eMLC stands for (Enterprise Multi-Level Cell). Just like the standard MLC NAND flash for consumer drives, the eMLC stores two bits of data per cell. However, the eMLC is designed specifically for commercial environments with higher levels of endurance, reliability, and performance. This is done with advanced error correction code, higher quality material, weal-leveling mechanisms, and many other things.
Normally, the SLC NAND flash is considered best for enterprise drives. However, eMLC was introduced as a middle ground between the SLC and consumer-grade MLC. The main benefits of eMLC over the SLC are lower cost and increased storage density. While SLC offers superior performance and endurance, it is expensive. eMLC was introduced to offer a balance, offering sufficient endurance and performance at a more affordable cost.
Data storage in eMLC NAND Flash
Just like the normal MLC NAND flash, eMLC also uses a single cell to store 2 bits of information. Inside a single floating gate transistor, there could be four different voltage levels representing a combination of two bits i.e. 00, 01, 10, 11. Compared to the SLC, where each cell is used to hold one bit of data, the eMLC increases the wear in the cell for the same amount of data. However, eMLC offers twice the storage space with the same number of NAND flash cells. This increases the storage density and reduces the price.
In the illustration, there are four possible bit values in the cells. Generally, a nearly 25% charged cell represents 11 while a nearly 100% charged cell represents the value of 10.
How is eMLC NAND Flash optimized for enterprise applications?
There are many ways manufacturers optimize the eMLC NAND flash to make it suitable for enterprise applications.
1. Quality of Material is Better
NAND Flash chips, controllers, DRAM chips, and many other components on the SSDs are made out of silicon. eMLC uses high-grade silicon material with little to no defects thus increasing reliability. Also, the manufacturing processes are under tight control which results in robust and more uniform memory cells. Even the PCB and the outer casing of these SSDs are way better compared to the consumer drives.
2. Advanced Error Correction Code
Because the eMLC NAND Flash stores 2 bits of data in each cell, it becomes more susceptible to errors because these varying voltage levels are much closer to each other compared to the SLC. A small disturbance in this charge can easily result in data errors. Also, as an eMLC ages, the chance for errors increases more. To overcome this issue, the eMLC SSDs use advanced error correction algorithms as compared to the normal MLC.
The most common ECC are BCH (Bose-Chaudhuri-Hocquenghem) Code and LDPC (Low-Density Parity-Check). In BCH code, multiple random bit errors can be corrected inside the block of NAND flash. The LDPC is much more powerful and can handle much more error, majorly related to the aging of flash memory. LDPC is very effective in keeping the NAND flash cells usable even when they are degrading.
3. Controlled Write Algorithms
These algorithms control how the data is written to the memory cells to lower the wear, increase lifespan, and enhance performance. Because of the increased data capacity in eMLC, the chance of errors and level of wear increases. To overcome these problems, the eMLC drives come with special write algorithms.
With these algorithms, multiple write steps are included like ISPP (Incremental Step Pulse Programming). In this programming, instead of programming a large pulse to store the charge in the floating gate, multiple small voltages are applied. This process makes sure the accurate level of voltage is pushed in the cell and there are no chances of errors. After the programming, there are multiple verification cycles are run to check for the desired voltage. If the voltage isn’t precisely at the point, more voltage is pushed to the cells.
4. Advanced Wear-Leveling and high over-provisioning
Wear leveling is the process of distributing the cell damage evenly across the NAND flash memory cells. This is the process of utilizing all the cells for data storage to ensure no specific cells are being overused and wearing faster than others. Wear leveling is an algorithm controlled by the controller that assigns the data locations for the incoming cells. In dynamic wear leveling, the data is constantly shuffled around cells to ensure the cells are evenly used. In static wear leveling, the infrequently used data is moved to ensure that even cells holding long-term data contribute to the wear distribution.
In over-provisioning, an extra space is kept aside for SSDs’ own internal tasks. It can also be used to replace the worn-out cells. The eMLC SSDs generally have higher space for over-provisioning.
Along with this, the eMLC SSDs go through rigorous testing and validation to ensure endurance and performance in enterprise environments
Higher P/E cycles in eMLC
The normal MLC NAND flash cells come with program/erase or P/E cycles of around 3,000 to 10,000 cycles. However, the eMLC NAND flash can have P/E cycles ranging from 10,000 to 30,000.
These comparatively higher P/E cycles are achieved mainly with the help of higher quality materials, optimized algorithms, write algorithms, advanced ECC, wear-leveling, increased over-provisioning, and optimized data placement.
