Mixing Memory To Speed Solid-State Drives

Korean researchers find that a little ferroelectric RAM goes a long way

IMAGE: Emily Cooper

Solid-state drives work faster when they contain a mix of flash memory [left] and ferroelectric RAM [right].

The pricey MacBook Air you covet, with its small, lightweight, shock-resistant solid-state drive (SSD), may have a secret. Despite their advantages, solid-state drives suffer not just from enormous price tags but also from slow performance during certain key operations. Now Korean engineers report that through a clever mix of two types of memory, they can give solid-state drives a boost without also jacking up their price.

Unlike a traditional hard-disk drive, which can write new data directly over recorded data, the NAND flash memory that makes up solid-state drives requires free memory space in which to write. That's usually not a problem when you have to write large chunks of sequential data, such as a video clip. But it is a problem when you have to make frequent small additions and changes to existing data. If, for instance, you need to update a file, the original data must be copied to a fresh memory block so that the first block can be erased. The new data can then be merged with the original and written back to the first block.

But as engineers at Seoul National University in South Korea report in a recent issue of IEEE Computer Architecture Letters , there's a better way. They developed a prototype solid-state drive, dubbed Chameleon, that employs a small amount of ferroelectric RAM (FRAM), a comparatively expensive niche nonvolatile memory, to more efficiently deal with such small data changes. ”Our motivation was to combine the benefits of NAND and FRAM so as to create a high-performance SSD,” says Sang Lyul Min, a professor in the department of computer science and engineering at Seoul National University who jointly led the drive's development with professor Yookun Cho.

Like flash memory, FRAM retains its data after the power is switched off; unlike flash, it can overwrite existing data. The mixed-memory prototype improves performance by more than 20 percent compared with an all-flash SSD in a standard suite of tests that included starting up Windows XP and loading applications. The FRAM also speeds the drive's own boot-up time by two orders of magnitude, to less than 7 milliseconds.

Flash memory stores its bits within transistors that are connected to each other serially. So though NAND flash is compact, it cannot easily overwrite itself. An FRAM cell takes up more space because it stores its bits as an electric field within ferromagnetic capacitors, but its arrangement allows for random overwriting.

Much of the performance gain comes from the way the FRAM handles maps and other information that keeps track of the data. Solid-state drives need these maps to make themselves appear to a computer to be a real hard-disk drive. The maps are frequently subject to small random updates, something FRAM is good at but flash is not.

Min and his colleagues are now working with SSD maker Mtron Storage Technology, located near Seoul, to bring a product to market.

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