I wrote something into the disk? Next time, how am I gonna find the thing I wrote?

Need some place to store the bookkeeping data

What is the layout of the contents in the disk?

partition table: $ sudo fdisk -l /dev/sda (what about mydisk.img?)

Before creating partitions: hexdump mydisk.img

Now give mydisk.img some partitions: fdisk mydisk.img

Create one big partition on the entire disk. hexdump again. What has been overwritten?

What about want a second partition?

Format the first partition to ext2: mkfs.ext2 mydisk.img?

Only partition has file system: verifiy it by $mount

How to format a partition in a virtual disk -- how to format a real partition

Expose partition as a device: losetup /dev/loop0 mydisk.img?

losetup -o [offset] /dev/loop0 mydisk.img. Now to the kernel, loop0 is a real partition

Now format loop0, mount loop0, write a file (again, to the kernel, it is a real fs/partition). hexdump again. umount, losetup -d