A typical flash memory comprises a memory array, which includes a large number of memory cells. Each of the memory cells includes a floating gate field-effect transistor capable of holding a charge. The data in a cell is determined by the presence or absence of the charge in the floating gate.
The cells are usually grouped into sections called "erase blocks." Each of the cells within an erase block can be electrically programmed in a random basis by charging the floating gate. The charge can be removed from the floating gate by a block erase operation, where all floating gate memory cells in the erase block are erased in a single operation.
One term frequently used to categorize "lifetime" is P/E. P/E refers to a Program/Erase cycle, when data is written to a cell, erased, and re-written. Different types of flash have differing lifetimes, or limits to the number of P/E cycles supported before the cell fails.
P/E cycle limitations are inherent to flash, and refer to program/erase cycle maximum numbers before information is not readable or writeable. This is the primary flash limitation. As with any memory, various semiconductor characteristics, fab density, and controller determine P/E lifetime, speed, cost, and transfer rates.
MLC vs. eMLC vs. SLC vs. TLC
There are four types of NAND flash, differing in number of P/E cycles per lifetime, and defined by their construction:
- SLC -- Single Level Cell: the most expensive, longest lived (high P/E), and generally fastest. Bits are stored only as 2 voltage levels, or a "1" or "0." In SLC less data is stored per cell, so the per unit storage cost is higher.
- MLC -- Multi-Level Cell: is consumer grade and used in phones, cameras, and USB sticks. The stored charge in MLC may be interpreted as a variety of values, 0 to 3, or 4 possible states, and may store 2 bits. With shorter lifetimes, usually 10x less than SLC, the advantage of this memory is that the cost is 2- 4x less than SLC, but with lower write speeds. MLC typically uses some form of error correction code per block.
- eMLC -- Enterprise (grade) Multi-Level Cell: is MLC with longer life, usually because of an advanced controller operating the cell and error recovery techniques, construction density, or some combination of the two. Violin has an interesting explanation of the practical differences between NAND types here.
- TLC -- Triple Level Cell: championed by Samsung, TLC has higher power and error correction requirements, and higher wear levels. TLC is targeted at environments with predominant read uses, and has not been commonly used.
NOR vs. NAND Architectures
The memory cells of both an EEPROM (electrically erasable programmable read-only memory) memory array and a flash memory array are typically arranged into either a "NOR" or "NAND" architecture:
- NOR architecture -- each cell directly coupled to a bit line, allows true random access.
- NAND architecture -- cells coupled into "strings" of cells, such that each cell is coupled indirectly to a bit line and requires activating the other cells of the string for access.
Flash memory is a type of EEPROM, but the term EEPROM usually refers to non-flash EEPROM, where data can be erased in small units, usually bytes. Most solid-state drives, USB flash, and memory cards use NAND flash. Erasing, usually a slow process, is much faster in flash than non-flash EEPROM, because of the large block sizes used in flash.
For a perspective of storage speeds, the hierarchy from fastest to slowest storage memory types is:
DRAM > NAND flash > Spinning Disk > Tape
NAND is about 1000x faster than spinning disks, but DRAM is 1000x faster than NAND. NAND is about 10x less expensive than DRAM, but more expensive than spinning disks.
Anyone who's tried one will attest, solid state drives (SSDs) sure are great! With faster load times for your favorite apps, higher overall efficiency, and added durability compared to traditional spinning hard drives, it's no wonder they're popping up in every new device that hits market.
But what makes certain SSDs more expensive than others? Like race cars, it's all about what's under the hood.
The Anatomy of an SSD
MyDigitalSSD BP4e mSATA SSD with two enclosed NAND flash memory chips installed. The controller chip is designed by PHISON
- A. NAND Flash: The part where your data is stored, in blocks of non-volatile (does not require power to maintain data) memory.
- B. DDR Memory: Small amount of volatile memory (requires power to maintain data) used to cache information for future access. Not available on all SSDs.
- C. Controller: Acts as the main connector between the NAND flash and your computer. The controller also contains the firmware that helps manage your SSD.
What is NAND Flash?
NAND flash memory is built up of many cells that holds bits, and those bits are either turned on or off through an electric charge. How those on/off cells are organized represents the data stored on the SSD. The number of bits in those cells also determine the naming of the flash, for example Single Level Cell (SLC) flash contains a single bit in each cell.
The reason behind SLC only being available at lower capacities is down to the physical real estate the NAND flash occupies on the Printed Circuit Board (PCB). Don't forget that the circuit board has to have the controller, DDR memory, and flash built to standard dimensions to fit inside your computer. MLC doubles the amount of bits per cell, whereas TLC triples, and this opens up for higher capacity SSDs.
There are particular reasons why manufactures build flash memory with a single bit per cell like SLC. SLC has the advantage of being the fastest, most durable but has the cons of being more expensive, and is not available in higher gigabyte storage capacity. That is why SLC is preferred for heavy enterprise usage.
MLC and TLC flash in comparison to SLC, is cheaper to produce, available in higher storage capacities, but at the tradeoff of relatively shorter life spans and slower read/write speeds. MLC and TLC are preferred for everyday consumer computer usage.
Understanding your own needs for computing and NAND flash basics will not only help you pick the right SSD, but will also help you figure out factors such as the price behind the product.
