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Solid-state drive

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An SSD in standard 2.5-inch (64 mm) form-factor
PCI attached IO Accelerator SSD

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 the present context, has been adopted to distinguish solid-state electronics from electromechanical devices. 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.



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;[1] 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.


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.

In 2008 low end netbooks appeared with SSDs. In 2009 SSDs began to appear in laptops.[2][3]

Enterprise Flash drives (EFDs) are designed for applications requiring high I/O performance (Input/Output Operations Per Second), reliability and energy efficiency.


At Cebit 2009, OCZ demonstrated a 1 TB flash SSD using a PCI Express x8 interface. It achieves a minimum write speed of 654MB/s and maximum read speed of 712MB/s.[4]

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.[5]

Architecture and function

An SSD is commonly composed of DRAM volatile memory or primarily NAND flash non-volatile memory.[6]

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.


  • 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.[7]

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.[8][9] 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.[10]

DRAM based drive

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.)

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.[11]

File:Open HDD and SSD.JPG
Open casing of 2.5-inch traditional hard disk drive (left) and solid-state drive (center)

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 to, DRAM 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/scratch files on a RAM SSD, as opposed to a traditional hard drive, therefore can increase performance significantly.

Comparison of SSD 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.[12]

File:Disassembled HDD and SSD.JPG
The disassembled components of a hard disk drive (left) and of the PCB and components of a solid-state drive (right)

Comparisons reflect typical characteristics, and may not hold for a specific device.


  • Faster start-up because no spin-up is required.
  • Fast random access because there is no read/write head[13]
    • Low read latency times for RAM drives.[14] In applications where hard disk seeks are the limiting factor, this results in faster boot and application launch times (see Amdahl's law).[15]
    • Consistent read performance because physical location of data is irrelevant for SSDs.[16]
    • File fragmentation has negligible effect.
  • Silent operation due to the lack of moving parts.
  • Low capacity flash SSDs have a low power consumption and generate little heat when in use.
  • High mechanical reliability, as the lack of moving parts almost eliminates the risk of "mechanical" failure.
  • Ability to endure extreme shock, high altitude, vibration and extremes of temperature.[17][18] This makes SSDs useful for laptops, mobile computers, and devices that operate in extreme conditions (flash).[15]
  • 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 form-factor. As of 2008 SSDs up to 256 GB are lighter than hard drives of the same capacity.[17]
  • Flash SSD's have twice the data density of HDD's (so far, with very recent and major developments of improving SSD densities), even up to 1TB disks[19][20] (currently more than 2TB is atypical even for HDD's)[21]). One example of this advantage is that portable devices such as a smartphone may hold as much as a typical person's desktop PC.
  • Failures occur less frequently while writing/erasing data, which means there is a lower chance of irrecoverable data damage.[22]


  • Wear leveling used on flash-based SSDs has security implications. For example, encryption of existing unencrypted data on flash-based SSDs cannot be performed securely due to the fact that wear leveling causes new encrypted drive sectors to be written to a physical location different from their original location -- data remains unencrypted in the original physical location. It is also impossible to securely wipe files by overwriting their content on flash-based SSDs.
  • As of early-2010, SSDs are still more expensive per gigabyte than hard drives. Whereas a normal flash drive is US$2 per gigabyte, hard drives are around US$0.10 per gigabyte for 3.5", or US$0.20 for 2.5".
  • The capacity of SSDs is currently lower than that of hard drives. However, flash SSD capacity is predicted to increase rapidly, with drives of 1 TB already released for enterprise and industrial applications.[20][23][24][25][26]
  • Asymmetric read vs. write performance can cause problems with certain functions where the read and write operations are expected to be completed in a similar timeframe. SSDs currently have a much slower write performance compared to their read performance.[27]
  • Similarly, SSD write performance is significantly impacted by the availability of free, programmable blocks. Previously written data blocks that are no longer in use can be reclaimed by TRIM; however, even with TRIM, fewer free, programmable blocks translates into reduced performance.[28]
  • Flash-memory drives have limited lifetimes and will often wear out after 1,000,000 to 2,000,000 write cycles (1,000 to 10,000 per cell) for MLC, and up to 5,000,000 write cycles (100,000 per cell) for SLC.[29][30][31][32] Special file systems or firmware designs can mitigate this problem by spreading writes over the entire device, called wear leveling.[33]
  • As a result of wear leveling and write combining, the performance of SSDs degrades with use.[34][35]
  • SATA-based SSDs generally exhibit much slower write speeds. As erase blocks on flash-based SSDs generally are quite large (e.g. 0.5 - 1 megabyte),[8] they are far slower than conventional disks during small writes (write amplification effect) and can suffer from write fragmentation.[36] Modern PCIe SSDs however have much faster write speeds than previously available.
  • DRAM-based SSDs (but not flash-based SSDs) require more power than hard disks, when operating; they still use power when the computer is turned off, while hard disks do not.[37]
  • Defragmentation cannot be performed on flash-based SSDs due to wear leveling (operating system cannot control the real physical location of disk sectors). Some SSDs compact free space when idle. However, this improves only writing speed -- not reading speed of existing fragmented data.


