Short answer: a huge System Data figure after updating to iOS 27 is usually temporary. In Settings → General → iPhone Storage, System Data (previously "Other") is the sum of caches, logs, offline media and leftover app container files, and iOS reclaims it automatically. A major update rebuilds indexes and caches, so it inflates for a while and normally drops back within a day or two. Your job is mostly to not panic — and above all not to trust a "one-tap cleaner", because iOS has no such official feature.
What System Data actually is
Open Settings → General → iPhone Storage and look at the coloured bar at the bottom. It is not a folder you can open and empty; it is a total across several categories:
- Caches — app image caches, web caches, temporary streaming files.
- Logs and diagnostics — system and app runtime logs.
- Offline media — audio and video kept for offline playback.
- App container leftovers — files an app leaves behind after updating or being removed.
The key point: iOS manages these on demand and reclaims them when space runs short. So a bigger number does not mean you must clean anything. To see where space goes in general, read Why is my phone always out of storage? and What is the "Other" storage category?.
Why it balloons after a major update
After an update installs, iOS does a few space-hungry things at once:
- Reindexing. Spotlight, Photos, Mail and Messages indexes are rewritten, and for a while old and new copies coexist.
- Rebuilding caches. Apps generate fresh caches the first time they run on the new system.
- Keeping installers and temporary files. The downloaded update package and unpacking scratch files sit there until they are reclaimed.
Stacked together, that can add several GB to System Data overnight. It is not a leak — it is more like boxes piled up in the hallway mid-move. They get cleared once the move is done. Read it alongside How long does iOS indexing take?.
The right order, starting with the gentlest step
① Wait, and set up good conditions. Plug in to power, join Wi-Fi, and use the phone normally for a day or two. Background work finishes faster and caches get reclaimed faster. This alone resolves most cases.
② Force restart once. Press Volume Up, press Volume Down, then hold the Side button until the Apple logo appears. It clears stuck background processes and temporary files that were never released — see How to force restart a stuck iPhone.
③ Update to the latest minor version. The first point release after a major version fixes bugs and often clears early-version caches and index leftovers along the way: Settings → General → Software Update.
④ Reinstall the apps that hoard cache. Long-video, social and map apps are the usual offenders. In the storage list, open one and check its Documents & Data size; reinstalling (deleting and downloading again) clears that cache and usually returns the space immediately. Note that removing an app deletes its local data, so confirm anything important is synced first.
⑤ Erase and restore — last resort only. Settings → General → Transfer or Reset iPhone → Erase All Content and Settings, then restore from backup. That is the nuclear option; consider it only when nothing above worked and the lack of space is actually blocking you.
When it is genuinely a problem
Do not let one bar graph scare you. These are the lines that matter:
- no sign of shrinking after a day or two, with free space still being squeezed enough to block installing apps or taking photos;
- storage growing in one direction only, even though you installed nothing and shot nothing new — that is the real signal;
- System Data sitting at a large share of your free space repeatedly, after you have already reinstalled cache-heavy apps;
- it comes with sustained heat and clearly faster drain, which means background work is still running; see iOS 27 battery drain after update.
If those apply, walking the list above is normally enough. Do not install unknown "cleaning" tools, and do not believe claims of one-tap System Data cleaning — those tools are either useless or bring risk onto your phone.