Power consumption in NAND flash storage
When designing a storage system, the power used by each component is an important consideration. Power dissipation in semiconductor devices has increased because of higher data rates and the smaller transistor sizes in modern chips. Here we will look at the implications for NAND flash-based storage, such as SSDs.
Managing power is important for reasons which vary for different markets and applications. In handheld consumer products and battery-powered IoT devices, battery life is likely to be the most important factor. For data centers with thousands of servers, each equipped with an SSD, provisioning for power is a significant part of the both installation and running costs. Reducing the power of each component can contribute to lower costs and a smaller environmental footprint.
Ultimately, increased temperatures affect the performance and lifetime of NAND flash based solutions. This is particularly important for industrial systems that operate at high ambient temperatures such as automotive solutions where high reliability is critical.
What factors contribute to power consumption in an SSD?
Chip power consumption can be categorized as either dynamic, which comes from the current used when transistors change state during operation, and static, which is caused by leakage current.
Dynamic power is proportional to clock speed and how much of the device is active. Leakage is independent of the function of the device and has become a much greater proportion of total power consumption as process geometries have shrunk. Leakage current increases dramatically with temperature.
The flash memory controller is a significant contributor to the power consumption because of the processing power needed to managing data transactions and implementing Error Correction Codes (ECC). The NAND flash memory itself also contributes to the power consumption. This has two parts: the power needed for transferring data between the controller and the memory, and the power required by the memory to function. The first is mainly dependent on the interface speed; faster interfaces consume more power.
Some SSDs also include DRAM as a cache for data and address mapping information. The high-speed interface to the DRAM consumes energy as does the DRAM itself. Therefore, an SSD with DRAM will use more power. It is also likely to be less robust to sudden power failure because important data is kept in volatile memory.
How can power consumption be minimized?
There are several ways to manage the power consumption of NAND flash storage.
If heat dissipation is the main consideration, then cooling systems such as heatsinks and fans may be part of the solution. However, these are not always appropriate because of size or cost constraints.
Choosing low power devices is important. Within a NAND flash storage system, the most critical decision for both power consumption and performance is choosing the right flash memory controller.
There are several circuit design tricks that can be used to minimize the power consumption of complex devices like the flash memory controller. A common technique is clock gating: this turns the clock off when circuits are not active hence reducing the dynamic power. This does not reduce the leakage current, so a technique that can be used in combination with this is power gating: turning off the supply voltage to inactive circuits. Rather than turning the clock and supplies off completely, it is also possible to dynamically adjust the clock speed and supply voltage. This allows performance and power consumption to be balanced to match the system requirements.
Whats the key take away?
Most NAND flash storage devices do not give a clear definition of their power consumption figures and comparing the datasheets for two products may not always be useful for your specific use case. Ultimately, a low-power flash memory controller that makes efficient use of the NAND flash memory ensures lower operational costs, higher performance, a longer lifetime and higher reliability, all with a reduced environmental impact.