Writing to a disk even when df says zero available space

Just a quick note to remind myself: There’s a gap between the size of a disk, the used space and the available space. It’s quite well-known that a certain percentage of the disk (that’s 200 GB on a 3.6 TB backup disk) is saved for root-only writes.

So the reminder is: No problem filling the disk beyond the Available = zero blocks point if you’re root. And it doesn’t matter if the files written don’t belong to root. The show goes on.

Also, the numbers shown by df are updated only when the file written to is closed. So if a very long file is being copied, it might freeze for a while, and then boom.

This is important in particular when using the disk just for backing up data, because the process doing the backup is root, but the files aren’t.

But whatever you do, don’t press CTRL-C while the extracting goes on. If tar quits in the middle, there will be file ownerships and permissions unset, and symlinks set to zero-length files too. It wrecks the entire backup, even in places far away from where tar was working when it was stopped.