Modernization of mobile storage

David Woolf, UNH-IOL

February 20, 2015

David Woolf, UNH-IOLFebruary 20, 2015

At the University of New Hampshire InterOperability Laboratory (UNH-IOL), we are fortunate to be able to follow technology developments in a number of areas, and watch as they go from sketches on a whiteboard to actual shipping products.

Sometimes these developments happen in their own bubbles, other times there are not so coincidental coincidences, where several industries are able to take advantage of technology originally developed for one community. We may be watching that happen right now as technologies originally developed for mobile are finding application in storage, and vice versa.

Mobile device makers tend to highlight high storage capacities of their devices. Lower end devices are typically in the 8-16 GB range, whereas higher end devices can have 128 GB or more. Most Android devices accept microSD cards, increasing their capacity further (currently microSD cards max out at 128 GB). But what are the technologies behind all this storage, and what’s in store for them and the future of mobile storage?

Today’s dominant mobile storage technologies
Today the dominant technologies for mobile storage are eMMC and microSD cards. Internal (i.e. non-user serviceable) storage is typically eMMC. Swappable and upgradeable microSD is what we know from use in cameras, phones, etc. The speed class on microSD refers to the read/write performance. The typical standard today is Class 10. Soon UHS (Ultra High Speed) cards will be available. Here are the key differences between eMMC and microSD:

     Embedded, not upgradeable
     400 MB/s

     microSD Cards:
     10 MB/s Class 10, up to 312 MB/s for future UHS cards
     Easily swappable, upgradable

Most users don’t think about the differences between these technologies. But when we look at the performance expectations on mobile devices, it’s worth asking if these technologies can handle the coming performance demands for mobile devices.

What are these performance demands? Stepping through an example may be helpful. Consider a 4K mobile display operating at 60 frames per second (fps). How much bandwidth will it need?

Here’s a simplified rough calculation:

     4096x 2160 = 8847360 pixels
     24 bits per pixel x 8847360 = 212336640 bits per frame
     60 frames per second x 212336640 = 12,700,000,000 bits per second
     12,700,000,000 bits per second = 1,590,0000,000 bytes per second

That's 1.59 GB per second, nearly 4 times the throughput available from eMMC today.

To be fair, we have greatly simplified the process that a system architect would go through to determine the necessary bandwidth. You can argue whether this simplification is valid or not, but we’re not trying to show how to build such a system, simply that existing technologies are at best being stretched to enable the demanded user experience and price point, and at worst, cannot meet the demand.

It’s important to keep in mind that system designers are juggling many competing factors, and we’ve ignored the effects of compression and buffering that can be used to save bandwidth and system cost. Architects will employ combinations of high performance (but constantly powered) DDR3 RAM, on board NAND Flash, and image processing and compression hardware to enable the best performance with minimal power draw. Surely a system with a lot of RAM could meet the necessary performance specs. But it would also have constant power draw and far exceed the sub $100 BoM of many devices. An expensive mobile device capable of showing a 4K movie is of little use if the battery dies at the climax of the movie.

The point is, today’s technologies aren’t enough. We need to ask what the new technologies are that will address this need. Is there one clear winner or will new technologies provide mobile device integrators and designers with more choice? Evolutions of technologies like NVMe, PCIe, UFS, and M-PHY (Figure 1) may lead to drastic performance increases for on-device storage. Each of these and how they can be used are examined below.

Figure 1: Mobile storage technology design choices

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