Hi, Thanks so much for your detailed explanation! Best regards, Richard Taubo

On 30 Nov 2019, at 10:25, Peter Krempa <pkrempa@redhat.com> wrote:

On Sat, Nov 30, 2019 at 01:12:31 +0100, ort_libvirt@bergersen.no wrote:

...

Hi!

I need to convert a KVM virtual machine from LVM to a qcow2 image, and wonder if this is the best way to do it (*1): [$] qemu-img convert -O qcow2 /dev/vg_name/lv_name/ /var/lib/libvirt/images/image_name.qcow2

If this is the best way to do this, will this keep the information on: /dev/vg_name/lv_name/ untouched?

As long as you also reconfigure the VM to use /var/lib/libvirt/images/image_name.qcow2 after this point the old logical volume will not be used.

The above approach is good if you are okay with some downtime on the VM while the image is converted over.

There are a few other approaches you can conisder when you want to do it live or for example save some space in the qcow2 image by using fstrim first.

For a live conversion, you can do an external disk-only snapshot first which creates a qcow2 overlay file:

virsh snapshot-create-as $VMNAME --no-metadata --disk-only --diskspec $DISK,file=/path/to/image.qcow2 --diskspec ...

Optionally if your disk frontend supports it [1] and you've configured it properly you can now run 'fstrim' on the filesystems. That records which parts of the disk are empty into the overlay file and thus will not be converted later

After that you pull the backing image into the qcow2:

virsh blockpull --wait --verbose $DOMNAME $DISK

if you executed the trim step the image will be smaller and also the conversion step will avoid copying unused parts. Optionally that step can be skipped and then the result will be the same as in the offline conversion approach.

Optionally if you don't care about the trim step above you can use also the 'blockcopy' operation instead of the two steps above to achieve the same:

virsh blockcopy $VMNAME $DISK --dest /path/to/new.qcow2 --wait --verbose --pivot --format qcow2 --transient-job

Also if you want to decrease the size after an offline coversion you can use the 'virt-sparsify' tool from the libguestfs suite on the image if the VM is offline.

[1] You must have at least qemu-4.0 if you want to use fstrim on a virtio-blk disk interface, or switch to virtio-scsi based disks. Also your disks must have the 'discard="unmap"' option in the <driver> element of your vm's XML.

I've used the above approach to convert few production VMs from LVs to qcow2 for moving them to a different host later on. Apart from a sub 1 minute downtime to restart the VM to switch to virtio-scsi from virtio-blk to enable support for trimming the images the conversion was then executed while the VM were running. Obviously if you don't require this you can easily go with the qemu-img approach.