Why will a 3D version of triple level cell flash, notoriously the poor man's flash, replace most higher-end multi-level cell flash?
The recording and sharing of data is fundamental to the advancement of society and a phenomenon that consumers seem to have embraced with insatiable desire. There is no question that the invention of writing, paper, newsprint, computers, Internet, and now the mobile Internet has resulted in huge leaps in technology. With more connections and users, ultra-fast speeds, and video everywhere, it is forecast that by 2018, 190 exabytes of data will be generated in a single year.1 To give some context—Eric Schmidt, when he was CEO of Google, stated that “5 exabytes is the total amount of data created between the dawn of civilization and 2003.”
In more recent times, the dawn of social media has triggered a mind-boggling increase in demand to store and share even more information than was anticipated just a few years ago. Popular sites such as YouTube have essentially changed the general population from content consumers to content generators. In fact, more video is uploaded to YouTube in a single month than what the 3 major networks created in 60 years.2 Correspondingly, the need for IT storage is growing exponentially—forecasted to grow over 20x in the 10 years from 2011 through 2020.
With the explosion in information and the relentless growth of information demand, identifying an efficient means of storage has become ever more important. Rotating magnetic media has served as the storage medium for the computing industry for decades, but has become a performance bottleneck as the rest of the computing system has gotten faster. There are three main components that dictate the speed of a computing system: the CPU, memory, and storage. While the CPU and memory are solid-state components that have increased in performance at an exponential rate, digital storage has been hampered by the physical speed of rotating disks. By going to solid-state storage, we are finally closing the gap with the CPU and memory.
The last 10 years has seen a spectacular rise in the use of flash-based (NAND) storage, which is now on the threshold of going mainstream in both client-computing and data center applications. This rise in deployment is due to improvements in technology as well as greater cost efficiency. In many cases, the most significant boost you can now give your computer is to upgrade your hard disk to a solid state drive (SSD). PC OEMs have taken notice and are now providing SSD options in many of their notebook offerings. In the data center, flash is increasing its footprint in servers as well as storage arrays. Many companies have been founded with the purpose of offering all-flash arrays.
So what are the characteristics of solid state drives that give them such an advantage over spinning, magnetic media? The main reason is the lack of moving parts. Magnetic media, whether in the form of removable discs or fixed platters in a hard disk drive, needs rotation to move the area of storage over a read/write head. There is a limitation in how fast you can physically move media, which are orders of magnitude slower than accessing information using electrical signals in solid state devices. Eliminating the rotating disk has a number of advantages, including:
- Faster, sustained, and more even performance because the file system does not fragment over time
- Improved reliability—mechanical items have an ‘order of magnitude’ higher failure rate compared to solid state devices
- Improved durability—shock rating 4x that of a mechanical system
- Reduced power consumption—mechanical parts in HDDs require much higher energy to operate
- Lower heat, hence elimination of cooling fans
- Smaller physical footprint
So why has solid state storage not completely eliminated magnetic storage? The main barrier is cost. As you can see in Figure 1.2 below, the cost of flash storage has dropped dramatically over the last several years. We are fast approaching a cross-over where SSDs are shown to be more economical than HDDs from a TCO (Total Cost of Ownership) perspective. Figure 1.3, reproduced from Wikibon, indicates that when you factor in considerations such as power, maintenance and space, the 4-year cost of SSDs will be more economical than HDDs in data centers—starting in 2016.
SSD challenges As an industry, flash-based memory has come down in cost due to increasing bit density. The more bits you can fit onto a silicon wafer, the lower the cost per bit. The process geometry has now come down to the mid-10nm-class range. To provide some context, 10nm-class is about 1/1000th the thickness of human hair. However, the closer you squeeze memory cells together, the more cell-to-cell interference occurs. This leads us to take a close look at the main challenge associated with flash-based memory: endurance.
Endurance is a measure of the total number of times a solid state drive can be overwritten. Each time that electrical currents are used to program a memory cell, the structure degrades slightly. The following factors primarily determine endurance:
- Process geometry—the smaller the process geometry, the less electrons available to hold the charge to determine the data held (as well as more inter-cell interference that can lead to data corruption)
- Number of bits per memory cell—holding more bits per cell results in greater memory densities, but involves the need to distinguish between finer voltage levels to determine actual data held
- Flash management firmware
- Wear-leveling—effectively spreads program/erase cycles evenly over available NAND in order to extend the life of the drive
- Bad block management and Error Correcting Code (ECC)—use of parity bits to compensate for failed bits
As discussed, the industry is at mid teen-nanometer process geometries for NAND memory. We are approaching a physical limit based on inter-cell interference as well as the electrons available to hold a charge at these small geometries. However, every challenge presents opportunities for those that can innovate. In the second segment of this product-to-market storage analysis, we will discuss recent leading-edge approaches to overcoming the challenges facing this increasingly important form of storage.
Part 1 of a 2-part series (Part 2 goes live on 10/14/15)
—Tien Shiah is a Product Marketing Manager for SSDs at Samsung Semiconductor. He serves as a product consultant and market expert, focused on the accelerating migration to SSDs in the client and enterprise marketplaces. He has over 15 years of product marketing experience in the semiconductor and storage industries. He has an MBA from McGill University (Montreal, Canada) and a BSEE from the University of British Columbia.
 Cisco VNI: Global Mobile Data Traffic Forecast Update, Feb 2015
 YouTube, Mar 2012