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SSD (Solid State Drives)

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Toshiba is an industry leader both in small form factor HDDs and in NAND flash technology (a type of non-volatile storage technology that does not require power to retain data), offering a broad range of digital media products based on innovative storage technologies. Toshiba storage products are used by major brands of applications such as notebooks, navigation systems, data centers and external storage solutions.

A solid state drive (SSD) is a data storage device based on semiconductor memory called NAND flash memory, which was first developed by Toshiba in 1987. SSDs are becoming increasingly popular in computer and other markets ranging from consumer electronics to enterprise and industrial equipment.

Unlike Hard disk drives, which are the more common storage solution used today, SSDs have no moving parts, therefore, SSDs have plenty of advantages: they are exceptionally silent, rugged, durable and reliable, and also offer a smaller, faster and lower power experience.

Solid state drives (SSD) and Hard disk drives (HDD) have their individual characteristics and strengths in terms of Input/Output per second (IOPS), capacity, sequential throughput, energy usage and cost effectiveness. Because SSDs and HDDs address different needs in the market, they complement each other and will co-exist long term. Toshiba is leading the industry as the sole total storage solution provider in both HDD and NAND flash-based SSD technology*1, and will continue to provide both storage solutions to be adopted in applications for which each is best suited.

*1: Toshiba survey as of November 2014.

Applications

The ruggedness, reliability and high-level performance of Toshiba NAND flash technology help increase mobility, functionality and confidence in a wide range of client and enterprise products: Mobile computing, gaming, home entertainment, industrial systems and data centre server. The application list continues to grow as MLC and TLC technologies enable higher capacities with cost competitiveness.

Toshiba have introduced the-state-of art SSD, cSSD and eSSD, into the market since 2007 and they are currently available in a wide variety of form factors, interface and capacities.

Client Solid State Drives (cSSD)

Notebooks demand not only thinner body but also lighter weight, longer battery life and higher processing performance. In order to meet the market demand of various types of notebooks from flip-flop to detachable, storage devices require higher data transmission speed, lower latency, small form factors and lower power consumption.

SSDs, a NAND flash based storage media, have recently received attention as a storage device for client notebook application. cSSDs are suitable for client applications in terms of fast read/write speed, small space, lower power and, shock and vibration resistance.

Enterprise Solid State Drives (eSSD)

With a seemingly exponential continued growth of data, the management of the data and the demands placed by today’s enterprise computing environments and applications, it becomes increasingly more vital to ensure accessibility, availability, performance and reliability criteria are met 24x7. Running in parallel with these expectations, advances in storage technology and techniques used in Solid State Drives (SSD) has meant that the quality of NAND based flash memory has now reached position, where it can provide mission-critical data reliability alongside data rates offering considerable improvements over traditional rotating HDD. SSD effectively places semiconductor memory behind a memory controller and interface electronics allowing the flash storage device to emulate the commands and operations of a HDD.

Toshiba’s Enterprise Class SSD (eSSD), is state-of-the art in terms of Enterprise class disk storage design. It is designed to complement and offer enhanced functionality into potentially any application environment (Server, Storage(DAS, SAN & NAS)), which was traditionally the stronghold of the mechanical based Enterprise class HDD offering a SAS2 interface. With advantages of reduced power consumption (less cooling and therefore a reduction in data centre support overheads), ultimate shock and vibration tolerance, and fundamentally faster access and data rates over HDD, eSSD is fast becoming the primary choice for Tier0 applications where IOPS and reliability are critical factors.

eSSD is predicted to replace the traditional mechanical spindle based technology in the majority of high-end performance led applications. It makes an excellent fit for mission critical applications where reliability, performance and long term expectations are desired.

* DAS : Direct Attached Storage, SAN : Storage Area Network, NAS : Network Attached Storage

* SAS : Serial Attached SCSI

 

Audio Equipment

Home Theatre

Broadcast Recording

Camcorders

CNC Devices

In Flight Entertainment

CT Scanners

Desktop PCs

Electronic Musical Instruments

HD Studio Camera

High-Speed Cameras

Navigation System

Kiosk Terminals

Laser Printers

Medical Measurement Instrument

Tablet PCs

Notebook PCs

Office Equipment

Arcade Games

POS Systems

Professional Camcorders

Set-Top Boxes

Studio Recording

Surveillance Cameras

Telephone Exchanges

Televisions

Video Editing Systems

Business Processing Servers

Financial Transactions Servers

Data Analysis Servers

How SSDs Work

An SSD is generally comprised of a printed circuit board, a set of NAND flash memory chips, a DRAM cache, a memory controller, an interface controller and an interface connector such as SATA, SAS etc.

