Will flash ever be cheap as disk?

The math seems simple enough: semiconductor prices keep plunging at a steady 40% annually. In the aughts disk drives exceeded that for a several years, but now are moving much slower.  Flash will catch up with disk and then goodbye, rotating rust!

But that will never happen - and yes, I know that forever is a long time. But why?

Several reasons.

Steep climb
The cheapest hard drives are now down to $0.03/GB. The cheapest flash chips now average about $0.80/GB on the spot market - or about 27x. Even if flash continues its 40% path, with disk at only 25%, it will take many years to close the gap.

Overhead
But the preceding example is an apples-to-oranges comparison. A flash chip is only equivalent to the disks and heads of a disk - not the entire disk. Much of the remaining overhead will remain the same: power; R/W chips; circuit boards; and packaging. So even if flash chips magically dropped to $0.03/GB they'd still be more expensive than disks, because of the device overhead.

Slower, worse & cheaper
Unlike other semiconductors that get faster, use less power and are more reliable as they shrink, NAND flash cells get slower and less reliable as they shrink while their power requirements remain the same. Write endurance has dropped from 10,000 to less than 1,000 in the last few years. Data retention time is also less. Three and 4 level cells will drop that even more.

Not a problem for consumer devices where the average cell sees a few dozen writes at most over its life. A big problem for SSDs, requiring all sorts of additional logic and over-provisioning at additional cost.

ReRAM to the rescue
In the next 5 years SSDs will have to move to a new technology. ReRAM (resistance RAM) is the likeliest candidate. Great stuff, but it will take years before it is as cheap as flash.

Disks aren't standing still
Will disks ever get back on the 40% declining cost curve? Not likely. With the slow collapse of PC sales, the investments needed for patterned media and advanced recording technologies are less inviting. But they'll get there.

The Storage Bits take
Despite the price differential, many consumers, like me, will choose a flash drive - or a flash-based mobile device - over a disk drive. But all those cloud-based services? Disks - lots and lots of disks - with perhaps some flash and/or DRAM based caching appliances on the front end.

Yes, flash is eating away at the disk market, but more in the form of flash-based smartphones and tablets than SSDs. PCs have been the largest single disk market for decades. As PC sales continue to decline, disk sales go with them.

The real threat to disks is not cheap flash - which will never be cheap enough to compete directly - but the next gen in solid-state storage: ReRAM. ReRAM will scale better, write faster and endure longer than current and future NAND generations. Storage people will pay extra for all those benefits.

But we'll see how that develops.

Comments welcome, as always.  Hybrid drives will boost disk performance at lower cost, making SSDs less attractive for consumers, don't you think?