A solid-state drive (SSD) is a data storage device that uses solid-state memory to store persistent data. Unlike flash-based memory cards, an SSD emulates a hard disk drive, thus easily replacing it in most applications. An SSD using SRAM or DRAM (instead of flash memory) is often called a RAM-drive. The original usage of the term solid-state (from solid-state physics) refers to the use of semiconductor devices rather than electron tubes, but has in this context been adopted to distinguish solid-state electronics from electromechanical devices as well. With no moving parts, solid-state drives are inherently less fragile than harddisks and therefore also silent (unless a cooling fan is used); as there are no mechanical delays, they usually enjoy low access time and latency.
History
SSDs have begun to appear in laptops, although they are at present substantially more expensive per unit of capacity than hard drives.History Please help improve this article or section by expanding it.Further information might be found on the talk page or at requests for expansion. (February 2007) In 1978, the StorageTek company developed the first solid-state drive. In the mid-1980s Santa Clara Systems introduced BatRam, an array of 1 megabit DIP RAM Chips and a custom controller card that emulated a hard disk. The package included a rechargeable battery to preserve the memory chip contents when the array was unpowered. The Sharp PC-5000, introduced in 1983, used 128 kilobyte (128 KiB) solid-state storage cartridges, containing bubble memory. RAM "disks" were popular as boot media in the 1980s, when hard drives were expensive, floppy drives were slow, and a few systems, such as the Amiga series and the Apple IIgs, supported such booting. At the cost of some main memory, the system could be soft-rebooted and be back in the operating system in mere seconds instead of minutes. Some systems were battery-backed so contents could persist when the system was shut down. In 1995 M-Systems introduced flash-based solid-state drives. (SanDisk acquired M-Systems in November 2006). Since then, SSDs have been used successfully as hard disk drive replacements by the military and aerospace industries, as well as other mission-critical applications. These applications require the exceptional mean time between failures (MTBF) rates that solid-state drives achieve, by virtue of their ability to withstand extreme shock, vibration and temperature ranges. The Gigabyte i-RAM uses standard DDR modules and connects to its host via Serial ATA. This card can use the system's standby power (also used for Wake-on-LAN and similar features) to maintain its RAM contents even with the system powered off, and includes a battery that can retain the data when the system is completely disconnected from power. Architecture and function An SSD is commonly composed of either NAND flash non-volatile memory or DRAM volatile memory. Most SSD manufacturers use non-volatile flash memory to create more rugged and compact alternatives for the consumer market. These flash memory-based SSDs, also known as flash drives, do not require batteries, allowing makers to replicate standard disk drive form factors (1.8-inch, 2.5-inch, and 3.5-inch). In addition, non-volatility allows flash SSDs to retain memory even during sudden power outages, ensuring data retrievability. Though flash SSDs are significantly slower than DRAM, they still perform better than traditional hard drives, at least with regard to reads. Flash SSDs have no moving parts, thus eliminating spin-up time altogether, and greatly reducing seek time, latency and other delays inherent in conventional electro-mechanical disks. SSDs based on volatile memory such as DRAM are characterized by very fast data access, generally less than 0.01 milliseconds (over 250 times faster than the fastest hard drives in 2004), and are used primarily to accelerate applications that would otherwise be held back by the latency of disk drives. DRAM-based SSDs sometimes incorporate internal battery and backup storage systems to ensure data persistence. If power is lost for any reason, the battery keeps the unit powered for sufficient time to allow the copying of all data from random access memory (RAM) to the back-up storage. Then, when the power is restored, data is copied back to RAM from the back-up storage, and the SSD resumes normal operation. (This is not unlike the "hibernate" function used in modern operating systems, which saves the entire contents of memory to nonvolatile storage before power-down, to be rewritten into memory upon power-up.) Solid-state drives are especially useful on computers that already have the maximum amount of supported RAM. For example, some computer systems built on the x86-32 architecture can effectively be extended beyond the 4 GB limit by putting the paging file or swap file on an SSD. Owing to the bandwidth bottleneck of the bus they connect to, SSDs cannot read and write data as fast as main RAM can, but they are far faster than any mechanical hard drive. Placing the swap file on an SSD, as opposed to a traditional hard drive, can therefore provide a significant performance increase. DRAM-based SSDs may also work as a buffer cache mechanism. Whenever data is written to memory, the corresponding block in memory is marked as dirty, and all dirty blocks can be flushed to the actual hard drive based on the following two criteria: Time (e.g. every 10 seconds, flush all dirty data); Threshold (when the ratio of dirty data to SSD size exceeds some predetermined value, flush the dirty data). Modern models carry lithium-ion batteries as back-up in case of a power outage, or to allow transfer to another computer.
