fs

OS CentOS 5.3

[root@Boot1935 /]# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/hda3 899G 34G 819G 4% /
/dev/hda1 99M 17M 78M 18% /boot
tmpfs 1009M 0 1009M 0% /dev/shm
/dev/hdc1 463G 33G 407G 8% /backup_device


34 + 819 = 899?

как понять