Cost and capacity

Until recently,[when?] flash based solid-state drives were too costly for widespread use in mobile computing.[citation needed] 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. It has been said that using said flash drive incurs an overall speed increase of 250%, which Professor Ray Johnston of Carleton University has described as "significantly faster".


Solid-state drive (SSD) technology has been marketed to the military and niche industrial markets since the mid-1990s[citation needed].

File:CompactFlash IDE Adaptor.jpeg
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 significantly to the price of the laptop, depending on the capacity, form factor and transfer speeds. As of 2008 some manufacturers have begun shipping affordable, fast, energy-efficient drives priced at $350 to computer manufacturers.[citation needed] For low-end applications, a USB flash drive may be obtained 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 as part of the 'One Laptop Per Child' project. Mass production of these computers, built for children in developing countries, began in December 2007. These machines use 1024 MiB SLC NAND flash as primary storage which is considered more suitable for the harsher than normal conditions in which they are expected to be used. Dell began shipping ultra-portable laptops with SanDisk SSDs on April 26, 2007.[2] Asus released the Eee PC subnotebook on October 16, 2007, and after a successful commercial start in 2007, it was expected to ship several million PCs in 2008, with 2, 4 or 8 gigabytes of flash memory.[38] On January 31, 2008, Apple Inc. released the MacBook Air, a thin laptop with optional 64 GB SSD. The Apple store cost was $999 more for this option, as compared to that of an 80 GB 4200 rpm Hard Disk Drive.[3] Another option—Lenovo ThinkPad X300 with a 64Gbyte SSD—was announced by Lenovo in February 2008,[39] and is, as of 2008, available to consumers in some countries. On August 26, 2008, Lenovo released ThinkPad X301 with 128GB SSD option which adds approximately $200 US.

The Mtron SSD

As of October 14, 2008, Apple's MacBook and MacBook Pro lines carry optional solid state hard drives of up to 256 GB at an additional cost. Dell began to offer optional 256 GB solid state drives on select notebook models in January 2009.

In late 2008, Sun released the Sun Storage 7000 Unified Storage Systems (codenamed Amber Road), which use both solid state drives and conventional hard drives to take advantage of the speed offered by SSDs and the economy and capacity offered by conventional hard disks.[40]

In May 2009 Toshiba launched a laptop with a 512 GB SSD[41][42].

In December 2009, Micron Technology announced the world's first SSD using a 6Gbps SATA interface. [43]

Quality and performance

SSD is a rapidly developing technology. A January 2009 review of the market by technology reviewer Tom's Hardware concluded that comparatively few of the tested devices showed acceptable I/O performance, with several disappointments,[44] and that Intel (who make their own SSD chipset) still produces the best performing SSD drive as of this time; a view also echoed by Anandtech.[45] In particular, operations that require many small writes, such as log files, are particularly badly affected on some devices, potentially causing the entire host system to freeze for periods of up to one second at a time.[46]

According to Anandtech, this is due to controller chip design issues with a widely used set of components, and at least partly arises because most manufacturers are memory manufacturers only, rather than full microchip design and fabrication businesses — they often rebrand others' products,[47] inadvertently replicating their problems.[48] Of the other manufacturers in the market, Memoright, Mtron, OCZ, Samsung and Soliware were also named positively for at least some areas of testing.

