Solid-state drive

This article is about flash-based, DRAM-based and other solid-state drives. For other flash-based solid-state storage, see USB flash drive. For software based secondary storage, see RAM disk.

An SSD in standard 2.5-inch (64 mm) form-factor.

DDR SDRAM based SSD

A solid-state drive (SSD) is a data storage device that uses solid-state memory to store persistent data. A 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.

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 employ low access time and latency.

SSDs have begun to appear in laptops, although as of 2009 they are substantially more expensive per unit of capacity than hard drives.

History

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.

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 2007, SSDs of a few gigabytes capacity gained mainstream popularity with netbook and subnotebooks.

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

Fusion-IO announced the 640GB version of ioDrive to be available in Q1 2008, but when ioDrive was finally released in 2008, the biggest unit only has the size of 320GB.

Toshiba, beginning in 2009 will begin to manufacture a 512 gigabyte SSD, the largest capacity yet reached for this technology. With the trend of achieving greater capacity into solid state drives, this technology will soon replace traditional hard drives in some notebook computers.

Architecture and function

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

Flash based

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-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 persistence. SSDs are slower than DRAM (and even traditional HDDs on big files), but may perform better than hard drives (at least with regard to reads) because of negligible seek time (flash SSDs have no moving parts, and thus eliminate spin-up time, and greatly reduce seek time, latency, and other delays inherent in conventional electro-mechanical disks).

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, the maximum the SATA interface can manage.

In 2008-03-31, Fusion-IO announced ioDrive, the company's first product, would begin shipping on 2008-04-07. It uses a PCI Express slot to overcome the bandwidth limitation of SATA and SAS interfaces, and the 80GB unit has a read speed of over 800MB/s and write speed of 694MB/s.

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. Still, MLC chips continue to be used in many commercially available SSDs.

DRAM based

See also: I-RAM and Hyperdrive (storage)

SSDs based on volatile memory such as DRAM are characterized by ultra fast 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 SDDs or traditional HDDs. DRAM-based SSDs usually incorporate internal battery 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 connection can be used as a RAM-based SSD.

File:Open HDD and SSD.JPG

Open casing of 2.5” 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 an 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 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.

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.

Advantages

• Faster start-up, as no spin-up is required (RAM & Flash).
• Typically, fast random access for reading, as there is no read/write head to move (RAM & Flash).
• Extremely low read latency times, as SSD seek-times are orders of magnitude lower than the best hard disk drives, as of 2008. (RAM) In applications where hard disk seeks are the limiting factor, this results in faster boot and application launch times (see Amdahl's law) (RAM & Flash).
• Extremely fast read (RAM, nearly the same for best modern flash).
• No noise: a lack of moving parts makes SSDs completely silent, unless, as in the case of some high-end and high-capacity models, they have cooling fans attached (RAM & Flash).
• For low-capacity flash SSDs, low power consumption and heat production when in active use, although high-end SSDs and DRAM-based SSDs may have significantly higher power requirements (Flash).
• High mechanical reliability, as the lack of moving parts almost eliminates the risk of mechanical failure (RAM & Flash).
  • Ability to endure extreme shock, high altitude, vibration and extremes of temperature: once again because there are no moving parts. This makes SSDs useful for laptops, mobile computers, and devices that operate in extreme conditions (Flash).
• Larger range of operating temperatures. Typical hard drives have an operating range of 5-55 degrees C. Most flash drives can operate at 70 degrees, and some industrial grade drives can operate over an even wider temperature range.
• Relatively deterministic read performance: unlike hard disk drives, performance of SSDs is almost constant and deterministic across the entire storage. This is because the seek time is almost constant and does not depend on the physical location of the data, and so, file fragmentation has almost no impact on read performance.
• 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 42.8×36.4×5 mm (1.7×1.4×.2 in) form-factor. Up to 256 GB, as of 2008 SSDs are lighter than hard drives of the same capacity.
• When failures occur, they tend to happen predominantly while writing, or erasing cells, rather than upon reading cells. With magneto-mechanical drives, failures tend to occur while reading. If a drive detects failure on write operations, data can be written to a new location. If a drive fails on read, then data is usually lost permanently.