Comparison with hard disk drives
A comparison (with benchmarks) of SSDs, Secure Digital High Capacity (SDHC) drives, and hard disk drives (HDDs) is given in the reference.
Advantages
Faster start-up (over 30 times faster[citation needed]) – as no spin-up is required. (RAM & Flash) Typically fast random access for reading – as there is no read/write head to move. (RAM & Flash) Extremely low read and write latency times – as SSD seek-times are orders of magnitude lower than the best current hard disk drives. (RAM) In applications where hard disk seeks are the limiting factor this results in faster boot and application launch times ( see Amdahl's law). (RAM) No noise – a lack of moving parts makes SSDs completely silent, unless, as in the case of some high-end and high-capacity models, they have cooling fans attached. (RAM & Flash) For low-capacity SSDs, low power consumption and heat production – although high-end SSDs and DRAM-based SSDs may have significantly higher power requirements. High mechanical reliability – the lack of moving parts almost eliminates the risk of mechanical failure. (RAM & Flash) Ability to endure extreme shock, high altitude, vibration and extremes of temperature – once again because there are no moving parts. This makes SSDs useful for laptops, mobile computers, and devices that operate in extreme conditions. (Flash) Relatively deterministic performance – unlike hard disk drives, performance of SSDs is almost constant and deterministic across the entire storage. This is because the seek time is almost constant and is not dependent on the physical location of the data, and so, file fragmentation has almost no impact on performance. For low-capacity SSDs, lower weight and size – although size and weight per unit storage are still better for traditional hard drives, and microdrives allow up to 20 GB storage in a CompactFlash 42.8×36.4×5 mm (1.7×1.4×.2 in) form-factor. Up to 256 GB, SSDs are currently lighter than hard drives of the same capacity.
Disadvantages
Price – as of mid-2008, flash memory prices are still considerably more costly per gigabyte than are comparable conventional hard drives: around USD 3.50 per GB compared to typically less than USD 0.40 for mechanical drives. Capacity – although currently far lower than that of conventional hard drives, SSD capacity is predicted to increase rapidly, with experimental drives of up to 1 TB in test. Higher vulnerability to certain types of effects, including abrupt power loss (especially DRAM based SSDs), magnetic fields and electric/static charges, in comparison to normal HDDs (which store the data inside a Faraday cage). Limited write cycles – flash-memory storage will often wear out after 300,000-500,000 write cycles[citation needed], while high endurance flash-memory storage may have an endurance of 1–5 million write cycles (many log files, file allocation tables, and other commonly used parts of the file system exceed this over the lifetime of a computer. Special file systems or firmware designs can mitigate this problem by spreading writes over the entire device (so-called wear levelling), rather than rewriting files in place. This problem is being improved all the time, with consequent increases in expected lifespans. Today's drives can last up to 20 years with average usage. An example for the lifetime of SSD is explained in detail here. SSDs based on DRAM, however, do not suffer from this problem. Slower write speeds – as erase blocks on flash-based SSDs generally are quite large, they are far slower than conventional disks for random writes, and in some cases for sequential writes. SSDs based on DRAM do not suffer from this problem.
Cost and capacity
Until recently, solid-state drives were too costly for mobile computing. As flash manufacturers transition from NOR flash to single-level cell (SLC) NAND flash and most recently to multi-level cell (MLC) NAND flash to maximize silicon die usage and reduce associated costs, "solid-state disks" are now being more accurately renamed "solid-state drives" – they have no disks but function as drives – for mobile computing in the enterprise and consumer electronics space. This technological trend is accompanied by an annual 50% decline in raw flash material costs, while capacities continue to double at the same rate. As a result, flash-based solid-state drives are becoming increasingly popular in markets such as notebook PCs and sub-notebooks for enterprises, Ultra-Mobile PCs (UMPC), and Tablet PCs for the healthcare and consumer electronics sectors. Major PC companies have now started to offer such technology. The capacity of these drives varies from 12 GB to 256 GB.
Availability
Even though solid-state drive (SSD) technology has been marketed to the military and niche industrial markets since the mid-1990s, it is only recently that the enterprise sector has taken notice of the benefits that SSDs can offer, as key SSD technologies emerge, prices drop and new case studies, along with analyst reports, are published.
