Android Storage 101
Not enough on-board storage? Darren Yates explains the options (and pitfalls) of adding external storage to your Android device.
You can think of smartphones as ‘never enough’ devices — there’s usually never enough battery life, never enough performance and almost always never enough storage. With new models pushing barriers into high-speed Full-HD and 4K video capture, we’re fast reaching the point where too much storage isn’t enough. As fewer phones arrive with a user-removable battery, external storage through microSD cards is one of the last options for bulking up most existing devices. But it’s not without pitfalls.
NEW ‘ADOPTABLE’ STORAGE
Android 6.0/Marshmallow’s arrival heralded new options for phones with a microSD slot. Google calls it ‘adoptable storage’ ( tinyurl.com/ya7k9kyk) and it triggers on most devices when Android 6.0 or later first detects a new card, offering you the option to ‘adopt’ the storage as internal or use it as ordinary ‘portable’ storage. Adopting the card formats and encrypts the card’s contents, locking the card to that particular device, but allows the card to store apps and private data, giving your device the appearance of more internal storage.
However, you can’t just pick up the cheapest high-capacity card you can find and expect perfection. Samsung’s 2017-flagship Galaxy S8 features 4K (3,840 x 2,160-pixel) video recording with 4K video-bit rates typically set to around 50 megabits per second (Mbps), meaning high-frame rate video capture is heading towards 10MB/second data speeds. At that rate, even the old Class-10 card speed designation barely keeps its head above water.
CARD SPEED
Figuring out flash card speeds and ratings is a minefield. The old SD/SDHC ‘class’ system showed the minimum write speed — Class-4 meant 4MB/ second minimum, Class-6 was 6MB/ second and so on. The introduction of UHS (Ultra High Speed) bus interfaces in various SDHC and SDXC cards changed how data clocks through the connection between card and device, giving rise to new UHS speed classes. These are labelled on the card as a ‘U’ with a number inside. A ‘U1’ card will write 10MB/second minimum, a ‘U3’ card 30MB/second. What’s made this more confusing is there are three UHS interface standards — UHS-I, UHS-II and UHS-III. These are bus interface indicators, not class speeds, but sadly, we’ve seen these interface indicators confused as speeds. The UHS interface standard is stamped as a standalone ‘I’, ‘II’ or ‘III’ on each card — it means you can have a UHS-I/U3 card or a UHS-II/ U1 card. The difference between them is that UHS-II/III cards have a second row of connection pads, while UHS-I cards have just one. These interfaces also have different bus speed limits — UHS-I tops out at 104MB/second, while UHS-II hits 156MB/second full-duplex (simultaneous read/write) or 312MB/
second half-duplex (read or write). New UHS-III claims up to 312MB/ second full-duplex and 640MB/ second half-duplex.
At time of writing, no phone supported UHS-II, let alone UHS-III, so there’s little point buying UHS-II cards just yet. A good-quality UHS-I Class-U3 card marks the sweet spot.
UFS CARDS
More recently, the UFS (Universal Flash Storage) standard, announced back in 2012, has been extended to external flash cards — UFS cards are similar in size but incompatible with microSD and have different guide-locks to prevent false-installation.
UFS cards promise peak write speeds of up to 170MB/second and read speeds of 530MB/second — in other words, near solid-state drive territory. Phone chips from Samsung and Qualcomm have supported UFS going back to at least the Qualcomm Snapdragon 821 and Samsung Exynos 8890. Samsung has reportedly been using UFS flash inside Galaxy phones since the S6 and introduced UFS cards in 2016, but no phone vendor has taken the UFS-card plunge yet. Still, UFS is an official JEDEC (Joint Electron Device Engineering Council) standard, so we reckon you could see phones with UFS cards in the next couple of years.
HOW TO FORMAT
Plug a new MicroSD card into most Android 6.0 or newer phones and it’ll automatically bring up the format wizard, but you can choose to reformat the card to internal storage at any time. Open the ‘Settings’ menu, select ‘Storage’, tap the MicroSD card listed under ‘Portable storage’, tap the three-dot options menu, choose ‘Storage settings’ and tap ‘Format as internal’. You’ll get a warning about encryption and erasing the card (backup everything first), then tap ‘Erase & Format’ when you’re ready. After it’s completed, you’ll get the option to move files to the card either now or later — your choice, however, some apps won’t work while the move is underway and in our experience, the process took two minutes to move around 1.5GB of data.
REASONS TO AVOID IT
The problem is reformatting the card from internal back to portable storage. You can, but you shouldn’t. Any apps, any personal or app data moved to the card has to be moved back to the phone or backed up before the card is reformatted as portable storage or it’ll be lost — and Android doesn’t move it for you. It’s said Android will uninstall any apps on the microSD card before the card is erased, but that’s it. Moreover, formatting a microSD card as ‘internal storage’ could well affect phone performance. Adding slow flash cards to higher-speed internal or ‘embedded’ flash storage slows down the storage overall and the phone’s performance.
You can’t use file-based encryption (FBE) together with adoptable storage, but there’s also the potential issue of microSD card endurance. All flash storage slowly kills itself every time you write to it and phones use wearlevelling techniques on internal storage to spread the wear evenly. When used as portable storage, microSD cards store files like videos and photos — typically, it’s a write-seldom-read-often data transfer model. When used to store apps and private data, however, it’s not clear how Android manages the extra write activity wear on adopted cards.
APPLICATION PERFORMANCE CLASS
The new Application Performance Class rating system appearing on new microSDHC/SDXC cards aims to rate cards for application speed — these cards are rated not just for sequential read/write speeds like standard cards, but also random read/writes, ratings more suited for apps. Class-A1 cards are rated at 1,500 IOPS (input/output operations per second) read and 500 IOPS write. Class-A2 cards are much faster, rated for minimum 4,000 IOPS random-read and 2,000 IOPS randomwrite. Both classes have a minimum sequential write speed of 10MBps.
CHOOSING THE RIGHT CARD
Embedded flash storage will always be faster than external cards, and right now, using adoptable storage will likely slow down your phone’s performance. The arrival of UFS or UHS-II cards with their higher write speeds still won’t match embedded storage, but as speeds overall increase, the speed differential should be less noticeable. In the meantime, know your phone’s fastest interface and buy as much storage for that interface as you can wisely afford — and keep it as portable storage.