the M.2 form factor. The SSD was designed to be installed permanently inside a computer.[218]
SSDs have very different failure modes from traditional magnetic hard drives. Because solid-state drives contain no moving parts, they are generally not subject to mechanical failures. Instead, other kinds of failure are possible (for example, incomplete or failed writes due to sudden power failure can be more of a problem than with HDDs, and if a chip fails then all the data on it is lost, a scenario not applicable to magnetic drives). On the whole, however, studies have shown that SSDs are generally highly reliable, and often continue working far beyond the expected lifetime as stated by their manufacturer.[219]
Flash-based solid-state drives can be used to create network appliances from general-purpose personal computer hardware. A write protected flash drive containing the operating system and application software can substitute for larger, less reliable disk drives or CD-ROMs. Appliances built this way can provide an inexpensive alternative to expensive router and firewall hardware.[citation needed]
SSDs based on an SD card with a live SD operating system are easily write-locked. Combined with a cloud computing environment or other writable medium, to maintain persistence, an OS booted from a write-locked SD card is robust, rugged, reliable, and impervious to permanent corruption. If the running OS degrades, simply turning the machine off and then on returns it back to its initial uncorrupted state and thus is particularly solid. The SD card installed OS does not require removal of corrupted components since it was write-locked though any written media may need to be restored.
Solid-state hybrid drives (SSHDs) are based on the same principle, but integrate some amount of flash memory on board of a conventional drive instead of using a separate SSD. The flash layer in these drives can be accessed independently from the magnetic storage by the host using ATA-8 commands, allowing the operating system to manage it. For example, Microsoft's ReadyDrive technology explicitly stores portions of the hibernation file in the cache of these drives when the system hibernates, making the subsequent resume faster.[227]
Typically the same file systems used on hard disk drives can also be used on solid state drives. It is usually expected for the file system to support the TRIM command which helps the SSD to recycle discarded data (support for TRIM arrived some years after SSDs themselves but is now nearly universal). This means that the file system does not need to manage wear leveling or other flash memory characteristics, as they are handled internally by the SSD. Some log-structured file systems (e.g. F2FS, JFFS2) help to reduce write amplification on SSDs, especially in situations where only very small amounts of data are changed, such as when updating file-system metadata.
An alternative to the kernel-level TRIM operation is to use a user-space utility called fstrim that goes through all of the unused blocks in a filesystem and dispatches TRIM commands for those areas. fstrim utility is usually run by cron as a scheduled task. As of November 2013[update], it is used by the Ubuntu Linux distribution, in which it is enabled only for Intel and Samsung solid-state drives for reliability reasons; vendor check can be disabled by editing file /etc/cron.weekly/fstrim using instructions contained within the file itself.[234]
Prior to version 7, Microsoft Windows did not take any specific measures to support solid state drives. From Windows 7, the standard NTFS file system provides support for the TRIM command. (Other file systems on Windows 7 do not support TRIM.)[251]
By default, Windows 7 and newer versions execute TRIM commands automatically if the device is detected to be a solid-state drive. However, because TRIM irreversibly resets all freed space, it may be desirable to disable support where enabling data recovery is preferred over wear leveling.[252] To change the behavior, in the Registry key HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\FileSystem the value DisableDeleteNotification can be set to 1. This prevents the mass storage driver issuing the TRIM command.
Defragmentation should be disabled on solid-state drives because the location of the file components on an SSD doesn't significantly impact its performance, but moving the files to make them contiguous using the Windows Defrag routine will cause unnecessary write wear on the limited number of P/E cycles on the SSD. The Superfetch feature will not materially improve performance and causes additional overhead in the system and SSD.[257] Windows Vista does not send the TRIM command to solid-state drives, but some third-party utilities such as SSD Doctor will periodically scan the drive and TRIM the appropriate entries.[258]
The following are noted standardization organizations and bodies that work to create standards for solid-state drives (and other computer storage devices). The table below also includes organizations which promote the use of solid-state drives. This is not necessarily an exhaustive list. 2ff7e9595c
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