Solid-state drive

This article refers to both flash and DRAM-based solid state drives. For discussions on other types of flash-based solid state storage, see flash drive and USB flash drive.

A solid state drive (SSD, also called solid state disk) is a data storage device that uses non-volatile memory (NAND) such as flash, or volatile memory such as SDRAM, to store data, instead of the spinning platters found in conventional hard disk drives. While not technically "disks" in any sense, these devices are so named because they are typically used as replacements for disk drives in situations where conventional drives are impractical.

Definition

A solid state drive is primarily a data storage device, for use in computing applications that traditionally use a hard disk drive.

A solid state drive is based on non-volatile memory instead of the spinning platter and mechanical-magnetic head found in a conventional hard disk drive. With no moving parts, a solid state drive eliminates seek time, latency and other electro-mechanical delays and failures associated with a conventional hard disk drive.

Design and function

A typical RAM-based SSD.

A flash SSD in standard 2.5-inch form factor.

File:Open HDD and SSD.JPG

Open casing of 2.5” traditional hard disk drive (left) and solid state drive (center), a drop-in replacement for hard disk drives for mobile computing.

The disassembled components of a hard disk drive (left) and of the PCB and components of a solid state drive (right).

SSDs based on volatile memory such as SDRAM and are categorized by fast data access, less than 0.01 milliseconds (over 250 times faster than the fastest hard drives in 2004) and are used primarily to accelerate applications that would otherwise be held back by the latency of disk drives.

DRAM-based SSDs typically incorporate internal battery and backup disk systems to ensure data persistence. If power is lost for whatever reason, the battery would keep the unit powered long enough to copy all data from random access memory (RAM) to backup disk. Upon the restoration of power, data is copied back from backup disk to RAM and the SSD resumes normal operation.

However, most SSD manufacturers use nonvolatile flash memory to create more rugged and compact alternatives to DRAM-based SSDs. These flash memory-based SSDs, also known as flash disks, do not require batteries, allowing makers to replicate standard disk drive form factors (1.8-inch, 2.5-inch, and 3.5-inch). In addition, nonvolatility allows flash SSDs to retain memory even during sudden power outages, ensuring data retrievability. Just like DRAM SSDs, flash SSDs are extremely fast since these devices have no moving parts, eliminating seek time, latency and other electro-mechanical delays inherent in conventional disk drives. (Though flash SSDs are significantly slower than DRAM SSDs).

Solid state disks are especially useful on a computer which already has the maximum amount of RAM. For example, some x86 architectures have a 4 GB limit, but this can effectively be extended by putting the swap file on a SSD. These SSD do not provide as fast storage as main RAM because of the bandwidth bottleneck of the bus they connect to, but would still provide a performance increase over placing the swap file on a traditional hard disk drive.

DRAM based SSDs may also work like a buffer cache mechanism. Whenever 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 two strategies: 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).

Advantages and disadvantages of SSDs

Flash based SSDs have several unique advantages:

• Faster startup - Since no spin-up required.
• Faster read time – In some cases, twice or more than that of the fastest hard drives.
• Low read and write latency (seek) time, hundreds of times faster than a mechanical disk.
• Faster boot and application launch time - Result of the faster read and especially seek time. But only if application already resides in flash and is more dependant on read speed than other issues, eg. OS bootup that detects devices will not be significantly sped up even with faster seeks & reads.
• Lower power consumption and heat production - no mechanical parts result in less power consumption.
• No noise - Lack of mechanical parts makes the SSD completely silent.
• Better mechanical reliability - Lack of mechanical parts result in less wear and tear. High level of ability to endure extreme shock, vibration and temperatures, which apply to laptops and other mobile devices, or when transported.
• Lower but improving read-write lifetime - In the past, flash based SSDs were limited in the number of read/write cycles, but modern flash technology and error correction allow Flash SSDs to operate several years without fail (some manufacturers claim as many as 10 years).
• Security - allowing a very quick "wipe" of all data stored.
• Deterministic performance - unlike mechanical hard drives, performance of SSDs is constant and deterministic across the entire storage. "Seek" time is constant, and performance does not deteriorate as the media fills up (See: Fragmentation).
• Lower weight and (depending upon type) size

Flash based SSDs also have several disadvantages:

• Price - As of late 2006 Flash prices are still considerably higher per Gigabyte than those of comparable conventional hard drives - around $25 per GB compared to about $0.25 for mechanical drives.
• Slower write time - Flash has a slower write time than high-end hard drives (around 18 MB/s compared to over 50 MB/s for hard drives), though this varies considerably based on the make and model.
• Shorter reliable lifetime - Flash based SSDs have relatively limited read/write cycles compared to hard drives which can last over a decade without mechanical malfunction. This is significant since in many systems, certain hard drive locations may regularly be accessed tens of thousands of times within even short periods.
• 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.
• Vulnerability against certain types of effects, including abrupt powerloss (especially DRAM based SSDs), magnetic fields and electric/static charges compared to normal HDDs (which store the data inside a Faraday cage).

History of the solid state disk

In the mid 1980s a company named Santa Clara Systems introduced a product named BatRam which consisted of an array of 1 megabit DIP RAM Chips and a custom controller card that emulated a hard drive. The package included a rechargeable battery to preserve the memory chip contents when the power was off.

msystems was the first company to introduce flash-based solid state disks in 1995 (SanDisk completed acquisition of msystem in November 2006). Since then, they have been used successfully as hard disk drive replacements by the military and aerospace industries, as well as other mission-critical applications that require the exceptional mean time between failure (MTBF) rates that solid state disks achieve based on their ability to withstand extreme shock, vibration and temperature ranges.

Commercialization of solid state drives

NAND flash cost and capacity considerations

Until recently, however, 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.

Availability

Currently flash-based SSDs are priced outside the mainstream consumer market and very limited in quantity. SSDs have been appearing in ultra mobile PCs and a few light weight laptop systems, adding a US$200–$800 premium to the systems, depending on the capacity, form factor and transfer speeds. Only a handful of companies offer large (>64 GB) SSD drives with write speeds adequate for replacing traditional drives, but these drives are available in limited quantities, very high priced and available by special request only.

• Samsung has promised to make its drives available to consumers in early 2007.
• Sandisk has released a 32GB solid state disk on January 4 2007 .
• Taiwanese A-DATA introduced at the Las Vegas CES 2007 SSD drives at capacities of 32GB, 64GB (1.8" model) and 128GB (2.5" model). It is expected to be commercially available by mid-2007.
• SimpleTech has announced a 64GB SSD that is only 9.5mm thick, half the size of competing SSDs.

See also

• Disk drive
• Hybrid drive
• Non-volatile memory
• Flash disk
• USB flash drive

External references

• STORAGEsearch.com SSD portal site
• 2005 SSD Survey by STORAGEsearch.com
• Samsung's Solid State Disk Drive Unveiled - An article and an interview with Don Barnetson from Samsung about their technology