Today’s article is by X-IO Technologies‘ 13-year veteran Storage Systems Engineer Jeff Boling. Jeff frequently serves as a storage subject matter expert for Citrix, SNIA, Microsoft, Cisco and VMware audiences, as well as being a SQL DBA, VCP5-DCV EMC SPEED Certified, MCT, CNI, CNNP, MCSE and SNIA Certified Storage Architect. You can keep up with Jeff on Twitter.
If you’ve been in IT for some time, you’ve heard it before: “Cheap, fast, or reliable. You can only pick two.” When it comes to flash technology, it’s no different. However, there are some winning flash array strategies that allow you to have it all when you aggregate many SSD drives together in the right architecture.
For flash drives in particular, there are essentially three current grades of flash: single-level cell (SLC) based flash, enterprise multi-level cell (MLC) based flash, (which is targeted for enterprise grade) and consumer-grade MLC. SLC is the most expensive, followed by eMLC, then MLC.
Comparing Flash Grades: MLC vs. SLC vs. eMLC
MLC is known as the “dirt cheap” consumer chip, which is all about high capacity and low endurance. Conversely, a pricier SLC has a much smaller capacity, higher performance, and longer life. Consumer-grade MLC has much larger capacities and longer latencies, but a shorter usable lifetime under normal operating conditions when used as an individual drive. eMLC is positioned to provide the best of both worlds by offering a larger capacity.
Additionally, eMLC has a longer usable lifespan by reserving more internal space for reallocating blocks of data and writing voltage states more carefully. eMLC is also touted to consist of cherry-picked wafers, which can maximize the number of good cells over the consumer-grade MLC wafers. Having more reserved internal space also improves performance by reducing or eliminating the need to periodically perform garbage collection to put reclaimed space back into production. Simply put, MLC wears out faster, eMLC mitigates this degradation by reserving more space, and SLC is off-the-chart expensive with low capacities.
The major difference between these grades at the bit level is the number of bits in each type of flash. SLC flash is able to store one bit per cell, while eMLC and MLC store two bits per cell. There is also new type of flash called TLC, which stores three and even four bits per cell; however, the drawbacks to using TLC are a loss in performance capability, reliability and longevity. The MLC and TLC variants are denser and more cost effective, but have significant drawbacks in enterprise computing as an SSD tier.
But what are the trade-offs among SLC, eMLC and MLC? How do they get worse as they get cheaper? Find out in our technical guide!