The memory used in SSDs is Non-volatile NAND flash memory and is produced in various features. NAND is designed based on using single-level cell (SLC), multi-level cell (MLC) or triple-level cell (TLC) technology. SLC stores one-bit-per cell, has longer endurance, but is significantly more costly to produce with higher capacities. MLC uses two bits per cell and TLC uses three bits per cell. These flash technologies have lower endurance, but hold larger capacities and can be produced at lower costs.

Configuration of SSDs

cSSD
Client SSD (HG6 Series)

eSSD
Enterprise SSD (HK3R & PX02SMB160)

(1) Connector

Like HDDs, SATA interface is used with SSDs. SATA connector has a mechanical configuration that is compatible with sockets for HDDs.

(2) Controller

The controller is a core device of SSDs, enabling fast read/write performance, high write endurance and enhanced reliability.

(3) NAND flash memory

Data is stored in a NAND flash memory, which leverages Toshiba's MLC and TLC technologies to realize high storage capacities and low costs.

SSD Features

SSDs offer several advantages for mobile device market because they have no moving parts and can withstand considerable shock and vibration. The high data transfer rate and low power consumption that they offer mean that SSDs are able to improve the performance of the devices with which they are used. Additionally, SSDs produce absolutely no mechanical noise, making them ideal for use in noise-sensitive devices.

Fast Read/Write Performance

SSDs do not require a mechanical operation which is a method of HDDs to access data and need only a read/write operation to access data through NAND flash memory. This electrical operation enables high speed access time which is the biggest advantage of SSDs. Furthermore, SSDs also enable pallarel access to multi NAND flash memories that improves read/write performance. The data trasmission speed of the SSD embedded 19nm NAND flash is faster than 500MB/s and the random access achieves 80,000IOPS.

Reagarding the read/write performance of NAND flash memory, SLC is faster than MLC, however a cost per bit of MLC is lower as MLC can retain two bits per cell. Therefore, MLC is widely used for Client PC application. Recently, TLC has started to be introduced for further improvement of cost effectiveness.

* Read and write speed may vary depending on the host device, read and write conditions, and file size.

Shock and Vibration Resistance

Another major advantage of SSDs is their lack of moving parts. They are impervious to impact and provide stable operation and can, therefore, withstand considerable shock and vibration. This means that they work reliably even in locations exposed to high levels of vibration and are ideal for use in notebooks and other smaller portable devices that require shock and vibration resistance.

Low Power Consumption

Because SSDs are not equipped with a motor, they consume less power than HDDs, helping to extend a computer's battery life. This is not only a big advantage for notebooks, but also especially in high-performance enterprise applications such as large servers, where SSDs replace several HDDs due to the significant performance advantages of solid state technology, the low power consumption is of high importance. With growing government regulation around the world, IT companies today are increasingly seeking ways to reduce power consumption without compromising performance. The heat generation per data proceeding performance of SSDs is significantly low, reducing data center cooling requirements, which is not only better for the environment, but also may result in substantial cost savings.

Lightweight and Compact

Client SSD
HG6 Series

Toshiba Client SSDs (cSSDs) are lightweight devices that weight approximately 9 g (Half Slim Module, mSATA Module). They are ideal for use with increasingly lightweight mobile devices that are demanded. Many of the SSDs presently come in 1.8- and 2.5-inch drive form factors to ensure that their shape allows them to be used as replacements for HDDs. However, because SSDs consist mainly of semiconductor devices, they offer greater design flexibility and can be designed to be directly plugged into a motherboard. M.2 type is one of the form factors that had recently put into practical use and SSDs support Type22110, 2280, 2260, 2242 and 2230. Thus, SSDs help reduce product size and offer greater flexibility in product design.

PCI Express M.2 standard
Figure : PCI Express M.2 standard (PCI Express M.2 Specification)

Silent

HDDs contain moving parts, which causes operation sounds. The benefit of not having any moving parts like SSDs, is the silence. SSDs produce absolutely no auditable noise, making them ideal for use with noise-sensitive devices such as televisions, home cinema systems, digital audio systems and camcorders.

