Think: if NAND flash storage arrays were being developed today, what is the chance that we’d put the flash into little bricks and then plug a bunch of them into a backplane? So why do it now?

It is a truism of design that when a new technology is developed, we use it to build what we have today. It is only in later generations that we realize the new possibilities enabled by the technology. And those generations can be long, even in computers.

For all out talk about the rapid pace of computer innovation, the market for the tried-and-true is much larger than the one innovators fight over.

Why SSD-based arrays are a bad idea
To be clear, this discussion covers storage arrays built with standards-based (i.e. SATA, SAS, 2.5″ or similar) SSDs.

  • Latency. Low compared to disks, but substantial compared to flash. SAS/SATA stacks were never optimized because disk latency was the big problem.
  • SSD bandwidth. There are wider options, especially close to the CPU.
  • Reliability. SSDs replace the head/media assembly in disk drives with NAND chips. The rest of the SSD has all the tender bits of a regular disk – bits that account for about half of all disk failures. Compare DIMM and disk replacement rates.
  • Cost. SSDs cost 50%-100% more than the raw flash, even after using all the high-volume disk components. Mounting directly on PC boards, like DIMMs or PCIe cards, is much more cost effective.
  • Flexibility. The good news with SSDs is that they take advantage of the huge tech infrastructure that supports disks. But that’s the bad news too, if an optimized clean-sheet architecture is the goal.

How big an issue is cost? DRAM on a DIMM is ≈98% of the DIMM’s cost, where the flash in an SSD ≈50%-65% of the cost. And since flash costs are dropping faster than the other component costs, so will its percentage of SSD cost.

Given the high cost of flash media compared to disk, efficient media usage is a major issue. Will flash SSDs pass that test?

A less important but related metric: rackspace. SSDs are inefficient users of racks, taking perhaps 2x the space of non-SSD flash arrays per TB. Few customers will care, but the ones who do write big checks.

The StorageMojo take
The massive technological momentum behind SSD-based arrays make them a popular option for both vendors and customers. After 20 years of RAID arrays, customers get the model. There’s a large raft of hardware and software support for disk drives that SSDs can use.

That cuts time-to-market and development cost. Given the performance advantages of SSDs over disks it is an easy win for customers even if the architecture is sub-optimal.

The squeeze comes later: if non-SSD architectures have significant advantages the SSD-based arrays will lose market share and gross margin. Flash-based SSDs make sense for many applications where their cost is a small percentage of the total solution.

Building storage arrays from SSDs is opportunistic, not strategic. It isn’t the future for high-end storage, but less-demanding mid-markets may not care.

Courteous comments welcome, of course. I’m really interested in any holes in the logic of this analysis. Please weigh in.