for rotating disks, above 90% full can no longer be defragged at all, and above 80% or so struggles with that if your files are relatively large. I would keep it in terms of %s and stick to 80% or so full on a rotating disk if you can.
I am not sure it matters at all for SSD. However you need some space free for the (esp win) OS and crapware like browsers to dump their garbage. So in that case, a fixed amount of free space is required to function, and that depends on what os and such what number is acceptable. If its not the disk with the OS on it, you can run until full for SSD on those, just the main OS disk needs a little room.
It depends on the file system and your usage. ZFS will tend to perform poorly after 80% capacity. All file systems will perform poorly at an unpredictable time that depends on how fragmented they are.
However, there are situations where the file system will perform acceptable even at 100% capacity. If the file system is used like an append-only archive (e.g. every day you back up another system and the backup is always around 4 GiB), then reads will always perform roughly identically. Obviously this won't hold true if the volume is a system drive, or if the computer is a desktop system.
for rotating disks, above 90% full can no longer be defragged at all
Nah, they can be. Whether the software is prepared to do it is a different matter, though.
I am not sure it matters at all for SSD.
SSDs respond to sequential requests faster, but they also perform their own internal reorganizations, so performance is less predictable compared to magnetic storage.
@helios @jonnin Thank you both for the informative replies. May I ask if either of you have dealt with a device making a constant beeping sound as a result of storage, if so, percentage-wise, how close was the device to the storage limit?
Does anyone know what might happen if one puts a balloon full of ethanol way, waaay up one's ass and it bursts?
I think they had an episode like that on 1000 Ways to Die. An alcoholic was in the hospital and had to be fed through tube. His drinking buddy snuck a bottle of Jack Daniels in and the administered it anally....
The guy died. It turns out that your rectum absorbs alcohol must faster than your stomach and he died of alcohol poisoning.