[libvirt-users] Converting LVM to qcow2

[ort_libvirt at bergersen.no]

Hi,

Thanks so much for your detailed explanation!

Best regards,
Richard Taubo


> On 30 Nov 2019, at 10:25, Peter Krempa <pkrempa at redhat.com> wrote:
> 
> On Sat, Nov 30, 2019 at 01:12:31 +0100, ort_libvirt at 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.