The Server Room, Ars Technica's new community for IT professionals, is sponsored by Dell's Future of Storage. This article on magnetic storage is part of our ongoing series of topics and discussions related to IT and storage technology.

My first computer was a 1990 i386 with 2MB of memory and an 80MB hard drive, scrounged from the offices of a local shipping company. Complete with serial mouse, IBM Model M keyboard, and 15" color VGA monitor, it was my parents' hope for making me into a competent writer, but it better succeeded in making me a PC gamer. This ancient machine, 17 years old, is incredibly outdated in the physical basis of every technological detail, except one: its hard disk.

What's wrong (and right) with the hard disk

The hard disk is still fundamentally the same device: magnetic reader heads fly over rotating platters. The disadvantages of this system, with its moving parts and associated rotational latency, are making themselves felt, and a number of considerations make it likely that solid state disks (SSDs) will rise to increasing prominence in the enterprise storage market as well as the laptop market. Some of those trends are explored below.



These two disks, manufactured 15 years apart, are much the same.

The leftmost still has Windows 3.1 on it.

Samsung's recent announcement of a 1TB disk at the $200 price point provides a dramatic illustration of just how rapidly the cost per unit of storage continues to fall. When 1TB hard disks were first introduced, in April of 2007, they launched at a cost of $400 at retail, and have fallen steadily in price since then, reaching $200 this month, halving in price in only a year to $.20 per gigabyte. This rapid decrease in price is part of a trend which spans the whole of the technology's history; the price of disks has fallen by a factor of 35,000 in the last 17 years.

At the launch of the 1TB hard disk, and through the tier's life so far, other disks have been available with lower costs per gigabyte and per transfer speed. In April 2007, 500GB disks had just hit $100, the $.20 per gigabyte level 1TB drives are just now reaching, and now 750GB drives can be had for $.16 per gigabyte. However, the cost of additional drives, in controller channels, server space, and electrical power, makes the highest-capacity hard disks optimal in all but extreme cases, and their price the relevant metric.

All roads lead to SSD

Other notable trends are at work in disk development. As capacities have risen, transfer speeds have risen and latencies have fallen, but not nearly as fast. A 1991 40MB Maxtor IDE disk was recently benchmarked reading at 0.6 MBps, whereas a modern 1TB disk may surpass 100MBps, a 160-fold improvement. Their capacities, however, differ by a factor of 25,000; the time to copy a complete disk rose from less than one minute to more than three hours in those same 17 years.

While capacities, fill times, and bandwidth have changed, latencies have, however, remained relatively static. Hard disks still use rotating platters with magnetic pits, and spin latencies can only decrease with increases in rotational speed. Spindle speeds have risen, from the 3500RPM Maxtor of yesteryear, through 4200, 5400, and today's 7200RPM spindle speeds, with the transition to 10,000 and 15,000 RPM speeds accomplished in some sectors of the server space and burgeoning in the desktop space. But this reliance on physical moving parts has made their development hard; this is only a doubling of spindle and a latency cut from about 30 milliseconds to about 10 milliseconds. While main memory latencies have fallen by a factor of ten, and CPU speeds have risen by factors of thousands, hard disk latencies have remained comparatively static in the milliseconds for those same 17 years.

The result of all of these trends is that it's easier and easier to store huge amounts of data at lower and lower cost, but the increasing data bandwidth and low latency that modern storage needs demand is harder to come by. I've heard from a source that a copy of the text of the entire Internet, stored by Google and its competitors and searched for text in web search queries, is in the vicinity of 20TB. In hard disks, this would now cost a mere $4,000 in disks (much more in servers) but would be completely unsuited to this kind of storage due to the long read time of the disks; a total read for a search would take hours. Instead, Google hosts its web servers from huge clusters of thousands of servers, storing complete copies of the text of the internet in RAM, at about half a million dollars in DDR2 per cluster.

Database applications are finding that the number of seeks that a hard disk can perform, largely latency-limited, is remaining comparatively constant as demands rise, and a means of circumventing this is not clear. 15,000RPM SAS and SCSI disks are scarcely an improvement next to the advances in other areas of storage. Again, content is sometimes served directly from RAM. The increasing capacity but relatively constant speed is great for archival uses of hard disk like the proliferating remote storage services, but harder on seek-limited and transfer-limited applications.

On multiple fronts, hard disks increasingly show themselves unable to answer the real challenges of many enterprise storage applications, and the kind of fundamental improvements which would resolve these problems are prevented by the fundamental physical limitations of hard disk technology. A new storage technology as part of the storage mosaic is the only way to serve the evolving needs of users.

Solid State will provide part of the answer to this dilemma, though it's not without its own limitations. Using flash memory, SSDs can provide much faster seek times and higher transfer rates, though at a much higher cost per gigabyte. If trends continue, and there is every reason to think they will, enterprise storage will transition to a mixed Flash and hard disks pool, adding SSDs as another tier of storage in between hard disks and memory to serve bandwidth and latency dependent loads. So in the long run, although changes are coming, enterprise storage will continue to be a mosaic of different technologies.