With the addition of Solid State Drives (SSD) to a growing list of data storage media options, it begs the question: “Which one is right for my application?”
For many years, industry favored flash memory due to its lack of rotating parts and resulting robustness against vibrations and other factors. In response to the frequent questions regarding data storage media, this blog will provide a summary of the most important issues. I will outline four main types of data storage media, and some differences between them. First, since all the options outlined are based on flash technology, I wanted to give some insight into the difference between two ways to store flash memory, Single Level Cell (SLC) and Multiple Level Cell (MLC). Then, I will start with the grandfather first, CompactFlash. This card, known as CF for short, gave birth to the CompactFast card (CFast), in both an SLC and MLC format. Finally, with the price of SSDs coming down, this has become a viable option.
SLC vs. MLC
For a long time, MLC-based flash drives were not suitable for industrial use because they allow a significantly lower number of write/erase cycles. This restriction was overcome by the development of intelligent wear leveling, which distributes write operations evenly across all available flash cells. SLC flash drives have a longer lifespan than MLC drives, but are also considerably more expensive.
The main difference between SLC (Single-Level Cell) and MLC (Multi-Level Cell) technology is in the way that data is stored. SLC flash drives store one bit per cell, while MLC drives are able to store two bits per cell.
MLC products have a clear price advantage, yet score lower when it comes to endurance (i.e. long-term life expectancy) compared to SLC products. MLC memory generally displays a loss of data retention (data storage when powered off) over time – from an initial 10 years to 1 year after the maximum number of write/erase cycles – which is not likely to be a problem for most end customers.
The biggest drawback compared to SLC memory is the significantly lower number of specified write/erase cycles per flash block. This factor has a direct impact on the specified TBW (terabytes written), so MLC products are not ideal for writing-intensive applications. If the specified TBW is not sufficient, it is possible to select a larger storage medium. This option is possible because the firmware used ensures that wear is distributed evenly across the blocks, as described above. Additionally, SMART functions can read out the average erase count per cell. This allows customers to monitor their application over a given interval and then estimate the predicted lifespan. Access to the SMART values is provided by freeware tools such as CrystalDiskInfo (http://crystalmark.info/?lang=en).
All flash memory suffers from wear, which occurs because erasing or programming a cell subjects it to wear due to the voltage applied. Each time this happens, a charge is trapped in the transistor’s gate dielectric and causes a permanent shift in the cell’s characteristics, which, after a number of cycles, manifests as a failed cell. Again, this has been lessened by the development of intelligent wear leveling.
CompactFlash, CFast, and SSD drives can have either of these formats to store data.
CompactFlash cards are based on the Parallel ATA (PATA) bus. It was originally built around Intel’s NOR-based flash memory, but has switched to NAND technology. CF is among the oldest and most successful formats. It has benefited from both a better cost to memory-size ratio and, for much of this format’s life, generally greater available capacity than other formats. We offer the SLC-based cards from 512 MB to 32 GB as a storage solution for industrial applications.
The CFast cards – based on the newer serial ATA (SATA) interface – are the successor to the CompactFlash cards. This supports a much faster maximum transfer rate. SATA supports transfer rates up to 600 MByte/s while PATA is limited to 167 MByte/s. We offer SLC-based CFast cards from 2 GB to 32 GB. In 2014, we added MLC-based variants in sizes 32, 64, and 128 GB.
Solid State Drives
The advantages of SSDs have made them increasingly popular in consumer devices. They are robust and provide high-speed data access. Most SSDs that are available are now based on MLC technology. Our products feature exclusively MLC-based SSDs. SSDs are currently available in the following sizes: 60 GB, 128 GB and 256 GB, with a 512GB drive coming soon.
With all the options available, it really comes down to what you want it to do and at what price you deem worth it. Below is a general overview of when SLC and MLC are more logical options:
SLC is preferred:
- Many writes to the flash (e.g. data logging)
- High ambient temperatures
- Special endurance requirements after many operating years
MLC is preferred:
- Standard applications without data logging
- Moderate ambient temperatures
If you compare the advantages and disadvantages of the various types of flash storage media, MLC technology is the clear winner in terms of price. However, this price advantage comes with some technical limitations. SLC technology is therefore still justified in certain applications, such as those with frequent write operations. If you would like assistance in how to choose the right data storage technology to ensure the integrity of your data, please reach out to us to have an IFP Application Engineer contact you: