Examining reports of slow write speeds on the 32GB iPhone 7

Reports are going around of late that the 32GB iPhone 7 has demonstrably slower write performance than its larger siblings. Under the provocative title "iPhone 7 - What Apple Doesn't Want You To Know," YouTuber Unbox Therapy has shown that the 32GB version of the phone has much slower write performance than the 128GB version. He concludes that Apple "probably went a little bit chintzier on the lower-capacity models in terms of the quality or caliber" of the devices' storage, leading to the performance difference.

When I saw these results, I immediately surmised that the write performance of the storage subsystem in the iPhone 7 is limited by the number of dies hooked up to the device's storage controller. In general, we know that the more flash devices one can hook up to a controller, the more performance one can extract from solid-state storage, at least to a point.

It appears that hunch could be correct. Thanks to the fearless folks at iFixit and Chipworks, we know some of the details of the iPhone's storage subsystem. At least some iPhone 7s use a 32GB SK Hynix H23QEG8VG2ACS storage chip. According to SK Hynix's data sheet, that chip contains four flash devices inside one package, for a total of 256Gb—or 32GB—of raw storage space. At least some 128GB versions of the iPhone 7 use the SK Hynix H23Q1T8QK2MYS chip, according to Chipworks, and that part has eight dies in a package. Chipworks also says the 256GB iPhone 7 uses an eight-die package with Toshiba 3D NAND inside.

We know next to nothing about Apple's storage controller in the iPhone 7, but it seems possible the controller isn't able to saturate all of its channels when it's paired with the 32GB SK Hynix chip. If that's true, the drop in write performance is regrettable, and it has unfortunate implications for moving data onto the device. Still, the difference seems to be a result of well-understood limitations in flash storage architectures, not because Apple "went a little bit chintzier in terms of the quality or caliber" of the 32GB phone's storage.

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