In the past year and a half, solid state drives have come from nowhere to take their place as the Next Big Thing in storage, especially in notebooks. The MacBook Air and the Asus Eee PC and OLPC XO-1 (One Laptop Per Child) netbooks were among the first consumer notebooks to utilize solid state drives. While SSDs are still most popular in netbooks, they have begun appearing in more mainstream notebooks and even high-end desktops.
SSDs have much higher read speeds than traditional drives, and with no moving parts, they’re more durable. They’re not susceptible to magnetic interference or vibration, and they use less power and run much more quietly than standard magnetic hard drives. Best of all, they come in standard 3.5-inch and 2.5-inch formfactors with SATA connectors and emulate traditional drives, so they’re compatible with existing architecture. Unfortunately, they’re also orders of magnitude more expensive per megabyte, thus limiting widespread adoption, at least for now.
Although the fastest solid state drives use DRAM for storage (with a battery backup to preserve data), this White Paper will focus on flash-based SSDs—the variety most commonly found in consumer gear.
As the name implies, a solid state drive’s first point of departure from a standard hard drive is that it has no moving parts. A mechanical hard drive uses a magnetic read/write head over rapidly spinning platters, like a super-high-speed record player, while a solid state drive writes data to NAND flash memory, similar to that used in other flash-based storage, such as memory cards and USB thumb drives.
While magnetic hard drives are marvels of modern engineering, solid state drives are much simpler—they are composed of just a SATA interface, a controller that emulates a hard drive and allocates reads/writes, and a collection of NAND flash modules that data is stored on. Since NAND modules don’t need to wait for a drive head to find the appropriate data sector on a moving platter to read data, their random-access times are extremely fast, as are their read times. And because they’re solid state, they neatly sidestep many of the failure points of traditional drives: Vibration, dust, magnets, and jarring are all potentially damaging to the read/write head and platters in a magnetic drive, but do not affect flash memory.
Flash-based SSDs come in two flavors: single-level cell (SLC) and multilevel cell (MLC). SLCs store one bit of data per cell, while MLCs store two bits per cell. SLCs are faster, provide less storage, and last longer. MLCs are cheaper and store more data, thus achieving better density rates, but they are susceptible to higher error rates and slower read/write times. Most cheap SSDs, especially those used in netbooks such as the Asus Eee PC, use MLCs for cost reasons, while performance SSDs use SLCs.
For an inside look at an SSD, check out last month’s Autopsy. For more on NAND flash memory, see the October 2007 White Paper.
Despite their many advantages over traditional hard drives, current-generation solid state drives have several drawbacks. Flash memory is still much more expensive than magnetic media, though as more manufacturers get into the SSD game, prices continue to go down. The average capacity of SSDs is much smaller than that of standard drives—the largest consumer HDDs on the market currently are 1.5TB; the largest SSDs are 256GB. Write times are still slower than those of top-end magnetic drives, because in order to write to NAND memory, the entire block of memory the data is being written to needs to be erased and rewritten.
Finally, flash memory cells are rated for a finite number of read/write cycles: around 10,000 for MLCs and 100,000 for SLCs, so SSDs have a limited life span. Fortunately, solid state drives include a wear-leveling algorithm to distribute read/write cycles over the entire drive evenly. When a solid state drive reaches its cycle limit, it doesn’t crash; the drive just stops allowing writes and becomes read-only, so you can still access your data, unlike a standard hard drive failure.
An SSD’s life span might sound bad, but take heart. No long-term studies on the life spans of consumer SSDs exist, but even an MLC drive should last at least five years with near-constant usage, about the average life span of a magnetic hard drive. Single-level-cell SSDs should last much longer—decades, hopefully.
In November 2008, SanDisk announced a system called ExtremeFFS that it claims will make random write speeds 100 times faster. It does this by keeping a number of small memory blocks that are marked for deletion actually erased. By making sure there are always small empty blocks to write to, ExtremeFFS ensures much speedier random writes. Since Windows makes lots of random writes in the course of its normal operation, this should increase overall system performance.
In the next few years, all major hard drive manufacturers will begin offering SSDs, and so will many flash-memory manufacturers. Drive capacity will increase and costs will decrease following roughly the same exponential pattern that standard storage has for years. And as the technology matures and SSDs become more prevalent, expect even wilder innovations. We wouldn’t be surprised if SSDs become nearly ubiquitous by 2014.