Difference between MLC and eMLC NAND Flash
Although the structure and working mechanisms of MLC and eMLC are the same, eMLC is way better in terms of endurance and performance. This is because of the things we discussed above.
These SSDs are different because their targeted markets are different. Consumer MLC SSDs can’t be used 24/7 in a data center. It is meant for home and office use. The eMLC, on the other hand, is made to handle heavy workloads. However, here is a detailed table comparing the both.
| Feature | MLC SSDs | eMLC SSDs |
|---|---|---|
| Target Market | Consumer SSD Market (Laptops, Desktops, etc) | Enterprise-grade applications |
| Endurance (P/E Cycles) | 3,000 to 10,000 cycles | 10,000 to 30,000 cycles |
| Data Retention Capabilities | 1 year at 25°C after reaching max P/E cycles | 3 to 5 years at 25°C after reaching max P/E cycles |
| Error Correction Code (ECC) | Corrects up to 1-3 bit errors per 512 bytes | Corrects up to 5-20 bit errors per 512 bytes |
| Programming Algorithms | Basic ISPP | Optimized ISPP with precise voltage control |
| Write Amplification | Typically 2-3x | Typically 1.1-2x |
| Wear Leveling | Basic dynamic wear leveling | Advanced dynamic and static wear leveling |
| Over-Provisioning | 5-10% | 20-28% |
| Cost | $0.10 to $0.30 per GB | $0.50 to $1.00 per GB |
| Raw Performance | Sequential read: 500-550 MB/s | Sequential read: 500-550 MB/s |
| Sequential write: 300-500 MB/s | Sequential write: 400-550 MB/s | |
| Latency | 40-100 microseconds (µs) | 20-40 microseconds (µs) |
| Temperature Range | Operating: 0°C to 70°C (some up to 80°C) | Operating: 0°C to 85°C (some up to 90°C) |
| Reliability (MTBF) | 1.5 to 2 million hours | 2 to 3 million hours |
| Power Loss Protection | Basic or not included | Advanced power loss protection with capacitors |
| Lifespan | 3-8 years under typical use | 5-10 years under heavy enterprise workloads |
| Write Endurance (Total Bytes Written, TBW) | 100-300 TB for a 1TB drive | 3,000-10,000 TB for a 1TB drive |
| Usage Scenario | Home, office, and other general purpose usage | Home, office, and other general-purpose usage |
eMLC vs SLC NAND Flash in enterprise environments
SLC SSDs are the best when it comes to the highest levels of performance and endurance. They are used in mission-critical applications or heavy-workload environments. The P/E cycles of SLC NAND flash can reach 1,000,000 cycles which is very high compared to the eMLC.
The best thing about the SLC SSDs is there is no need for advanced algorithms for error correction and programming. Because of the simple structure of the memory, the chances of errors are close to none. Also, there is no need to optimize the write operations because there are just two voltage levels high or low.
The write amplification in SLC NAND flash is close to 1x. Wear leveling is required but not to a very high extent. The wear just has to be spread across cells and nothing else. The raw read/write performance is almost similar in eMLC and SLC NAND flash but the SLC has lower latency compared to the eMLC.
The common MTBF numbers for eMLC are 2 to 3 million hours while 3 to 5 million hours for SLC.
eMLC is more affordable, typically ranging from 0.50$ to $1.00 per GB. The SLC, on the other hand, can cost easily from 2 to 5$ per GB. So, this can make a major impact in a budget-tight environment.
Top Enterprise SSDs with eMLC NAND Flash
These are some very popular enterprise SSDs offering great services in the market. Some of the common names are Intel SSD DC S3700 Series, Samsung PM863a Series, Seagate Nytro 1200, and Micron 5100 Eco Series. These SSDs offer a good balance between performance and price as compared to the SLC which are just focused on performance. But, for demanding tasks and high-stakes environments, SLC is the best option.
Usage areas of SLC and eMLC SSDs
eMLC SSDs are generally used in database and virtualization enterprise storage systems. Public and private cloud systems, data warehousing, enterprise backups, and web servers also use the eMLC SSDs. Generally, eMLC SSDs are employed along with high redundancy to mitigate the chances of data loss due to drive failures. Also, we can say that eMLC SSDs are used in less critical commercial environments.
The SLC SSDs are used in mission-critical environments and high-end data center environments. SLC SSDs are also used in industrial automation, aerospace and defense, telecommunication, and many other places where failure is not an option.
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