What Is Flash Storage?
Flash storage is a type of non-volatile memory, which can be electrically erased and programmed. What was the event that precipitated the introduction of this new storage medium? Well, it started in the mid-1980s, when Toshiba was working on a project to create a replacement for the EEPROM, a low-cost type of non-volatile memory, which could be erased and reprogrammed. The problem with the EEPROM was its cumbersome erasure process; it needed to be exposed to an ultraviolet light source to perform a complete erasure. To overcome this challenge, the E2PROM was created. The E2PROM type of memory cell was block erasable, but it was eight times the cost of the EEPROM. The high cost of the E2PROM led to rejection from consumers who wanted the low cost of EEPROM coupled with the block erasable qualities of the E2PROM.
This market desire led to the creation of what became known as the NOR architecture at Toshiba. While Toshiba was the first to create a NOR-based flash memory cell, the first commercially successfully design did not arrive until the late 1980s and was known as the ETOX Cell. The ETOX cell was slow to write and slow to erase, but was quick to read. This low-capacity NOR architecture became the industry standard replacement for read-only memory.
From there came new advances in storage technology, which has had a radical impact on the storage market as well as on data center economics. This paper discusses both the technical aspects of flash storage as well as the economic impact it has had.
In the late 1980s, Toshiba introduced a new kind of architecture — the NAND architecture — which had a significantly lower cost-per-bit, a much larger capacity, and performance improvements in every area. The larger capacity made it suitable for storing data. Today’s flash storage systems are direct descendants of this architecture. In the following sections, discover the technology underpinnings that make these speedy systems work in the modern data center.
Flash Storage Cells
The key component of a flash storage device is the cell. Information is stored in groups of memory cells comprised of floating-gate transistors (Figure 1). Information is comprised of 1s and 0s, which are stored as electrons inside the floating-gate transistor. Think of this transistor as a light switch that can be either on or off. The light will stay on until turned off just as data will reside in flash media until it is erased. This statefulness (the ability to retain information) of the data is made possible by the floating gate.
Figure 1 — Floating-gate transistor
The floating-gate is isolated, insulated all around by an oxide layer, with no electrical contacts. This means that any electrons placed on the floating-gate are literally trapped until removed. This persistence is the foundation for using flash memory as storage.
For electrons to be placed on the floating gate, they must penetrate the insulating material used for isolation. In order to penetrate the insulation, the electrons must be exposed to a high voltage in a process called tunneling. Once the charged electron passes through the insulation, it lands on the floating-gate where it is then stored. The process of tunneling is the Achilles’ heel of a flash storage cell because it causes physical degradation to the insulation material. Each time a data store on the floating-gate needs to be reused, it has to be completely erased and then rewritten. The more this cycle of programming and erasing (called a P/E Cycle) is invoked, the more the material degrades. Thus, each flash cell has a limited write endurance before the cell is completely degraded. This phenomenon is the reason that flash storage has a finite life span.
Modern storage companies are taking steps to further overcome such challenges. Between these advances, it’s clear that flash media use will continue to grow in the data center.
Learn how to:
- Differentiate between single level (SLC), multi-level (MLC), and triple-level cell (TLC) flash media
- Understand write amplification, wear leveling, and garbage collection
- Select the right type of flash storage for your needs
- How data reduction technologies can lower the cost of flash while improving performance