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 interface, thus easily replacing it in most applications. An SSD using SRAM or DRAM (instead of flash memory) is often called a RAM-drive , not to be confused with a RAM disk.
The original usage of the term solid-state (from solid-state physics) refers to the use of semiconductor devices rather than electron tubes, but in this context, has been adopted to distinguish solid-state electronics from electromechanical devices as well. With no moving parts, solid-state drives are less fragile than hard disks and are also silent (unless a cooling fan is used); as there are no mechanical delays, they usually enjoy low access time and latency.
Development
The first ferrite memory SSD devices, or auxiliary memory units as they were called at the time, emerged during the era of vacuum tube computers. But with the introduction of cheaper drum storage units, their use was discontinued. Later, in the 1970s and 1980s, SSDs were implemented in semiconductor memory for early supercomputers of IBM, Amdahl and Cray; however, the prohibitively high price of the built-to-order SSDs made them quite seldom used.
In 1978 StorageTek developed the first modern type of 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 not powered. The Sharp PC-5000, introduced in 1983, used 128 kilobyte (128 KB) 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, the Apple IIgs, and later the Macintosh Portable, supported such booting. Tandy MS-DOS machines were equipped with DOS and DeskMate in ROM, as well. 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.
Intermediate
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.
SSDs have begun to appear in laptops, although as of 2009 they are substantially more expensive per unit of capacity than hard drives ($580 for a 256 GB SSD, vs. $50 for a similar size external USB HDD).
Enterprise Flash drives (EFDs) are designed for applications requiring high I/O performance (Input/Output Operations Per Second), reliability and energy efficiency.
On September 25, 2007, Fusion-io announced the ioDrive to be available in Q4 2007, with capacities of 80GB, 160GB and 320GB. The ioDrive actually did not begin shipping until April 7, 2008.
Contemporary
At Cebit 2009, OCZ demonstrated a 1 TB flash SSD using a PCI Express x8 interface. It achieves a minimum read speed of 654MB/s and maximum read speed of 712MB/s.
On March 2, 2009, Hewlett-Packard announced the HP StorageWorks IO Accelerator, the world's first enterprise flash drive especially designed to attach directly to the PCI fabric of a blade server. The mezzanine card, based on Fusion-io's ioDrive technology, serves over 100,000 IOPS and up to 800MB/s of bandwidth. HP provides the IO Accelerator in capacities of 80GB, 160GB and 320GB.
In April 2009, Texas Memory System announced the highest capacity rack mounted flash storage unit to date, a 5TB RamSan-620. It has a throughput of 3GB/s and a sustained random read/write of 250,000 I/Os per second (IOPS). It utilizes high-speed Fibre Channel or InfiniBand interface for data transfers.
On May 4, 2009, DDRdrive LLC introduced the PCI Express based DDRdrive X1. It integrates both 4GB DRAM and 4GB NAND for a total drive capacity of 4GB and targets IOPS intensive enterprise storage, achieving up to 300,000+ Random 512B Read IOPS, a power efficiency of 30,000+ IOPS/W, and a cost effectiveness of 200+ IOPS/$. The next day Photofast announced the G-Monster-PROMISE PCIe SSD with capacity choices from 128GB to 1TB, with 1000MB/s of read/write speeds.
Architecture and function
An SSD is commonly composed of DRAM volatile memory or primarily NAND flash non-volatile memory.
Flash drives
Most SSD manufacturers use non-volatile flash memory to create more rugged and compact devices for the consumer market. These flash memory-based SSDs, also known as flash drives, do not require batteries. They are often packaged in standard disk drive form factors (1.8-, 2.5-, and 3.5-inch). In addition, non-volatility allows flash SSDs to retain memory even during sudden power outages, ensuring data persistence. Flash memory SSDs are slower than DRAM SSDs and some designs are slower than even traditional HDDs on large files, but flash SSDs have no moving parts and thus seek times and other delays inherent in conventional electro-mechanical disks are negligible.
Components:
- Cache: A flash-based SSD uses a small amount of DRAM as a cache, similar to the cache in Hard disk drives. A directory of block placement and wear leveling data is also kept in the cache while the drive is operating.
- Energy storage: Another component in higher performing SSDs is a capacitor or some form of batteries. These are necessary to maintain data integrity such that the data in the cache can be flushed to the drive when power is dropped; some may even hold power long enough to maintain data in the cache until power is resumed.
The performance of the SSD can scale with the number of parallel NAND flash chips used in the device. A single NAND chip is relatively slow, due to narrow (8/16 bit) asynchronous IO interface, and additional high latency of basic IO operations (typical for SLC NAND - ~25 μs to fetch a 4K page from the array to the IO buffer on a read, ~250 μs to commit a 4K page from the IO buffer to the array on a write, ~2 ms to erase a 256 KB 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/Intel SSD made faster flash drives by implementing data striping (similar to RAID0) and interleaving. This allowed creation of ultra-fast SSDs with 250 MB/s effective read/write.
SLC versus MLC
Lower priced drives usually use multi-level cell (MLC) flash memory, which is slower and less reliable than single-level cell (SLC) flash memory. This can be mitigated by the internal design structure of the SSD, such as interleaving and more excess capacity for the wear-leveling algorithms to work with. For instance, in a recent ATTO benchmark a single PCIe Single-Level Cell storage device manufactured by Fusion Multisystems, Inc was able to outperform four MLC-based Intel X-25Ms in RAID 0.
DRAM based drive
See also: I-RAM and Hyperdrive (storage)SSDs based on volatile memory such as DRAM are characterized by ultrafast data access, generally less than 0.01 milliseconds, and are used primarily to accelerate applications that would otherwise be held back by the latency of Flash SSDs or traditional HDDs. DRAM-based SSDs usually incorporate either an internal battery or an external AC/DC adapter and backup storage systems to ensure data persistence while no power is being supplied to the drive from external sources. If power is lost, the battery provides power while all information is copied from random access memory (RAM) to back-up storage. When the power is restored, the information is copied back to the RAM from the back-up storage, and the SSD resumes normal operation. (Similar to the hibernate function used in modern operating systems.)
These types of SSD are usually fitted with the same type of DRAM modules used in regular PCs and servers, allowing them to be swapped out and replaced with larger modules.
A secondary computer with a fast network or (direct) Infiniband connection can be used as a RAM-based SSD.
DRAM based 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 a SSD. Owing to the bandwidth bottleneck of the bus they connect
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