Mysql error 28 no space left on the device
"no space left on the device" could also mean that the partition is out of inodes. Use df -i to check inode usage.

Edit: Each file, directory, and symbolic link requires one inode. So the idea is to remove some files from the / partition. It doesn't matter how large the files are. You can of course just pick some files on the partition and move them to another partition, if you can find some that are convenient to move.

It's unusual to run out of inodes, so it's possible you have some directory full of small or zero-length files somewhere that you're not aware of. Unfortunately, I don't know of an easy way to find this sort of thing. Running find / -xdev would print the name of every file and directory on the partition. You might be able to spot some suspicious directory full of empty files.

If you can add additional disk partitions to the system, you could pick some directory on the / partition and make it into a separate partition. Copy all of the files in that directory to the new partition, delete them from the / partition, then mount the new partition on top of the old (and now empty) directory.

The nuclear option would be to back up the / partition, wipe it, and rebuild it with more inodes. That's an advanced topic and I won't try to explain it here.