Sunday, January 20, 2008

Solid state disk
A solid state drive (SSD), or solid state disk, is a data storage device that uses solid-state memory to store persistent data. SSD emulates conventional hard disk drive, thus easily replacing it in any application.
With no moving parts, a solid state drive largely eliminates seek time, latency and other electro-mechanical delays and failures associated with a conventional hard disk drive.
While an SSD is not technically a disk, the term solid state disk emphasises its typical use as an alternative to a disk drive.

Architecture and function
However, this is not always the case, as explained below. Flash memory is organised in blocks which can be erased, written or read, but only as whole blocks. The access time is the same for each block. If one or more blocks are used as Access Unit (AU), fragmentation has no harmful effect on access speed. However, for high capacity flash memories the AU would be too big, causing a lot of wasted bytes due to unused space within allocated AU's. Hence, in these cases each block is split up in a number of AU's. Initially AU's will be used sequentially within blocks. So a file with a size of N blocks will use no more than N+2 blocks, the first and last block only partially. However, after some time a situation will occur where no block is available of which all AU's are free, so that one or more extra blocks are needed The result is that said file will need more than N+2 blocks and accessing more blocks takes more time. In the worst case only one AE per block is free and said file will need S*N blocks, where S is the number of AE's per block. The conclusion is that, with bigger flash memory, fragmentation has a detoriating effect on access time.

Faster startup - Since no spin-up required.
Far faster than conventional disks on random I/O
Extremely low read and write latency (seek) times, roughly 15 times faster than the best current mechanical disks.
Faster boot and application launch time when hard disk seeks are the limiting factor. See Amdahl's law.
In some cases, somewhat longer lifetime - Flash storage typically has a data lifetime on the order of 10 years before degradation. If data is periodically refreshed, it can store data indefinitely. Flash drives have limited endurance (typically, 100,000-300,000 write cycles), which, if a single block is written once per second, leads to failure in a few days at most. However, all flash drives employ a technique known as wear levelling, where writes are smoothly distibuted over all blocks. This means that if one write occurs per second, and n is the number of writes before failure and m is the number of blocks on the disk, failure no longer occurs in n seconds, but in (n*m) seconds. Given that blocks are typically on the order of 1kb and an 8 GB disk will have 8,192 blocks, this gives about 9,500 days before failure; remember also this is with one write per second for that entire time
Relatively deterministic performance [8] - unlike mechanical hard drives, performance of SSDs is almost constant and deterministic across the entire storage. "Seek" time can be constant, and performance does not deteriorate as the media fills up (See: Fragmentation).
For very low-capacity drives, lower weight and size. Size and weight per unit storage are still better for traditional hard drives, and microdrives allow up to 20GB storage in a CompactFlash 42.8×36.4×5 mm (1.7×1.4×.2 in) form factor. Advantages
Flash based SSDs also have several disadvantages:

Price - As of early 2007, flash memory prices are still considerably higher per gigabyte than those of comparable conventional hard drives - around US$8 per GB compared to about US$0.25 for mechanical drives.
Lower recoverability - After mechanical failure the data is completely lost as the cell is destroyed, while if normal HDD suffers mechanical failure the data is often recoverable using expert help. Subsequent investigations in to this field, however, have found that data can be recovered from SSD memory.
Vulnerability to certain types of effects, including abrupt power loss (especially DRAM based SSDs), magnetic fields and electric/static charges compared to normal HDDs (which store the data inside a Faraday cage).
Somewhat slower than conventional disks on sequential I/O, the latest perpendicular hard disks doing about 150 MB/s read, with the latest SSDs doing about 120 MB/s read. History of the solid state drive

Commercialization of solid state drives
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.


RAM disk
Computer storage
Disk drive
Flash drive
Hybrid drive
Non-volatile memory
Random access memory
Semiconductor memory
USB flash drive
Volatile memory

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