Non-volatile storage (NVS) is a broad collection of technologies and devices that do not require a continuous power supply to retain data or program code persistently on a short- or long-term basis.
Three common examples of NVS devices that persistently store data are tape, a hard disk drive (HDD) and a solid-state drive (SSD). The term non-volatile storage also applies to the semiconductor chips that store the data or controller program code within devices such as SSDs, HDDs, tape drives and memory modules.
Many types of non-volatile memory chips are in use today. For instance, NAND flash memory chips commonly store data in SSDs in enterprise and personal computer systems, USB sticks, and memory cards in consumer devices such as mobile telephones and digital cameras. NOR flash memory chips commonly store controller code in storage drives and personal electronic devices.
Non-volatile storage technologies and devices vary widely in the manner and speed in which they transfer data to and retrieve data or program code from a chip or device. Other differentiating factors that have a significant impact on the type of non-volatile storage a system manufacturer or user chooses include cost, capacity, endurance and latency.
For example, an SSD equipped with NAND flash memory chips can program, or write, and read data faster and at lower latency through electrical mechanisms than a mechanically addressed HDD or tape drive that uses a head to write and read data to magnetic storage media. However, the per-bit price to store data in a flash-based SSD is generally higher than the per-bit cost of an HDD or tape drive, and flash SSDs can sustain a limited number of write cycles before they wear out.
Volatile vs. non-volatile storage devices
The key difference between volatile and non-volatile storage devices is whether or not they are able to retain data in the absence of a power supply. Volatile storage devices lose data when power is interrupted or turned off. By contrast, non-volatile devices are able to keep data regardless of the status of the power source.
Common types of volatile storage include static random access memory (SRAM) and dynamic random access memory (DRAM). Manufacturers may add battery power to a volatile memory device to enable it to persistently store data or controller code.
Enterprise and consumer computing systems often use a mix of volatile and non-volatile memory technologies, and each memory type has advantages and disadvantages. For instance, SRAM is faster than DRAM and well suited to high-speed caching. DRAM is less expensive to produce and requires less power than SRAM, and manufacturers often use it to store program code that a computer needs to operate.
By contrast, non-volatile NAND flash is slower than SRAM and DRAM, but it is cheaper to produce. Manufacturers commonly use NAND flash memory to store data persistently in business systems and consumer devices. Storage devices such as flash-based SSDs access data at a block level, whereas SRAM and DRAM support random data access at a byte level.
Like NAND, NOR flash is less expensive to produce than volatile SRAM and DRAM. NOR flash costs more than NAND flash, but it can read data faster than NAND, making it a common choice to boot consumer and embedded devices and to store controller code in SSDs, HDDs and tape drives. NOR flash is generally not used for long-term data storage due to its poor endurance.
Trends and future directions
Manufacturers are working on additional types of non-volatile storage to try to lower the per-bit cost to store data and program code, improve performance, increase endurance levels and reduce power consumption.
For instance, manufacturers developed 3D NAND flash technology in response to physical scaling limitations of two-dimensional, or planar, NAND flash. They are able to reach higher densities at a lower cost per bit by vertically stacking memory cells with 3D NAND technology than they can by using a single layer of memory cells with planar NAND.
Emerging 3D XPoint technology, co-developed by Intel Corp. and Micron Technology Inc., offers higher throughput, lower latency, greater density and improved endurance over more commonly used NAND flash technology. Intel ships 3D XPoint technology under the brand name Optane in SSDs and in persistent memory modules intended for data center use. Persistent memory modules are also known as storage class memory.
Using non-volatile memory express (NVMe) technology over a computer's PCI Express (PCIe) bus in conjunction with flash storage and newer options such as 3D XPoint can further accelerate performance, and reduce latency and power consumption. NVMe offers a more streamlined command set to process input/output (I/O) requests with PCIe-based SSDs than the Small Computer System Interface (SCSI) command set does with Serial Attached SCSI (SAS) storage drives and the analog telephone adapter (ATA) command set does with Serial ATE (SATA) drives.
Emerging non-volatile storage technologies currently in development or in limited use include ferroelectric RAM (FRAM or FeRAM), magnetoresistive RAM (MRAM), phase-change memory (PCM), resistive RAM (RRAM or ReRAM) and spin-transfer torque magnetoresistive RAM (STT-MRAM or STT-RAM).