Technology for Life Time Expansion

As a result of repeating read/write i.e. electron injection into a floating gate of NAND memory cell, the oxide layer is degraded and this degradation causes limitation of read/write endurance shortening a life time of SSD. Various technologies have been applied to overcome this limitation and to extend the life time of SSD, there are three major techniques widely employed with current SSDs, “Wear Leveling”, “Over Provisioning” and “ECC & Refresh”.

Wear Leveling is managed through the flash controller algorithms which monitor and reassign data blocks that are frequently accessed and have met a predefined access threshold to maintain performance. Over Provisioning method - by which the number of logical blocks assigned to the device - exceeds the marketed capacity to provide the required life expectancy through re-assignment using the Wear Leveling technique. With regard to the adoption of Error correction codes (ECC) and Refresh, ECC is redundant codes added to user data to correct errors and Refresh is a mechanism which relocates data to prevent an error before the limit of error correction by ECC is exceeded. Error rates of NAND memory increase when Erase/Write cycle increases. ECC and Refresh techniques avoid error rates getting worse and help SSDs to expand their life time.

Key Words

  • SLC (Single-Level Cell)

    SLC is a memory element capable of storing data in individual memory cells. SLC accesses at high speed and writes to SSDs by using "simpler" control logic with 1 bit versus 2 bits used by Multi-level Cell (MLC). Therefore, SLC has the advantage of lower power consumption. In addition, the write operations in SLC chips last 100,000 cycles, approximately ten times longer than MLC endurance. Due to the fast transfer rate and high reliability, SLC memory is used in high-performance SSDs.

    * Read and write speed may vary depending on the host device, read and write conditions, and file size.

  • MLC (Multi-Level Cell)

    NAND devices can store more than 1 bit of information per memory cell by choosing between multiple levels of electrical charge to apply to the floating gates of its cells. MLC retains 2 bits of information per cell. This technology enables higher density storage in a small form factor and more cost-effective storage per gigabyte.

    * Read and write speed may vary depending on the host device, read and write conditions, and file size.

  • TLC (Triple-Level Cell)

    TLC stores 3 bits per cell and provides more cost-effectiveness into SSDs. TLC has started to be used in cSSD and it is predicted that a demand of TLC introduction becomes strong as low-cost client applications such as low-end notebooks and tablet PCs are getting popular.

    * Read and write speed may vary depending on the host device, read and write conditions, and file size.

  • Endurance

    The maximum number of Erase/Write cycles that reaches a product life time as degradation of NAND cell occurs, which causes loss of data retention and reliability.

  • Data Retention

    This is a time period that a stored memory cell data is expected to remain recoverable and uncorrupted during a life of a storage device. A cell degrades as it approaches the maximum endurance and the cell Erase/Write count gets to a value representing total life expectancy.

  • Over-provisioning

    This is an essentially excess capacity allocated to a storage device to allow for cells reaching 100% endurance (The life expectancy reached) within the period of warranted use of a storage device. An excess capacity is brought on-line and allocated to offset the loss of usable storage cells. Generally an acceptable figure is dependent on each product.

  • Wear-leveling

    Each memory cell has a finite life, their structure breaks down and the cells are worn out. To ensure that each cell is subjected to equal wear and tear, Erase/Write count is maintained. Once the count has reached a pre-determined threshold, the collective cell data content is re-assigned to a younger cell collective / sector.

  • Error Correction Codes (ECC)

    ECC is redundant codes which are added to user data, are read out with user data then correct an error included in the data.

  • Refresh

    As SSDs are used for a long period of time, they wear out. Refresh is a mechanism to relocate data to prevent an increase of error and reassign the data to expand a usage life time of SSDs.

  • Process Technology Shrink of NAND Flash Memory

    Shrink of Process technology enables larger capacities of semiconductor memories and an increase of the number of chips in one wafer improves cost performance. Therefore, shrink of Process technology is one of the factors to evaluate the competitiveness of NAND flash memory process technology. The minimum line pitch of circuits (Half pitch) is used as an index of process technology shrink.

    Advancement of the process technologies of Toshiba NAND flash memory embedded in Client SSDs is following below:

    43 nm — 2008

    32 nm — 2009

    19nm — 2012

    19nm second process — 2014

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