SSDs Cheaper Than Hard Drives? Not In This Decade

I regularly hear people, including many that should know better, predicting that in just a few years we’ll evict all the hard drives from our data centers as SSDs become less expensive than spinning disks. While the decline in SSD prices has been dramatic over the last year or so, I’m betting that hard drives will still be going strong in 2020.

Today, if you shop carefully online, you can buy a general purpose enterprise SSD, such as Intel’s DC S3700 for about $2.65/GB or a read oriented drive like the Intel DC S3500 for $1.30/GB. By comparison, a 4TB nearline SATA hard disk such as Western Digital’s RE or Seagate’s Constellation cost under $400 or $0.09/GB. Interestingly, consumer/laptop SSDs are well below the magic $1/GB level with Crucial’s M500 selling for about $0.59/GB -- about what hard drives cost in 2005.

Over the past few years, flash memory prices have fallen at an average of 35% a year. Hard drive prices declined at a similar rate or even greater rate in the past, but the trend has flattened out of late. In part, this is a one-time perturbation caused by the flooding in Thailand two years ago, which caused a shortage of both drives and drive components. Prices have just recently returned to their pre-flood levels as the drive vendors have re-filled their supply and distribution channels.

The truth is the cost of a hard disk hasn’t really fallen in 10 years or more. Most SATA hard drives sell for somewhere between $70 and $200, as they have for years. Of course, the capacity of those drives has increased tenfold since 2005, when a 320GB hard disk cost $130. Today, that $130 will buy you a 3TB desktop hard disk, or a 1TB nearline drive.

Another factor affecting drive hard drive prices is the reduced competition resulting from the consolidation of the industry to essentially three vendors: Seagate, Toshiba and Western Digital. However, Western Digital runs HGST as an arms-length subsidiary, which theoretically makes HGST a fourth player.

[Read Howard Marks' analysis of Seagate's new Kinetic Open Storage Platform in "Seagate Boosts Disk Drive Intelligence."]

More than a simple competition, the cost of hard drives is driven by increases in drive capacity, which have stalled recently as drive vendors reach the limits of perpendicular recording technology’s ability to squeeze more bits per square inch of drive platter. The most recent density advancements, such as helium-filled drives from HGST, are one-time tricks that get the drive vendors a little breathing room. But once you play that card and cram seven platters into a standard size drive ,there’s no nine platter or 11 platter follow up.

Still, if we assume that SSD prices will fall at their historical 35% annual rate and hard drive prices will fall at a more conservative 15% by 2020, the enterprise SSD will cost almost 13 cents a gigabyte, more than the hard drive costs today, while the 20TB drives the hard drive vendors are promising for 2020 will cost under 3 cents a GB. The price difference will have shrunk from 30:1 to around 5:1. If drive prices fall at a closer to historical 25%, they’ll still be a tenth the cost of SSDs at the end of the decade.

Nonetheless, SSDs will continue to displace spinning disks in primary storage applications, where their low cost per IOP and low latency are more important than simple capacity costs. In fact, for greybeards like this humble reporter, SSDs -- even enterprise SSDs -- have reached the point where their prices are in line with our gut feel for what storage should cost for OLTP and other latency sensitive applications. Spinning disks will be relegated to secondary storage and applications such as media stores, where streaming throughput is more important than random IOPS.