StatsDawg

Disk Usage shows the utilization and type of all the system partitions currently mounted on the system. This output, by default, includes special partitions such as the udev and shm filesystems, although this various among Linux distributions. The most useful aspects of this output are the physical partitions, which are mounted at various points in the filesystem:

Filesystem    Type    Size  Used Avail Use% Mounted on
/dev/hda2 reiserfs     72G   47G   25G  66% /
udev         tmpfs    249M   72K  249M   1% /dev
/dev/hda3     ext3    1.9G   33M  1.8G   2% /tmp
/dev/sda1 reiserfs    150G   49G  101G  33% /home
shm          tmpfs    249M     0  249M   0% /dev/shm

In the above example, there are three partitions on two physical drives, /dev/hda (an IDE drive) and /dev/sda (a SCSI or SATA drive). The root partition is 66% full, with 25GB remaining; the /tmp partition is 2% full with 1.8GB remaining, and so on. The root and /home partitions are both using the reiserfs filesystem while the /tmp partition is using the extended-3 filesystem. A good rule of thumb is to keep your filesystem no more than 90% full to avoid getting stuck in a situation where the partition has reached 100% which can cause sometimes severe problems for running applications, such as MySQL, Apache, etc.

Note that because Linux purposely sets aside spare space for the root user, a drive that is 100% full will not allow writable access by anyone except for the root user, although this can depend on the type of filesystem used. The amount of writable space for the root user is very small and is only there to allow you to clear out space and use various system functions in an emergency situation, although you should never let a partition get 100% full.