SSD (Solid State Drives)

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.

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.

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.

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

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.

(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.

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.

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.

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.

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.

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.

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.