Out of inodes: “No space left on device” with free space
On a 128 MB ext4 /var, 7,830 PHP sessions exhaust all 8,192 inodes while df -h shows 72%: what an inode is, df -i, du --inodes to find the culprit folder, PHP session cleanup on Debian, mke2fs -i and -N, and how XFS raises its inode cap online.