Queue - July/August 2008

DRAM (dynamic memory) and hard drives; flash is interesting because it falls in a sweet spot between those two components for both cost and performance in that flash is significantly cheaper and denser than DRAM and significantly faster than disk (see figure 1). Flash can accordingly augment the system to form a new tier in the storage hierarchy—perhaps the most significant new tier since the introduction of the disk drive with RAMAC in 1956.

PROPER TIES OF FLASH

Flash has two distinct categories: NAND and NOR— designations that refer to the way the flash cells are arranged. NOR flash allows for random access and is best suited for random access memory, while NAND must be treated as blocks and is ideal for persistent storage. The rest of this article examines only NAND flash, the cheaper and more common variety, of which again there are two types: SLC (single-level cell) and MLC (multilevel cell). SLC stores a single binary value in each memory cell. The binary value is distinguished by two threshold voltages. MLC supports four or, recently, eight distinct values per memory cell, corresponding to two or three bits of storage. Because of its improved longevity and performance, the conventional wisdom is that SLC is best suited for enterprise (i.e., nonconsumer-grade) solutions, so our focus here is on

SLC flash, its cost, power dissipation, performance, and longevity as compared with DRAM and disk drives.

The cost per unit of storage is what has brought flash to the forefront in recent years (see figure 2). Earlier this decade, flash costs were on par with those of DRAM; now, flash devices are much less expensive: $10-$35 per gigabyte for an SLC flash device compared with around $100 per gigabyte for DRAM. The cost trend appears to be continuing to widen the gap between flash and DRAM. Disk drives are still much cheaper than flash, weighing in at less than $1 per gigabyte for 7200-RPM drives and in the neighborhood of $3 per gigabyte for 15,000-RPM drives.

The other exciting attribute of flash is its low power consumption. As the cost of power and the impetus toward green computing rise, so does the attractiveness of lower-power solutions. While completely accurate comparisons among flash, DRAM, and hard drives are difficult because of differences in capacity and interfaces, it’s fair to say that flash consumes significantly less power than those other system components, especially on a per-gigabyte basis. Table 1 records the power consumption for some typical components to provide a broad sense for each type of device.