The first image I made was DD'd from an 8GB SD card that I had already created and tested. I formatted the SD card, tested in the Pi and then took an image with DD. Then, rewrote it to the card and tested again. All good. Then a few people commented that the image was slightly too large to fit on other 8GB cards which is when I checked and my card it was indeed a few sectors large than some others. So how do you create images which are useful for more people without going out and buying an army of various sized SD cards? The answer is creating loopback devices which are basically file-systems in a single file (which is what you create when you DD a working image from an SD card, for example).
1. Decide what size you would like your image to be. For this example and the OpenELEC image I posted previously, I used 1GB.
2. Create your empty container file using DD. You can replace the expression after seek with 1024000000 if you prefer - this is the size of your image in bytes.
dd if=/dev/zero of=~/1gb_file_image.img bs=1024 count=0 seek=$[1000*1000]
3. Mount this device as a loopback device
sudo losetup /dev/loop0 ~/1gb_file_image.img
4. This will give you a loopback block device under /dev/loop0. You can now create your partitions using fdisk / parted. I've used the examples from the OpenELEC wiki here.
sudo parted -s /dev/loop0 mklabel msdos
sudo parted -s /dev/loop0 unit cyl mkpart primary fat32 -- 0 16
sudo parted -s /dev/loop0 set 1 boot on
sudo parted -s /dev/loop0 unit cyl mkpart primary ext2 -- 16 -2
sudo mkfs.vfat -n System /dev/loop0p1
sudo mkfs.ext4 -L Storage /dev/loop0p2
5. Now you have a filesystem in a file with two partitions. Things can get a little tricky here when you need to mount them individually. You need to umount the current loopback device so you can remount it with some sector offsets and size limits. If you search and go by the scraps of mailing lists that mention this, it can be a little daunting but it's not as bad as it seems. Lets look at an fdisk output of our new filesystem in a file.
sudo fdisk -l ~/1gb_file_image
Disk 1gb_file_image.img: 1024 MB, 1024000000 bytes
255 heads, 63 sectors/track, 124 cylinders, total 2000000 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0xd00251a6
Device Boot Start End Blocks Id System
1gb_file_image.img1 * 2048 264191 131072 c W95 FAT32 (LBA)
1gb_file_image.img2 264192 1999999 867904 83 Linux
What we need to note here are the start and end sectors as we will use these to calculate the offset and sizelimit parameters to pass to losetup when mounting the partitions in the file. Taking the first partition as an example (shown as an actual file with a different number at the end, that's just how fdisk rolls I guess). We have a starting sector of 2048 and and end sector of 264191. We need to multiply each of these by 512 to get the total amount of bytes instead of sectors (bytes per sector is in the output - line 4 above). So, our starting offset becomes 1048576, and the size being (end - start, multiplied by 512 again) 134217216.
We'll add these to our next command.
6. Mount the loopback image file again, specifying the offset and sizelimit for the first partition.
sudo losetup /dev/loop0 ~/1gb_image_file.img -o 1048576 --sizelimit 134217216
7. Now, you can mount this loopback device as a block device with the standard mount command
sudo mount /dev/loop0 /mnt/my_mount_point
This should give you the first FAT partition (if you used the OpenELEC example anyway) mounted under /mnt/my_mount_point (make sure it exists first!). Then you can copy files into your loopback device to populate your image. To mount the second partition, follow the same procedure as in steps 5-7 but use a new loopback device (/dev/loop1 is a good starting point) and calculate the new offsets and sizelimits based on the start and end sectors of the second partition. When you're finished, make sure to unmount the loop devices, then detach the loopback devices with
sudo losetup -d /dev/loop0
sudo losetup -d /dev/loop1
It can be a finicky procedure, but once you've got it nailed it's pretty handy. Let me know if any parts don't work as advertised (especially seeing as it took several attempts to get mine right too!).
