Way back in the 1980s, it seemed inevitable that solid state drives (based on memory chips) would eventually replace hard drives (based on rotating magnetic platters). The only question was when, and 2000 seemed like a nice round number. Little did we know that the HDD industry would progress from selling 10 megabyte drives for $1,000 to selling terabyte drives for $50, and holding on to the market by offering vast amounts of storage at unbeatable prices. Even though SSDs are finally gaining a foothold in ultrathin devices, where their small size and low power requirements compensates for the high price of SSD storage, victory is not assured. The hybrid SSD/HDD (or SSHD) is now appearing, and it offers both SSD speeds and HDD capacities at a reasonable price.
The SSD has obvious advantages in speed of response, small size, light weight, and robustness, all of which become more significant when choosing drives for Ultrabooks and tablet PCs. However, SSDs come in relatively small capacities -- typically 64GB and 128GB -- at high prices. As I recently found, you can now buy an internal 3.5-inch terabyte hard drive for as little as £35, which means you get 931 gigabytes of file storage space for about 4p per gigabyte. SSDs cost about £1 per gigabyte, which is 25x more.
The hybrid drive is an attempt to offer the best of both worlds. It combines an SSD with a significant amount of storage -- perhaps 4GB or more -- with a traditional hard drive. The SSD is use most of the time, with the HDD only spinning up to copy data from the SSD to the hard drive when required. This is not a full SSD replacement, because the hybrid drive still uses the HDD to store most programs and possibly the operating system. The HDD will also need to spin up when users access stored files. However, this should be less of a problem if users are mostly using online or server-based data.
Happily, users rarely need to boot their PCs nowadays. Also, even using the Microsoft ReadyDrive system introduced with Windows Vista and continued in Windows 7 speeds up boot times by about 10%.
Enter the SSHD
Seagate and Samsung introduced the first hybrid SSHD drives in 2007, when Vista was launched. However, it wasn't until the 3.5-inch Seagate Momentus XT was introduced that Anandtech was able to announce Finally a Good Hybrid HDD in May 2010. Today, a 500GB Momentus XT sells for about $100-$125, which is about a fifth of the price of SSD storage.
Another significant problem is that SSDs and HDDs currently use different kinds of optimised controllers, which adds to the cost of the hybrid approach. However, new commands are being introduced in the ATA-8 standard to manage on-drive memory, and should therefore be widely supported.
Pending that, however, OCZ has just launched the RevoDrive Hybrid, which uses a high-speed PCIe bus connection. The RevoDrive combines a 100GB SSD with a terabyte HDD for about $475.
Intel's Z68 Express chipset also features Smart Response technology to help HDDs and SSDs work together.
Ultrabooks and tablets
There is no doubt that the SSD has helped the popularity of the latest versions of Apple's MacBook Air. There's also no doubt that most people don't want to pay that much for a PC. Using a hybrid drive instead of an SSD is one way to bring down the price, and Acer has been the first to try it with its $899 Aspire S3 Ultrabook. This combines a 20GB SSD with a 320GB hard drive, though it's not clear how. It's not, as I first hoped, the 2.5-inch version of the Seagate Momentus XT.
In a recent Seagate Storage Blog post, Seagate explained 5 reasons Ultrabooks should include hybrid drives. Alongside a photo of the Acer S3, the post said: "Today, the Momentus XT drives are standard 9.5mm drives. I can expect with 7mm becoming more and more prevalent, we’ll see the hybrid versions in the future."
Seagate has sold more than a million hybrid drives, and they are used in laptops from Asus, Dell, Sony, Toshiba and other suppliers. A 7mm-thin version that could be used in Ultrabooks and tablets could greatly expand that market. The idea is certainly worth a look.
It may still seem inevitable that SSDs will eventually replace HDDs. However, if SSHDs take off, rotating platters could be around for another decade or two longer than predicted.
OCZ's RevoDrive Hybrid