free disk vs available disk

Solution 1:

available = free - reserved filesystem blocks(for root)

Solution 2:

The ext filesystems (ext2, ext3, ext4) all, by default, reserve 5% of the space on a filesystem for use only by the root user. This is to ensure that the root user can perform necessary maintenance in the event that users fill up the filesystem.

You can change the amount of reserved space when you create the filesystem with the -m flag to mkfs:

mkfs -t ext4 -m 0 /dev/sda1

You can change the reservation on an existing filesystem using tune2fs:

tune2fs -m 0 /dev/sda1

In both cases, the argument to -m is a percentage.