Disadvantages

• Cost: As of mid-2008, SSD prices are still considerably higher per gigabyte than are comparable conventional hard drives: consumer grade drives are typically US$2.00 to US$3.45 per GB for flash drives and over US$80.00 per GB for RAM-based compared to about US$0.38 or less per gigabyte for hard drives.
• Capacity: As of 2008, far lower than that of conventional hard drives (Flash SSD capacity is predicted to increase rapidly, with experimental drives of 1 TB,, hard drive capacity also continues to expand, and hard drives are likely to maintain their capacity edge for some time).
• Limited write (erase) cycles: Flash-memory cells will often wear out after 1,000 to 10,000 write cycles for MLC, and up to 100,000 write cycles for SLC, while high endurance cells may have an endurance of 1–5 million write cycles (many log files, file allocation tables, and other commonly used parts of the file system exceed this over the lifetime of a computer). Special file systems or firmware designs can mitigate this problem by spreading writes over the entire device (so-called wear levelling), rather than rewriting files in place. In 2008 wear levelling was just beginning to be incorporated into consumer level devices. However, effective write cycles can be much less, because when a write request is made to a particular memory block, all data in the block is overwritten even when only part of the memory is altered. The write amplification, as referred by Intel, can be reduced using write memory buffer. In combination with wear leveling, over-provisioning SSD flash drives with spared memory capacity also delays the loss of user-accessible memory capacity. NAND memory can be negatively impacted by read and program (write) disturbs arising from over accessing a particular NAND location. This overuse of NAND locations causes bits within the NAND block to erroneously change values. Wear leveling, by redirecting SSD writes to lesser-used NAND locations, thus reduces the potential for program or write disturbs. An example for the lifetime of SSD is explained in detail in this wiki. SSDs based on DRAM, however, do not suffer from this problem.
• Slower write speeds: As erase blocks on flash-based SSDs generally are quite large (e.g. 0.5 - 1 megabyte), they are far slower than conventional disks during small writes (the smaller, the worse) and can suffer from write fragmentation, and in some cases for sequential writes. SSDs based on DRAM, which do this several orders of magnitude faster than conventional disks, do not suffer from this problem.
• Lower storage density: Hard disks can store more data per unit volume than DRAM or flash SSDs, except for very low capacity/small devices.
• DRAM based SSD require more power than hard disks, when operating; and they still use power when the computer is turned off, while hard disks do not.

Commercialization

Cost and capacity

Until recently, solid-state drives 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 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.

Availability

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

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. For low-end applications, a USB memory stick may be used as a Flash hard drive 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. Asus released the Eee PC subnotebook on October 16 2007, and after a successful commercial start in 2007, expects to ship several million PCs in 2008, with 2, 4 or 8 gigabytes of flash memory. 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. Another option - Lenovo ThinkPad X300 with a 64Gbyte SSD - was announced by Lenovo in February 2008, and is, as of 2008, available to consumers in some countries. (Now X301 with 128GB).

File:IMG 0085.jpg-m.jpg

The Mtron SSD

Apple's MacBook and MacBook Pro lines, announced on October 14, 2008, carry optional 128 GB solid state hard drives for an additional $550. Apple's MacBook Pro 17" laptop announced on January 6, 2009 also carried optional 256 GB solid state drives for an additional $900. Dell began to offer optional 256 GB solid state drives on select notebook models in January 2009 for an additional $400.

Applications

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

Main article: Hybrid drive

A hybrid disk uses a 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 savings, 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. These devices would be slightly more energy efficient but could not prove to be any better in performance.

DRAM-based SSDs may also work as a buffer cache mechanism (see hybrid RAM drive). When data are 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:

1. Time (e.g., every 10 seconds, flush all dirty data);
2. Threshold (when the ratio of dirty data to SSD size exceeds some predetermined value, flush the dirty data).
3. Loss of power/computer shutdown

SSDs and Microsoft Windows

Windows is optimized for hard disk drives rather than SSDs. Microsoft created ReadyBoost to exploit characteristics of USB-connected flash devices, but Windows operating systems are not optimized for use of SSDs. Microsoft's latest OS Windows 7 which is still in beta, is optimized for SSDs as well as hard disks.

Tests by an SSD maker show that Windows 2000 is markedly faster than Windows XP, Windows Vista, Mac OS X or Linux, when it is run from an SSD using NAND flash memory. Windows 98 is even faster, but does not support wear leveling, so would cause the drive to wear out quickly.

See also

• Hybrid drive - A hard disk which uses a solid-state drive to improve performance.
• Computer storage
• Flash file system

Further reading

• Netbook Trends and Solid-State Technology Forecast (PDF). pricegrabber.com. p. 7. Retrieved 2009-01-28.

External links

History

• STORAGEsearch.com

Technology Adoption

• JEDEC Continues SSD Standardization Efforts