Linux – How to merge 2 disk images into one with 2 partitions

command linefdisklinuxqemuroot

I have 2 (relatively small) disk images for qemu (raw format) containing different filesystems (say fat and ext4).

I want to put them into an usb stick as two different partitions so that I can plug it in a pc and boot from there.

I could do it with fdisk and dd but that would require root access and manual intervention (this is going to be a frequent activity in the next months.

How can I automate the task?
Can I avoid sudo and root access?

Currently the disk images are created during our continuous integration process and do not require root access or manual intervention but I cannot find a way to merge these two disks to satisfy this new requirement.

edit

What I need to automate is not the partition of an actual usb key, but the creation of a raw disk image that can be written into the usb key with a single sudo dd .... The disk image should already contains the partitions filled with the data (preserving permissions).

What I cannot grasp is why I should need root to write a raw disk image.

Best Answer

You dont need root. You can run "fdisk someimage.hdd" and as long as you have access to that file, you're fine. You can also dd to parts of that file that correspond to partitions, using offset and notrunc parameters to dd.

Specifying the offsets manually is quite more inconvenient compared to using partition devices with dd though. This is where root access can come in handy - you can use the "losetup" command to create loopback block devices for your image and its partitions.

Still, those offset calculations won't vary each time you repeat the exercise if you don't change the partition sizes, so if you figure them out once you can reuse them for the future.

And there is one other way - after you make the image once and have figured out the offsets, you can extract the start of the disk image containing the partition table as a separate file and not need to use fdisk and dd for your future image rebuilds - just concatenate ("cat") the partition table and the two partition images into a disk image. (In some cases you might need to cat also a disk-suffix file after the partitions similar to the disk-prefix file containing the partition table, as there might be a backup partition table at the end of the disk.)

Related Solutions

mdadm – Rebuilding IMSM RAID-0 Array from Disk Images Using mdadm

Looking at the partition table for /dev/loop0 and the disk image sizes reported for /dev/loop0 and /dev/loop1, I'm inclined to suggest that the two disks were simply bolted together and then the partition table was built for the resulting virtual disk:

Disk /dev/loop0: 298.1 GiB, 320072933376 bytes, 625142448 sectors

Device       Boot   Start        End    Sectors   Size Id Type
/dev/loop0p1 *       2048    4196351    4194304     2G  7 HPFS/NTFS/exFAT
/dev/loop0p2      4196352 1250273279 1246076928 594.2G  7 HPFS/NTFS/exFAT

and

Disk /dev/loop1: 298.1 GiB, 320072933376 bytes, 625142448 sectors

If we take the two disks at 298.1 GiB and 298.1 GiB we get 596.2 GiB total. If we then take the sizes of the two partitions 2G + 594.2G we also get 596.2 GiB. (This assumes the "G" indicates GiB.)

You have already warned that you cannot get mdadm to recognise the superblock information, so purely on the basis of the disk partition labels I would attempt to build the array like this:

mdadm --build /dev/md0 --raid-devices=2 --level=0 --chunk=128 /dev/loop0 /dev/loop1
cat /proc/mdstat

I have a chunk size of 128KiB to match the chunk size described by the metadata still present on the disks.

If that works you can then proceed to access the partition in the resulting RAID0.

ld=$(losetup --show --find --offset=$((4196352*512)) /dev/md0)
echo loop device is $ld
mkdir -p /mnt/dsk
mount -t ntfs -o ro $ld /mnt/dsk

We already have a couple of loop devices in use, so I've avoided assuming the name of the next free loop device and instead asked the losetup command to tell me the one it's used; this is put into $ld. The offset of 4196532 sectors (each of 512 bytes) corresponds to the offset into the image of the second partition. We could equally have omitted the offset from the losetup command and added it to the mount options.

Linux – use a read-write root filesystem disk image as an OverlayFS upperdir

Create an additional level of nesting in the image file:

mkdir   /tmp/upper/upper
mkdir   /tmp/upper/work
mount -t overlay -o rw,lowerdir=/tmp/lower,upperdir=/tmp/upper/upper,workdir=/tmp/upper/work overlay /tmp/union

Best Answer

Related Solutions

mdadm – Rebuilding IMSM RAID-0 Array from Disk Images Using mdadm

Linux – use a read-write root filesystem disk image as an OverlayFS upperdir

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