df -h

root@OpenWrt:~# df -h
Filesystem                Size      Used Available Use% Mounted on
/dev/root                 2.5M      2.5M         0 100% /rom
tmpfs                    29.3M      1.1M     28.2M   4% /tmp
/dev/mtdblock4            3.8M      1.3M      2.4M  35% /overlay
overlayfs:/overlay        3.8M      1.3M      2.4M  35% /
tmpfs                   512.0K         0    512.0K   0% /dev