SLC (Single Level Cell)
The Single Level Cell flash is so called for it's single bit that can either be on or off when charged. This type of flash has the advantage of being the most accurate when reading and writing data, and also has the benefit of lasting the longest data read and write cycles. Program read/write life cycle is expected to be between 90,000 and 100,000. This type of flash has done exceptionally well in the enterprise market because of it's life span, accuracy and overall performance. You won't see too many home computers with this type of NAND due to its high cost and low storage capacities.
- Has the longest lifespan and charge cycles over any other type of flash.
- More reliable smaller room for read/write error.
- Can operate in a broader temperature range.
- The most expensive type of NAND flash on the market.
- Often only available in smaller capacities.
- Industrial use and workloads that require heavy read/write cycles such as servers.
eMLC (Enterprise Multi Level Cell)
eMLC is MLC flash, but optimized for the enterprise sector and has better performance and lastability. Read/write data life cycles are expected between 20,000 and 30,000. eMLC provides a lower cost alternative to SLC, yet maintains some of the pros of SLC.
- Cheaper alternative than SLC for an enterprise SSD.
- Has better performance and endurance over standard MLC.
- Does not match SLC NAND flash SSDs in performance.
- Industrial use and workloads that require heavy read/write cycles such as servers.
MLC (Multi Level Cell)
MLC flash as it's name suggests stores multi bits of data on one cell. The big advantage of this is the lower cost of manufacturing versus manufacturing SLC flash. The lower cost in flash production is generally passed onto you as the consumer, and for that reason is very popular among many brands. MLC flash is preferred for consumer SSDs for it's lower costs but the data read/write life is less in comparison to SLC at around 10,000 per cell.
- Lower production costs are passed onto you the consumer.
- Is more reliable than TLC flash.
- Not as durable and reliable as SLC or enterprise SSDs.
- Everyday consumer use, gamers, and enthusiasts.
TLC (Triple Level Cell)
Storing 3 bits of data per cell, TLC flash is the cheapest form of flash to manufacture. The biggest disadvantage to this type of flash is that it is only suitable for consumer usage, and would not be able to meet the standards for industrial use. Read/write life cycles are considerably shorter at 3,000 to 5,000 cycles per cell.
- Cheapest to manufacture which in turn leads to cheaper to market SSDs.
- Cells will survive considerably less read/write cycles compared to MLC NAND. This means that TLC flash is good for consumer use only.
- Everyday consumer use, web/email machines, netbooks, and tablets.
The SSD Life Cycle
Like all good things, an SSD does not last forever. As noted above, a solid state drive's life cycle can be directly attributed to the NAND flash it comes with. SLC flash, for example, will last longer than MLC or TLC flash but that comes at a hefty price tag.
With MLC and TLC flash commonly used/found in consumer SSDs, the real question is how long will they last?
TechReport.com has tested several available consumer-grade SSDs, most of which were MLC NAND with one being TLC NAND, and the results are promising. All of the devices tested lasted at least 700 terabytes (TB) of writes before failing, and a couple even pushed passed a petabyte (PB).
This is a lot of data, but let's put that into perspective in writing 1 PB to an SSD.
1 petabyte (PB) = 1,000 terabytes (TB) / 1,000,000 gigabytes (GB) / 1,000,000,000 (MB)
That 1 PB could net you:
- 222,222 movie DVDs at 4.5GB a DVD
- 333,333,333 mp3 songs at 3MB a song
- 500,000,000 jpg photos at 2MB an image
- 15,384 installs of the game Grand Theft Auto V at 65GB an install
Looking at those numbers should really put to rest any doubts about your SSD failing in any short amount of time.
If you are considering an MLC or TLC SSD for everyday consumer use like; storing music, photos, software, personal documents or play games then you should feel assured that your SSD should last several years. This kind of usage is considered light compared to the ongoing heavy read/write usage of enterprise servers and computers as outlined in the next section below.
Note: For anyone worried about the lifespan of their SSD, features such as Self-Monitoring Analysis and Reporting Technology, or S.M.A.R.T. for short, can help you better keep track of your SSD's longevity.
Enterprise vs. Consumer SSDs
The difference and demands expected of enterprise SSDs set them a world a part from consumer SSDs. Enterprise SSDs are designed to meet a higher standard, and consistently perform in high-tech services, military, science and any area that would require a large amount of reading and writing data.
Database servers are an example of where you might see an enterprise SSDs, these servers are on 24/7 and that includes: longer read/write life cycle, faster read/write speeds, increased reliability and durability in harsh environments.
Consumer SSDs are less expensive, and are stripped down versions of enterprise SSDs. This may sound like you are missing out on certain features, but the benefits of a cheaper product with larger storage capacity are worth it. Besides manufactures are always increasing the performance of SSDs while bringing down the price.
At this point, you probably have a good idea on the difference between SLC, MLC, and TLC NAND flash. The basics we discussed here, with insight into why some cost more than others, should clear up any confusion as to what type of flash best fits your needs.
Single Level Cell
|Bit Per Cell||1||2||2||3|
The important thing to take away from this guide is that modern SSDs are built to last a considerable amount of time. While their life-cycle should be taken into account, it should by no means prevent you from buying faster and more efficient storage.