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A Look at Cloud Storage Component Technologies Trends and Future Projections

Cloud trends projections

Future Storage Landscape

Future trends for storage technologies rely on predicting the bits per unit area or areal density increases for these technologies. Improvements in areal density are reflected in increased capacities for storage components. Bit scaling strategies for NAND HDD and LTO Tape Media are illustrated in Figure 4. Absent in Figure 4 is reference to optical storage bit cells on
BD-XL multi-layer media. Although proposed for cold storage archive applications, manufacturing volumes and density scalability for optical BD disks have not been realized.

For LTO Tape Media, large bit cells, in comparison with present-day HDD cell sizes, are retained during scaling. This suggests successful scaling to higher densities will be achieved since sensors and bit cells would be in the 500 nm length scale range, significantly larger than present-day HDD products. Conversely, the scaled bit cells in HDD must approach the 20 nm length scale (25 to 50 times smaller than LTO Tape media length scales), which implies sensor widths and mechanical positioning in the sub 10 nm range. These dimensions suggest that future scaling would be challenging for HDD as demonstrated by this technology’s present low areal density annual rate increase of 10 percent. NAND is approaching scaling differently.

Recognizing processing issues associated with producing bit cells below the 20 nm length scale, NAND is designing multi-layer bit cell structures or 3-D cells. Simply stated, NAND is increasing the physical bit size area by a factor of N but adding N layers of cells in a stack and, most critically, processing all N layers simultaneously. In Figure 4, for a NAND areal density of 1500 Gbit/in2, a traditional planar cell with an area of 19 nm x 19 nm or 379 nm2 is equivalent to an 84 nm x 84 nm area 3D cell of 7056 nm2 if 20 layers of cells are stacked above this cell (i.e., the effective area is 7056 nm2 / 20 = 352 nm2). Cost efficiency is achieved since all cells in the 20 layer stack are processed simultaneously as is the case with a planar cell.

LTO Tape Media progress will use straightforward planar scaling since length scale limits don’t exist. 4x areal density increases are realizable for LTO Tape in the next four- to six-year horizon. HDD scaling is problematic. Scaling must overcome minimum feature limits that stress fabrication and mechanical dimension control. Historical 10 percent annual areal density trends imply the four-year horizon for HDD is a 1.5 to 1.7x density increase. NAND scaling depends on the ability to manufacture multi-layer cells efficiently. It’s reasonable to assume that in a four-year timeframe areal densities will support 4x increases by moving the layer number from 20 to 80 (see Figure 4). Aggressive increases of 6x would assume moving from 3 bit per cell to 4 bit per cell designs.

Using these density growth projections, Table 3 shows the component capacity changes for NAND chips, LTO tape cartridges and HDDs from 2016 present values to possible 2020 values. In particular, note that LTO tape cartridge capacity will exceed HDD capacity in this time frame. Also, tape $/GB would improve significantly over HDD. This assumes manufacturing efficiencies scale with cell size, and then project the following $/GB values for storage components in 2020 using the 2016 Table 1 $/GB values as a baseline: NAND $0.066/GB to $0.097/GB, HDD $0.029/GB, LTO Tape Media $0.004/GB.

Density Increasing

Storage trends for HDD, NAND and LTO Tape Media show that in classical Moore’s Law scaling, storage density increasing by 40 percent each year and cost per bit dropping by 30 percent each year, is not isn’t being achieved. Scaling projections in four years suggest that both NAND and LTO Tape Media densities will increase by 4x while HDD increases will approach 2x resulting in less favorable $/GB environments for HDD relative to NAND and LTO Tape Media. More critically, the projected $/GB values for tape from density increases in these next four years makes tape even more essential for the economics of the cloud infrastructure.

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