Johannesburg, 17 Sep 2013
There's been some hype in the industry recently around the belief that an array designed with consumer-grade flash cannot be as resilient as an array built from enterprise-grade flash. At Pure Storage, we have actually found that the opposite is true.
In order to substantiate this statement, we need to consider the various options:
The SLC flash
The SLC flash is single-level cell where each cell is charged and can determine a single voltage. Although the SLC can only store one bit of data, its flash cells can be used much longer as only one voltage level has to be determined. The use of SLC for solid state storage array is rare now as they have become expensive, and are not necessary because of the flash optimisations vendors have made.
The MLC flash
The MLC flash is programmed to be able to determine multiple voltage levels, and is thus able to store two bits of data. MLC devices have fewer write cycles (longevity) than SLC, but as they store more data per cell, they are much cheaper.
eMLC
Then there are enterprise MLC devices or eMLC. These use exactly the same flash chips, but are being marketed as a more reliable SSD. They generally contain more sophisticated controllers, have more flash over-provisioning to manage writes, and may include exotic cache layers with power-safe mechanisms.
Most of the legacy disk arrays which have retrofitted flash choose eMLC as it has evolved to provide the longevity in un-optimised systems vendors used to choose SLC for, and it's cheaper. Examples include EMC VMax/VNX, NetAPP FAS and HP 3Par arrays. These vendors have not been able to push their designs to leverage MLC flash as the technology required to make consumer MLC work with enterprise reliability requires a storage OS written from scratch.
cMLC
Finally, there are consumer MLC or cMLC devices, which are SSDs specifically optimised for consumer devices. Of the four types, these have the highest volume, they undergo the most extensive testing, and are the least expensive.
Cost - the case for MLC and cMLC
Both MLC and cMLC are much cheaper than the other two options, but they achieve this with a reduction in write endurance. Most all-flash array devices which were built from the ground-up for flash have chosen MLC flash, such as Pure Storage, Violin Memory, SolidFire and Whiptail. But, in order for an enterprise-class array to successfully leverage MLC or cMLC, it must implement sophisticated write coalescing/avoidance technologies, array-wide garbage collection and data integrity features.
At Pure Storage, we achieve this with the FlashCare layer within the Purity Operating Environment.
Why choose consumer-grade flash?
Now, back to whether an array designed with consumer-grade flash can be as resilient as an array built from enterprise-grade flash. At Pure Storage, in our testing of all four, we have found consumer-grade MLC drives to be far more reliable than eMLC and SLC drives: we have achieved a really low SSD annual failure rate, much less than 0.1% per year.
Because more than 10 times the number of consumer-grade MLC drives are produced than either eMLC or SLC drives, they are shipped in large bulk, and are therefore thoroughly tested.
Purity operating environment
Another reason for their reliability is that our operating environment is gentle to SSDs. Inside our technology we have an I/0 virtualisation layer that works out the unique I/0 fingerprint of each SSD we use, and understands how to send data to that SSD in the most efficient manner: it maximises its reliability, performance and lifespan. We can literally take an SSD, run it past its manufacturer-designated write cycle endurance, put it in a FlashArray, and find no material difference in performance between that drive and a new one.
Future of consumer flash
The consumer flash will continue to be cheaper, denser and have less write endurance, and will always be at the cutting-edge of cost. Any product which was architected to use eMLC or SLC flash will just not be able to compete price-wise.
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