CompactFlash card used as SSD
Along with the emerging enterprise market, SSDs have been appearing in ultra-mobile PCs and a few lightweight laptop systems, adding a US$ $600 to $1000 premium to the price of a HDD-equipped laptop, depending on the capacity, form factor and transfer speeds. Only a handful of companies offer large (128 GB or larger) SSD drives with write speeds adequate for replacing traditional drives, and these drives are available in limited quantities and are very expensive. Already some manufacturers have begun shipping affordable, fast, energy-efficient drives priced at $350 to computer manufacturers. For low-end applications, a USB memory stick may be used as a Flash hard drive for $10 to $100 or so, depending on capacity, or a CompactFlash card may be paired with a CF-to-IDE or CF-to-SATA converter at a similar cost. Either of these requires that write-cycle endurance issues be managed, either by not storing frequently written files on the drive, or by using a Flash file system. Standard CompactFlash cards usually have write speeds of 7 to 15 megabytes per second while the more expensive upmarket cards claim speeds of up to 40 MB/s One of the first mainstream releases of SSD was the XO Laptop built under the 'One Laptop Per Child' project. Mass production of these computers built for children in developing countries begun in December 2007. These machines use 1024 MiB SLC NAND flash as primary storage solution which is considered more suitable for the harsher than normal conditions they are expected to be used in. Dell has begun shipping ultra-portable laptops with SanDisk SSDs on April 26, 2007. Asus released the Eee PC subnotebook on October 16 2007, and after a successful commercial start in 2007, expects to ship several million PCs in 2008, with 2, 4 or 8 gigabytes of flash memory. On January 31, 2008 Apple Inc. released the MacBook Air, a thin laptop with optional 64 GB SSD. The cost is $599 more for this option if configured in the Apple Store, as compared to that of an 80 GB 4200 RPM Hard Disk Drive. Another option - IBM Lenovo ThinkPad X300 with a 64Gbyte SSD - was announced by Lenovo in February 2008, and is currently available to consumers in some countries. Architecture and function An SSD is commonly composed of either NAND flash non-volatile memory or DRAM volatile memory. Most SSD manufacturers use non-volatile flash memory to create more rugged and compact alternatives for the consumer market. These flash memory-based SSDs, also known as flash drives, do not require batteries, allowing makers to replicate standard disk drive form factors (1.8-inch, 2.5-inch, and 3.5-inch). In addition, non-volatility allows flash SSDs to retain memory even during sudden power outages, ensuring data retrievability. Though flash SSDs are significantly slower than DRAM, they still perform better than traditional hard drives, at least with regard to reads. Flash SSDs have no moving parts, thus eliminating spin-up time altogether, and greatly reducing seek time, latency and other delays inherent in conventional electro-mechanical disks. SSDs based on volatile memory such as DRAM are characterized by very fast data access, generally less than 0.01 milliseconds (over 250 times faster than the fastest hard drives in 2004), and are used primarily to accelerate applications that would otherwise be held back by the latency of disk drives. DRAM-based SSDs sometimes incorporate internal battery and backup storage systems to ensure data persistence. If power is lost for any reason, the battery keeps the unit powered for sufficient time to allow the copying of all data from random access memory (RAM) to the back-up storage. Then, when the power is restored, data is copied back to RAM from the back-up storage, and the SSD resumes normal operation. (This is not unlike the "hibernate" function used in modern operating systems, which saves the entire contents of memory to nonvolatile storage before power-down, to be rewritten into memory upon power-up.)
Solid-state drives are especially useful on computers that already have the maximum amount of supported RAM. For example, some computer systems built on the x86-32 architecture can effectively be extended beyond the 4 GB limit by putting the paging file or swap file on an SSD. Owing to the bandwidth bottleneck of the bus they connect to, SSDs cannot read and write data as fast as main RAM can, but they are far faster than any mechanical hard drive. Placing the swap file on an SSD, as opposed to a traditional hard drive, can therefore provide a significant performance increase. DRAM-based SSDs may also work as a buffer cache mechanism. Whenever data is written to memory, the corresponding block in memory is marked as dirty, and all dirty blocks can be flushed to the actual hard drive based on the following two criteria: Time (e.g. every 10 seconds, flush all dirty data); Threshold (when the ratio of dirty data to SSD size exceeds some predetermined value, flush the dirty data). Modern models carry lithium-ion batteries as back-up in case of a power outage, or to allow transfer to another computer.
