Why is `volumeAvailableCapacityForImportantUsage` zero?



I'm following the Apple's documented example to figure out how to query the available disk space on my device.

I'm using the code below in my applicationDidFinishLaunchingWithOptions:

let fileURL = URL(fileURLWithPath:"/")
do {
let values = try fileURL.resourceValues(forKeys: [
print("Available Capacity: \(Float(values.volumeAvailableCapacity!)/1000000000)GB")
print("ImportantUsage Capacity: \(Float(values.volumeAvailableCapacityForImportantUsage!)/1000000000)GB")
print("Opportunistic Capacity: \(Float(values.volumeAvailableCapacityForOpportunisticUsage!)/1000000000)GB")
print("Total Capacity: \(Float(values.volumeTotalCapacity!)/1000000000)GB")
} catch {
print("Error retrieving capacity: \(error.localizedDescription)")

This logs the following:

Available Capacity: 3.665879GB
ImportantUsage Capacity: 0.0GB
Opportunistic Capacity: 0.0GB
Total Capacity: 63.989494GB

Why are volumeAvailableCapacityForImportantUsage and volumeAvailableCapacityForOpportunisticUsage zero and under what circumstances does this happen?


  • I'm running this experiment on my own 64GB iPhone SE via xCode 10.2.1 (so the total capacity looks correct)
  • My iPhone is running iOS11
  • iTunes claims my device has 10.27GB 'free'
  • I'm trying to figure this out so I know whether my user will have enough space to download a large (40MB+) in-app purchase

Note: This is not the same as this question. I know how to query available space. I want to understand the results of that query.

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