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solid state drive



Abstract - A SSD (solid-state drive or solid-state disk) is a storage device that stores
persistent data on solid-state flash memory. SSDs actually aren't hard drives at all, in the traditional sense of the term, as there are no moving parts involved. Instead, an SSD has an array of semiconductor memory organized as a disk drive, using integrated circuits (ICs) rather than magnetic or optical media.

Solid state is term that refers to electronic circuitry that is built entirely out of semiconductors. The term was originally used to define those electronics such as a transistor radio that used semiconductors rather than vacuum tubes in its construction. Most all electronics that we have today are built around semiconductors and chips. In terms of a SSD, it refers to the fact that the primary storage medium is through semiconductors rather than a magnetic media such as a hard drive. SSDs have no moving mechanical components, which distinguish them from traditional electromechanical magnetic disks such as hard disk drives (HDDs) or floppy disks, which contain spinning disks and movable read/write heads. Compared with electromechanical disks, SSDs are typically less susceptible to physical shock, much quieter, have lower access time, and less latency. However, while the price of SSDs has continued to decline in 2012, SSDs are still about 10 times more expensive per unit of storage than HDDs. This arrangement has many advantages. Data transfer to and from solid-state drives is much faster than electromechanical disk drives. Seek time and latency are also substantially reduced. Users typically enjoy much faster boot times as well. In general, SSDs are also more durable and much quieter, with no moving parts to break or spin up or down. SSDs do, however, have slower write times and a set life expectancy, as there is a finite number of erase/write cycles before performance becomes erratic. Development and adoption of SSDs has been driven by a rapidly expanding need for higher input/output performance. High performance laptops, desktops or any application that needs to deliver information in real-time or near real-time can benefit from SSDs. Historically, SSDs have been much more expensive than conventional hard drives. Due to improvements


in manufacturing technology and expanded chip capacity, however, prices have dropped, leading both consumers and enterprise-level customers to re-evaluate SSDs as viable, if still expensive, alternatives to conventional storage. In recent years, SSDs have been used in enterprise storage to speed up applications and performance without the cost of adding additional servers. According to storage expert Rick Cook, one of the most potent uses of SSDs is to employ them as a "super cache" in a SAN, dramatically speeding up access to frequently accessed files or applications. While SSDs have many advantages over HDDs, they also have some drawbacks. Since the SSD technology is much newer than traditional hard drive technology, the price of SSDs is substantially higher. As of early 2012, SSDs cost about 10 times as much per gigabyte as a hard drive. Therefore, most SSD drives sold today have much smaller capacities than comparable hard drives. They also have a limited number or write cycles, which may cause their performance to degrade over time. Fortunately, newer SSDs have improved reliability and should last several years before any reduction in performance is noticeable. As the SSD technology improves and the prices continue to fall, it is likely that solid state drives will begin to replace hard disk drives for most purposes. [5]

The key components of an SSD are the controller and the memory to store the data. The primary memory component in an SSD had been DRAM volatile memory since they were first developed, but since 2009 it is more commonly NAND flash non-volatile memory. Other components play a less significant role in the operation of the SSD and vary among manufacturers.

2.1 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:
      

Error correction (ECC) Wear leveling Bad block mapping Read scrubbing and read disturb management Read and write caching Garbage collection Encryption

2.2 Memory 2.2.1 Flash memory-based
Most SSD manufacturers use non-volatile NAND flash memory in the construction of their SSDs because of the lower cost compared with DRAM and the ability to retain the data without a constant power supply, ensuring data persistence through sudden power outages. Flash memory SSDs are slower than DRAM solutions, and some early designs were even slower than HDDs after continued use. This problem was resolved by controllers that came out in 2009 and later. [1]

2.2.2 DRAM-based
SSDs based on volatile memory such as DRAM are characterized by ultrafast data access, generally less than 10 microseconds, 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).SSDs of this type are usually fitted with DRAM modules of the same type used in regular PCs and servers, which can be swapped out and replaced by larger modules.


