Axiom Storage Diag

• iOS file system handles billions of files daily across all apps
• System behavior is documented and predictable
• 99% of issues are location/protection mismatches

Mandatory First Steps

ALWAYS check these FIRST (before changing code):

// 1. Check WHERE file is stored
func diagnoseFileLocation(_ url: URL) {
    let path = url.path
    if path.contains("/tmp/") {
        print("⚠️ File in tmp/ - system purges aggressively")
    } else if path.contains("/Caches/") {
        print("⚠️ File in Caches/ - purged under storage pressure")
    } else if path.contains("/Documents/") {
        print("✅ File in Documents/ - never purged, backed up")
    } else if path.contains("/Library/Application Support/") {
        print("✅ File in Application Support/ - never purged, backed up")
    }
}

// 2. Check file protection level
func diagnoseFileProtection(_ url: URL) throws {
    let attrs = try FileManager.default.attributesOfItem(atPath: url.path)
    if let protection = attrs[.protectionKey] as? FileProtectionType {
        print("Protection: \(protection)")
        if protection == .complete {
            print("⚠️ File inaccessible when device locked")
        }
    }
}

// 3. Check backup status
func diagnoseBackupStatus(_ url: URL) throws {
    let values = try url.resourceValues(forKeys: [.isExcludedFromBackupKey])
    if let excluded = values.isExcludedFromBackup {
        print("Excluded from backup: \(excluded)")
    }
}

// 4. Check file existence and size
func diagnoseFileState(_ url: URL) {
    if FileManager.default.fileExists(atPath: url.path) {
        if let size = try? FileManager.default.attributesOfItem(atPath: url.path)[.size] as? Int64 {
            print("File exists, size: \(size) bytes")
        }
    } else {
        print("❌ File does not exist")
    }
}

Decision Tree

Files Disappeared

Files missing? → Check where stored

├─ Disappeared after device restart
│   ├─ Was in tmp/? → EXPECTED (tmp/ purged on reboot)
│   │   → FIX: Move to Caches/ or Application Support/
│   │
│   ├─ Was in Caches/? → System purged (storage pressure)
│   │   → FIX: Move to Application Support/ if can't be regenerated
│   │
│   └─ Protection level .complete? → Inaccessible until unlock
│       → FIX: Wait for unlock or use .completeUntilFirstUserAuthentication
│
├─ Disappeared randomly (weeks later)
│   ├─ In Caches/? → System purged under storage pressure
│   │   → EXPECTED if re-downloadable
│   │   → FIX: Re-download when needed, or move to Application Support/
│   │
│   └─ In Documents or Application Support/?
│       → Check if user deleted app (purges all data)
│       → Check iOS update (rare, but check migration path)
│
└─ Only some files missing
    → Check isExcludedFromBackup + iCloud sync
    → Check if file names have special characters
    → Check file permissions

Files Inaccessible

Can't access file?

├─ Error: "No permission" or NSFileReadNoPermissionError
│   ├─ Device locked? → Check file protection
│   │   └─ .complete protection? → Wait for unlock
│   │       → FIX: Use .completeUntilFirstUserAuthentication
│   │
│   └─ Background task accessing? → .complete blocks background
│       → FIX: Change to .completeUntilFirstUserAuthentication
│
├─ File exists but read returns empty/nil
│   └─ Check actual file size on disk
│       → May be zero-byte file from failed write
│
└─ File exists in debugger but not at runtime
    → Check if using wrong directory (Documents vs Caches)
    → Check URL construction

Backup Too Large

App backup > 500 MB?

├─ Check Documents directory size
│   └─ Large files (>10 MB each)?
│       ├─ Can they be re-downloaded? → Move to Caches + isExcludedFromBackup
│       └─ User-created? → Keep in Documents (warn user if >1 GB)
│
├─ Check Application Support size
│   └─ Downloaded media/podcasts?
│       → Mark isExcludedFromBackup = true
│
└─ Audit backup with code:
    ```swift
    func auditBackupSize() {
        let docsURL = FileManager.default.urls(
            for: .documentDirectory,
            in: .userDomainMask
        )[0]
        let size = getDirectorySize(url: docsURL)
        print("Documents (backed up): \(size / 1_000_000) MB")
    }
    ```

Common Patterns by Symptom

Pattern 1: Files in tmp/ Disappear

Symptom: Temp files missing after restart or even during app lifecycle

Cause: tmp/ is purged aggressively by system

Fix:

