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On top of all of the above advancements, the modern trend towards increasing storage densities has a convenient side
benefit – with higher capacities also comes higher performance, because increasing the number of NAND chips in an SSD
allows for more parallelization, helping to overcome the inherently slow program times that MLC NAND suffers compared
to its SLC predecessor (this same phenomenon also explains why the same or similar NAND flash can deliver very different
performance, lifespan, and reliability among various NAND-based devices). The additional free space has the added benefit
of providing the controller with unused capacity to use as a kind of non-official over-provisioning space.
Together, all of the progress SSD vendors have made in NAND technology has led to lower cost drives with performance,
endurance, and reliability that can satisfy the demands of an ever-increasing audience.
Industry Trends
History is set to repeat itself thanks to the above and other continuous
advances in NAND technology. The entire Information Technology
and Consumer Electronics industries, which voraciously consume
NAND chips for devices like MP3 players, smart phones, memory
cards, USB flash drives, car navigation systems, and the other devices
that make up this digital age, have benefited greatly from increased
capacity at more favorable pricing. It is now time for PC storage,
which is rapidly moving away from traditional Hard Disk Drive (HDD)
technology in favor of SSDs, to enjoy similar benefits. The “bit growth”
phenomenon discussed above is pushing the industry towards wider
3-bit MLC NAND adoption. In fact, we may soon see 3-bit MLC NAND-based products begin to dominate the SSD industry.
In reality, 2-bit MLC NAND’s performance and lifetime characteristics are such that they far exceed the requirements of most
computing tasks. There are even fewer applications that require SLC-level endurance and reliability these days. Thus, the
real-world benefits are well worth any tradeoffs in performance and endurance we must make as we move towards denser
memory technology. Today’s NAND-based storage will still far outlive the useful lifespan of the devices it powers. Choosing to
sacrifice a bit of excess lifetime and performance in favor of dramatic cost benefits will allow manufacturers to deliver NAND
technology and its many advantages over traditional storage technology to a far wider audience.
Samsung has taken the first step by introducing 3-bit MLC NAND to the SSD market with its 840 Series SSD, made possible by
its fully integrated design approach, proprietary controller architecture and firmware algorithms, and superior NAND quality. As
mentioned previously, increased SSD capacity leads to increased performance and endurance, so SSDs using this technology
will only improve as densities grow. Thus, 3-bit MLC may represent the beginning of the next personal storage revolution.
Asynchronous vs. Synchronous NAND
At the heart of all electronic devices is the concept of a regulatory signal, which coordinates the rate at which instructions
are executed and acts as a kind of conductor to keep everything in sync. Historically, NAND devices used two regulatory
signals: the “RE signal” (for Read Events) and the “WE signal” (for Write Events). As NAND evolved and increased in
speed, however, it became necessary to introduce a new signal, called a “strobe.” Present in all modern DDR NAND
implementations, the strobe helps the controller to handle read and write data at high speed. It starts after receiving
a signal from the host indicating a read or write event – think of a stopwatch, which can start and stop on demand.
Depending on its current task, the strobe plays slightly different roles. During a write event, it is directly managed by the
controller, whereas it plays a more supporting role during a read event.
With ONFI 2.0, synchronous logic was introduced to the ONFI NAND specification. Synchronous devices operate a
“free-running” clock, which means that, as long they are receiving power, the clock will continuously run – much like a
standard wall clock. This clock is used as a reference for the strobe. Modern ONFI NAND implementations eschew the use
of a clock once again in favor of using only the strobe. Every generation of Toggle NAND was asynchronous.
NAND Bit Portion by Tech
Asynchronous Synchronous