2.2.3 Cache or buffer
A flash-based SSD typically 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. Data is not permanently stored in the cache. One SSD controller manufacturer, SandForce, does not use an external DRAM cache on their designs, but still achieves very high performance. Eliminating the external DRAM enables a smaller footprint for the other flash memory components in order to build even smaller SSDs. [3]

2.3 Battery or super capacitor
Another component in higher performing SSDs is a capacitor or some form of battery. 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. In the case of MLC flash memory, a problem called lower page corruption can occur when MLC flash memory loses power while programming an upper page. The result is that data written previously and presumed safe can be corrupted if the memory is not supported by a super capacitor in the event of a sudden power loss. This problem does not exist with SLC flash memory. Most consumer-class SSDs do not have built-in batteries or capacitors; among the exceptions are the Intel 320 series and the more expensive Intel 710 series.

The size and shape of any device is 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 or optical disc along with the spindle motor inside. If an SSD is made up of various interconnected integrated circuits (ICs) and an interface connector, then its shape could be virtually anything imaginable because it 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.


For general computer use, the 2.5" form factor (typically found in laptops) is the most popular. For desktop computers with 3.5" hard disk slots, a simple adapter plate can be used to make such a disk fit. Other types of form factors are more common in enterprise applications. A SSD can also be completely integrated in the other circuitry of the device, as in the Apple MacBook Air (starting with the fall 2010 model).

2.4.1 Box form factors
Many of the DRAM-based solutions use a box that is often designed to fit in a rackmount system. The number of DRAM components required to get sufficient capacity to store the data along with the backup power supplies requires a larger space than traditional HDD form factors.

2.4.2 Ball grid array form factors
In the early 2000s, a few companies introduced SSDs in Ball Grid Array (BGA) form factors, such as M-Systems' (now SanDisk) DiskOnChipand Silicon Storage Technology's NANDrive (now produced by Greenliant Systems), and Memoright's M1000 for use in embedded systems. The main benefits of BGA SSDs are their low power consumption, small chip package size to fit into compact subsystems, and that they can be soldered directly onto a system motherboard to reduce adverse effects from vibration and shock.

There is one drive which combines HDD and SSD on one unit and that is Hybrid Drive. SSD have faster starting facility because as mention earlier it does not contain disks to spin for booting. It has fast random access because it does not need any seeking motion like hard disk drives. Plus it has silent functioning due to less physical work. It consumes less power than


hard disk drives due to this laptop manufacturer keep them as an additional optional to standard HDD. Lack of physical work reduces the risk of mechanical failure. This technology is immune to magnets. Its size varies according to the requirement. Writing/reading data is secure as less failure occur which means there is less chance of data damage. SSD can perform parallel reads on multiple sections of memory which decrease the seek time of hard disk.

In mobile computing flash bases, solid state drives were costly for it. SSD are becoming popular in markets for notebook PC due to its less size and physical work. SSDs have been appearing in mobile PCs and a few lightweight laptop systems, adding significantly to the price of the laptop, depending on the capacity and transfer speeds.

This is a rapid developing technology. That is why it is been used in many applications. Until 2009, SSDs were used in mission critical applications where the speed of the storage system needed to be as fast. Since Flash memory has become a common, the falling prices have made it financially attractive for many other applications. Organizations that can benefit from faster access of system took this technology in hands. The list of applications which could benefit from faster storage is vast. Any company can assess the ROI from adding SSDs to their own applications to best understand if that will be cost effective for them. Flash-based Solid-state drives can be used to create network appliances from normal PC hardware. A flash drive which is writing protected containing the operating system and application software can substitute for larger and less reliable disk. Appliances built this way can provide an inexpensive alternative to expensive router. Thus this technology helps in any possible way to replace hard disk drives.


[1] Devesh Agrawal, Deepak Ganesan, Ramesh K. Sitaraman, Yanlei Diao, and Shashi Singh. Lazy-Adaptive Tree: An Optimized Index Structure for Flash Devices. PVLDB, 2(1):361–372, 2009. [2] Stephan Baumann, Giel de Nijs, Michael Strobel, and Kai-Uwe Sattler. Flashing Databases: Expectations and Limitations. In DaMoN, 2010.

[3] Jaeyoung Do and Jignesh M. Patel. Join Processing for Flash SSDs: Remembering Past Lessons. In DaMoN, pages 1–8, 2009. [4] Geoff Gasior. Intel’s X25-E Extreme Solid-state Drive. Technical report, The Tech Report, 2008.

[5] Goetz Graefe. The Five-Minute Rule 20 Years Later (and How Flash Memory Changes the Rules). Commun. ACM, 52(7):48–59, 2009.


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