// ❌ WRONG: Using tmp/ for anything that should persist
let tmpURL = FileManager.default.temporaryDirectory
let fileURL = tmpURL.appendingPathComponent("data.json")
try data.write(to: fileURL)  // WILL BE DELETED

// ✅ CORRECT: Use Caches/ for re-generable data
let cacheURL = FileManager.default.urls(
    for: .cachesDirectory,
    in: .userDomainMask
)[0]
let fileURL = cacheURL.appendingPathComponent("data.json")
try data.write(to: fileURL)

Pattern 2: Caches Purged, Data Lost

Symptom: Downloaded content disappears weeks later

Cause: Caches/ is purged under storage pressure (expected behavior)

Fix: Either re-download on demand OR move to Application Support if can't be regenerated

// ✅ CORRECT: Handle missing cache gracefully
func loadCachedImage(url: URL) async throws -> UIImage {
    let cacheURL = getCacheURL(for: url)

    // Try cache first
    if FileManager.default.fileExists(atPath: cacheURL.path),
       let data = try? Data(contentsOf: cacheURL),
       let image = UIImage(data: data) {
        return image
    }

    // Cache miss - re-download
    let (data, _) = try await URLSession.shared.data(from: url)
    try data.write(to: cacheURL)
    return UIImage(data: data)!
}

Pattern 3: .complete Protection Blocks Background

Symptom: Background tasks fail with "permission denied"

Cause: Files with .complete protection inaccessible when locked

Fix:

// ❌ WRONG: .complete protection for background-accessed files
try data.write(to: url, options: .completeFileProtection)
// Background task fails when device locked

// ✅ CORRECT: Use .completeUntilFirstUserAuthentication
try data.write(
    to: url,
    options: .completeFileProtectionUntilFirstUserAuthentication
)
// Accessible in background after first unlock

Pattern 4: Backup Bloat from Downloaded Content

Symptom: App backup >1 GB, app rejected or users complain

Cause: Downloaded content in Documents/ or not marked excluded

Fix:

// ✅ CORRECT: Exclude re-downloadable content
func downloadPodcast(url: URL) async throws {
    let appSupportURL = FileManager.default.urls(
        for: .applicationSupportDirectory,
        in: .userDomainMask
    )[0]

    let podcastURL = appSupportURL
        .appendingPathComponent("Podcasts")
        .appendingPathComponent(url.lastPathComponent)

    // Download
    let (data, _) = try await URLSession.shared.data(from: url)
    try data.write(to: podcastURL)

    // Mark excluded from backup
    var resourceValues = URLResourceValues()
    resourceValues.isExcludedFromBackup = true
    try podcastURL.setResourceValues(resourceValues)
}

Production Crisis Scenario

SYMPTOM: Users report lost photos after iOS update

DIAGNOSIS STEPS:

1. Check storage location (5 min):

   // Were photos in Caches/?
   let photosInCaches = path.contains("/Caches/")
   // If yes → system purged them (expected)
   

2. Check if backed up (5 min):

   // Check if excluded from backup
   let excluded = try? url.resourceValues(
       forKeys: [.isExcludedFromBackupKey]
   ).isExcludedFromBackup
   // If excluded=true AND not synced → lost
   

3. Check migration path (10 min):

   • Did app container path change?
   • Did we migrate data from old location?

ROOT CAUSES (90% of cases):

• Photos in Caches/ (purged under storage pressure)
• Photos excluded from backup + no cloud sync
• Migration code missing after major iOS update

FIX:

• User photos MUST be in Documents/
• Never exclude user-created content from backup
• Always have cloud sync OR backup for user content

Quick Diagnostic Checklist

Run this on any storage problem:

func diagnoseStorageIssue(fileURL: URL) {
    print("=== Storage Diagnosis ===")

    // 1. Location
    diagnoseFileLocation(fileURL)

    // 2. Protection
    try? diagnoseFileProtection(fileURL)

    // 3. Backup status
    try? diagnoseBackupStatus(fileURL)

    // 4. File state
    diagnoseFileState(fileURL)

    // 5. Directory size
    if let parentURL = fileURL.deletingLastPathComponent() as URL? {
        let size = getDirectorySize(url: parentURL)
        print("Parent directory size: \(size / 1_000_000) MB")
    }

    print("=== End Diagnosis ===")
}

Related Skills

• axiom-storage — Correct storage location decisions
• axiom-file-protection-ref — Understanding protection levels
• axiom-storage-management-ref — Purge behavior and capacity APIs

Last Updated: 2025-12-12 Skill Type: Diagnostic