Manage partitions on a raw device file

Every once in a while there comes a time when you need to use a file as raw device
for example when using qemu qcow file or if you want to have your home directory portable and encrypted
in this example i am using one file named my-hd-image.file and 2 Linux partitions inside it . lets start

Creating the raw device file is simply by using dd ( can also be use via qemu-img )

dd if=/dev/zero of=my-hd-image.file bs=1M count=1024

Create partitions inside the new disk using fdisk , in my file
I had created 2 partitions of 512M

Device Boot Start End Blocks Id System
my-hd-image.file1 2048 1050623 524288 83 Linux
my-hd-image.file2 1050624 2097151 523264 83 Linux

creating the file system using loop devices to be linked to specific offset inside the file
so that each partition will have its own loop device . the offset calculation is in bits
and i am using bc to calculate it fast for me . the offset of each partition is its start block times 512
witch is the block size ( unless you have set a different one upon creation )

Partition my-hd-image.file1 starts at offset 2048

echo "2048 * 512" | bc
1048576

Partition my-hd-image.file2 starts at offset 1050624

echo "1050624 * 512" | bc
537919488

use the calculation offset to map to the loop device

losetup -o 1048576 /dev/loop1 my-hd-image.file
losetup -o 537919488 /dev/loop2 my-hd-image.file

Now we can format the partition

mkfs.ext4 /dev/loop1
mkfs.ext4 /dev/loop2

all is left to do is mount the partitions

[email protected]# mkdir /mnt/my-hd-partition-1
[email protected]# mkdir /mnt/my-hd-partition-2
[email protected]# mount /dev/loop1 /mnt/my-hd-partition-1
[email protected]# mount /dev/loop2 /mnt/my-hd-partition-2

To remove the device just use unmount and to release the loop device
you will need to use losetup

umount /mnt/my-hd-partition-2
umount /mnt/my-hd-partition-1

losetup -d /dev/loop1
losetup -d /dev/loop2
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