My iPhone 11 Pro 512GB has just 11.3GB left, thanks to 147,301 photos & videos taking up 459.2GB
I'm looking forward to upgrading my iPhone 11 Pro 512GB to iPhone 13 Pro 1TB soon because I have nearly run out of storage space. This is primarily due to me taking tons of photos every day and keeping them on the phone. I had a similar experience three years ago on an iPhone X 256GB, and was saved by iPhone XS having a 512GB option.
At the beginning of the week, my phone running iOS 14.8 had about 5 to 6GB of space left. This may seem a lot for those using a smaller storage tier, but having lots of files apparently cause lots of temporary data to build up. As a result, the remaining space could drop to 2GB or less depending on what you're doing and so it was becoming dangerous. Then iOS 15 came out on Tuesday and I installed it right away. After completion, I was left with just 2GB of space on the phone and it dropped to nearly zero when I attempted to do a local backup.
With the space filled up, apps slowed down or stopped working. I tried rebooting, but the phone would hang at the Apple logo indefinitely. So I put the device into recovery mode, connected it to my Mac, and did the "Update" procedure. Thankfully, the phone could boot up again after doing that and I was told that there was now more than 30GB of space available. It turned out that the photo database was flushed to make such room and so the Photos app didn't show any photos initially. Over the course of more than 24 hours the database was automatically rebuilt with just minor hiccups and everything came back. The iPhone now has about 11GB left as seen above, which should give it much more breathing room until a new device arrives.
The whole experience was unsettling. Although I didn't lose anything important this time, it should serve as a reminder to have ample storage space left at all times, proportional to the overall size.
Documents & Sync consumed 6.6GB of cellular data on my iPhone last month
I usually use somewhere between 5 to 7GB of cellular (mobile) data per month on my iPhone because my data plan gives me 6GB and I make sure not to go over too much. But since last December, I started to notice an unintended, excessive data consumption which caused my monthly usage to hover around 9 to 12GB until April. It then subsided for a while, but it came back last month. I didn't want to pay the carrier more than what I actually used, so I decided do something about it.
The root of this problem has been pinpointed to "Documents & Sync" activity under "System Services" category from fairly early on. You can see that it consumed 6.6GB of data by itself last month, out of 11.51GB total. The difficult part was determining what exact services were causing it, since there were many iOS features depending on synchronizing data and documents to iCloud. After much trial and errors, I was able to find two of them which caused most, of not all, of the issues.
Disabling Photos and iCloud Drive from using cellular data solved the overuse
One was the Photos app using the data connection to update Shared Albums and iCloud Photos. This was a bit strange because I have disabled iCloud Photos and my Shared Albums see barely any activity. Still, it had caused a burst of data consumption from time to time, spending up to nearly 1GB in about 10 minutes in the worst case and making the phone hot in the process. After disabling cellular data option within the Photos entry in the Settings app, these bursts were no more.
The other was the iCloud Drive. When I disabled its cellular data use by using the option buried at the bottom of the Cellular entry in the Settings app, the remaining excess usage stopped. Come to think of it, the data consumed during normal use, be it using the camera, browsing the internet, or interacting with the social media, was roughly twice the previously normal level. So whatever I did, the iCloud Drive was trying to sync some undetermined data in parallel, even though I had no intention of letting it do that. The available space in the Drive did not change, so I would guess it was some sort of system-level stuff.
By the way, the Internet searches I did while trying to fix this problem revealed that a lot of people had similar experiences, but with slightly different solutions. Some people had success by adjusting settings related to iMessages or Keychain, to name a few. So my solution may not be definitive for everyone and you should treat it as a starting point.