The overall conclusion by Tom's Hardware however, was that "none of the [non-Intel] drives were really impressive. They all have significant weaknesses: usually either low I/O performance, poor write throughput or unacceptable power consumption".[44]

OCZ has recently unveiled OCZ Vertex 2 Pro which is currently the fastest MLC SSD drive with a Sandforce Controller onboard performing more or less as the Intel X25-E series SSD drives. [49]


A use for flash drives is to run lightweight operating systems designed specifically for turning general-purpose PCs into network appliances comparable to more expensive routers and firewalls. In this situation, a write protected flash drive containing the whole operating system is used to boot the system. A similar system could boot from CD, floppy disk or a traditional hard drive but flash memory is a good choice because of very low power consumption and failure rate.

Hybrid drive

A hybrid disk uses an SSD as a buffer for a larger hard disk drive. The hard disk may be spun down more of the time if data is available in the SSD.

NAND Flash based SSDs offer a potential power saving; however, the typical pattern of usage of normal operations result in cache misses in the NAND Flash as well leading to continued spin of the drive platter or much longer latency if the drive needed to spin up.[citation needed] These devices would be slightly more energy efficient but could not prove to be any better in performance.[citation needed]

DRAM-based SSDs may also work as a buffer cache mechanism (see hybrid RAM drive). When 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 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);
  • Loss of power/computer shutdown.

Microsoft Windows and exFAT

Versions of Windows prior to Windows 7 are optimized for hard disk drives rather than SSDs.[50][51] Windows Vista includes ReadyBoost to exploit characteristics of USB-connected flash devices. Windows 7 is optimized for SSDs[52][53] as well as for hard disks. It includes support for the TRIM command.

Microsoft's exFAT file system is optimized for SSDs.[54] According to Microsoft, "The exFAT file system driver adds increased compatibility with flash media. This includes the following capabilities: Alignment of file system metadata on optimal write boundaries of the device; Alignment of the cluster heap on optimal write boundaries of the device."[55] Support for the new file system is included with Vista Service Pack 1 and Windows 7 and is available as an optional update for Windows XP.[55]


Solaris, as of 10u6 (released in October 2008), and recent versions of OpenSolaris and Solaris Express Community Edition on which OpenSolaris is based, can use SSD drives as a performance booster for ZFS. There are two available modes—using an SSD for the ZFS Intent Log (ZIL), which is used every time a synchronous write to the disk occurs, or for the L2ARC (Level 2 Adaptive Replacement Cache), which is used to cache data for reading. When used either alone or in combination, large increases in performance are generally seen.[56]

See also


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  48. ^ "Intel X25-M SSD: Intel Delivers One of the World's Fastest Drives". Anandtech. September 8th, 2008. pp. Enter the Poorly Designed MLC. http://www.anandtech.com/cpuchipsets/intel/showdoc.aspx?i=3403&p=7. Retrieved 2009-04-21. 
  49. ^ http://www.anandtech.com/showdoc.aspx?i=3702
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  51. ^ "Samsung, Microsoft in talks to speed up SSDs on Vista". http://www.computerworld.com/action/article.do?command=viewArticleBasic&articleId=9111939. Retrieved 2008-09-22. 
  52. ^ David Flynn. "Windows 7 gets SSD-friendly". http://apcmag.com/windows_7_gets_ssdfriendly.htm. Retrieved 2009-01-29. 
  53. ^ e7blog. "MSDN Blogs: Support and Q&A for Solid-State Drives". http://blogs.msdn.com/e7/archive/2009/05/05/support-and-q-a-for-solid-state-drives-and.aspx. Retrieved 2009-05-27. 
  54. ^ "FAT32 Gets Steroids Boost – No Limitations". Splash in Flash Memory. 2009-02-23. http://www.getflashmemory.info/fat32-gets-steroids-boost-no-limitations/. Retrieved 2009-10-18. "The exFAT file system is a new file format system to address the growing demand and size of mobile storage like USB sticks, PDAs, and solid state hard drives." 
  55. ^ a b "Description of the exFAT file system driver update package". KB955704. Microsoft Corporation. 2009-09-29. http://support.microsoft.com/kb/955704. Retrieved 2009-10-15. 
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