On 6/3/2012 1:44 AM, Tom Metro wrote:
The Flash chips have commodity volume production in their favor (which can be significant). If the SSD's with high-end controllers are also using high-end Flash chips that are produced in smaller volumes, then this advantage is nullified.
There ain't no such thing as "high end" flash chips in the way that you're thinking. High end in the NAND flash arena is a matter of sustained write performance. That's it. Not quality. Not reliability. Speed. The thing is, flash read and write speeds have a technical plateau so you won't see much difference in the raw performance of different manufacturers' chips of a given generation. The difference is how different vendors optimize their controllers.
Here is an important point: SandForce doesn't make SSDs. They make SSD controllers. You can stick pretty much any manufacturer's MLC flash chip on a SandForce controller and get reasonable performance out of it.
So how do the likes of EMC and Violin go so much faster for so much longer (5-10 years vs. 6-24 months for commodity SSD)? Several things.
They use battery-backed DRAM cache on their controllers. One of the reasons why SandForce SSD controllers are relatively inexpensive is because they don't use DRAM cache. Throw a big DRAM cache in front of your flash chips and depending on I/O load you can reap a big performance boost and reduce flash write cycles.
The typical consumer-grade SSD has a single big flash chip in it. An enterprise class SSD has banks of chips arranged in something similar to a RAID 0 configuration to reap the benefits of more "spindles". Enterprise-class SSDs also layer error correction onto the write process to ensure data integrity.
Typical consumer-grade flash devices are over-provisioned by 7-8% to provide a quantity of spare cells to cover those that are bad at manufacture time and those that die during use. Enterprise-class SSDs can have 100-200% over-provisioning or more. Enterprise-class SSDs are more likely to use that spare capacity before cells fail because a failed cell is a read/write fault is a performance hit.
All of this comes at a premium. EMC will gladly sell you a 120GB flash-based SSD for 100 times the cost of the 120GB SSD you'd get at MicroCenter.
I leave the rest to the reader, but I suggest comparing STEC's literature to your favorite commodity SSD vendor's literature.
-- Rich P. _______________________________________________ Discuss mailing list [email protected] http://lists.blu.org/mailman/listinfo/discuss
