An interesting blog post from the developer of evergreen app Instapaper, Marco Arment, went up yesterday in which he warns his followers about a new quirk in iOS 5: If you recently updated your device to Apple's latest operating system, the software will automatically erase your cached data (such as Instapaper's offline articles) when you start to run out of space on your device--without so much as a single prompt.
Documents and Caches/tmp App Directories
There's a bit of developer jargon in there, but Arment explains that basically, Apple allows each iOS app two partitions of space: a home directory called Documents for storing files which are created by the app (and impossible to regenerate), and a Caches/tmp directory for storing files that would remain "untouchable" (that is, files here don't get backed up to iTunes when you sync your device). The idea was that if a user ever did a complete restore or accidentally wiped their device, data could still be reconstructed after being redownloaded from the Caches/tmp folder.
Instapaper took advantage of this setup by filing away its users' "Read Later" articles in the Caches/tmp directory. Here they would remain safe, available offline, and users would be freed from the burden of slowed down syncs.
But with the arrival of iCloud, Apple decided to crack down on apps which store large files in the Documents folder, since these would back up to their servers. Developers had been receiving instructions to store "database cache files and downloadable content, such as that used by magazine, newspaper, and map applications" in the Caches/tmp directory. Instapaper had already been doing this anyway.
However, there is one crucial difference. Apple now apparently considers the contents of the caches directory low priority. In iOS 5, it gets routinely cleaned out--without warning--when you start to approach the limits of your device's capacity. Sure, you can easily redownload the contents, but only with an Internet connection. So if you ever find yourself in a situation where unwarily you get a large file onto your device after caching a lot of Instapaper articles, there's an unfortunate likelihood that these will disappear from your device without your consent. The same goes for all other apps that cache offline content, according to Arment: Ebook apps, offline Wikipedia apps, offline mapping programs, etc.
How To Avoid This from Happening to You
Always be aware of the space you have left on your device. This is relatively easy to check. Just connect your iOS device to your computer, click on the Summary tab in iTunes, and check how much free space you have left.
Beyond this, all we can really do is sit back and hope that Apple institutes another directory that works like Caches used to. It's interesting to note, however, that Apple's own Reading List needs an Internet data connection of its own to function. We also wonder if iBooks is immune to this restriction.