Lvm – How to mount/resize/modify LVM partition in KVM qcow2 or raw images

You may simply have an off-by-one math error:

3146776576 (size, in bytes of your .raw file)  / 1024 (kb) / 1024 (mb) == 3001MB

But you made your LV exactly 3000MB ?

There's nothing wrong with DD'ing one partition onto another. Even though your vmbuilder partition list added up to 3000, your .qcow file is a hard drive image, which includes a partition table + mbr @ the front of the disk.

The answer to your first question depends on your requirements and priorities. Using an LVM volume for a guest disk will definitely perform a little better, especially under high disk usage, but qcow2 virtual disks are -- as you point out -- more flexible and easily portable. We use qcow images for just about everything for this reason, and pass through entire hard disks as block devices where performance is more important. Of course, where performance is absolutely critical it may be best to forgo virtualization altogether.

In regard to your second question, to use your example of an Ubuntu host/guest combination with a qc2 image, we would select the install option to create and install to a single LVM volume using the entire (virtual qcow2) disk. This is relatively quick and painless and should work for most scenarios, but be advised that if you've selected an encrypted LVM volume for the host, do NOT choose encryption for the guest's volume, as this generally isn't necessary and has caused problems for us in the past. In fact, even encrypting the home directories on the guest has caused freeze-ups for us when the guest VM image files were run from on an encrypted LVM host volume.