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This article is in need of attention from an expert on the subject. WikiProject Computing may be able to help recruit one. |
This article refers to both flash and DRAM-based solid state drives. For other flash-based solid state storage, see flash drive and USB flash drive.
A solid state drive (SSD) is a data storage device that uses solid-state memory to store persistent data. An SSD emulates a hard disk drive, thus easily replacing it in any application. An SSD using SRAM or DRAM (instead of flash memory) is often called 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, a solid state drive is usually more robust, effectively eliminating the risk of mechanical failure, usually silent, and usually enjoys reduced seek time and latency by removing mechanical delays associated with hard disk drives.
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A SSD is commonly composed of either NAND flash non-volatile memory or DRAM volatile memory.
Most SSD manufacturers use nonvolatile 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 in regards to reads. Flash SSDs have no moving parts, eliminating seek time, latency and other electro-mechanical delays inherent in conventional disks.
SSDs based on volatile memory such as DRAM are characterized by fast data access, 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 typically incorporate internal battery and backup disk systems to ensure data persistence. If power is lost for whatever reason, the battery would keep the unit powered long enough to copy all data from random access memory (RAM) to the backup disk. Upon the restoration of power, data is copied back from backup disk to RAM and the SSD resumes normal operation.
Solid state drives are especially useful on a computer that has already come with maximum amount of 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 a SSD. These SSDs do not provide as fast storage as main RAM because of the bandwidth bottleneck of the bus they connect to, but would still provide a performance increase over placing the swap file on a traditional hard disk drive.
DRAM based SSDs may also work like 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 strategies:
Modern models carry lithium-ion batteries as backup in case of a power outage or for transfer to another computer.
A detailed comparison (with bench marks) of SSD, SDHC, and hard disk drives (HDDs) is given here.
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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 solid-state disk 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 failure rates that solid state drives achieve based on 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.
Until recently, solid state disks 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 technology 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, with Dell leading, started to offer such technology. The capacity of these drives varies from 16 GB to 256 GB.
Even though solid state disk (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 several case studies, along with analyst reports, are published.
Along with the emerging enterprise market, SSDs have been appearing in ultra mobile PCs and a few light weight 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 around $10-$100, 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.Dell Gets Flash With SSD Option for Laptops Asus released the Eee PC subnotebook on October 16 2007 which, after a successful commercial start in 2007, is expected to ship several millions PCs in 2008 with 2, 4 or 8GB of soldered-in flash memory.Chen, Shu-Ching Jean. "$199 Laptop Is No Child\'s Play", Forbes, 2007-06-07. Retrieved on 2007-06-28. On January 31 2008 Apple Inc. released the MacBook Air, a thin laptop with optional 64GB SSD. The cost is $1000 more for this option, over the 80GB 4200RPM Hard Disk Drive.
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Toshiba Launches High Performance Solid State Drives With MLC NAND Flash Memory
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