[Discuss] SSD: enterprise vs. consumer Flash

[richard.pieri at gmail.com (Richard Pieri)]

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.