The following instructions will show you how to create either a growable (consumes space as needed) or reserved (consumes all space in the beginning) drive image file, format it, and mount it as a directory. This usable on systems that are already partitioned and you need to work with a new "partition" (say one with/without quota support), or just want a portable filesystem that you can transfer between systems.

Create a "growable" 80GB disk image called virt.img.

dd if=/dev/zero of=virt.img  bs=1M seek=81920 count=1

(note that 81920 = 80 * 1024)

or create a "reserved" 80GB disk image called virt.img

dd if=/dev/zero of=virt.img  bs=1M count=81920 seek=1

(note that 81920 = 80 * 1024)

Format your virtual partition (choose yes to continue):

/sbin/mkfs.ext3 virt.img

Mount the virtual partition:

mkdir /mnt/image
mount -o loop virt.img /mnt/image

The /etc/fstab entry (assuming virt.img is stored in the root partition "/" ):

/virt.img        /mnt/image         ext3    rw,loop,auto    0 0