[libvirt-users] Questions regarding migration from QEMU to libvirt

Tue Nov 6 09:15:51 UTC 2018

On Mon, Oct 22, 2018 at 12:07:13PM -0300, Rodrigo Siqueira wrote:
>Hi,
>
>First of all, thanks for your feedback :)
>

Sorry for the slow reply, lot of work around KVM Forum conference.

>On 10/17, Martin Kletzander wrote:
>> On Wed, Oct 03, 2018 at 01:04:50PM -0300, Rodrigo Siqueira wrote:
>> > Hi,
>> >
>> > My colleagues and I have a set of scripts that we use to automate our
>> > daily tasks related to the Linux Kernel. As a result, most of our code
>> > relies on the QEMU features; and recently we decided use libvirt instead
>> > of QEMU. However, we have some questions, and I would like to know if
>> > someone could help us. Follows:
>> >
>>
>> Could you explain how much do you rely on the VM looking the same from the guest
>> perspective (Guest ABI stability)?
>
>Actually, we do not use any special feature from QEMU. Just basic
>things. Just for illustrate, follows the command that we use for start a
>VM:
>
>qemu-system-x86_64 -enable-kvm -daemonize -m 3G -smp cores=4,cpus=4 -net nic \
>-net user,hostfwd=tcp::2222-:22,smb=/home/USER /home/USER/p/DISK.qcow2
>
>> If you only need the disk to stay the same and you want to easily migrate
>> existing machines, then virt-v2v might be good to try.
>
>Thanks! I will take a look on this.
>
>> > 1) Import our QEMU images with virsh
>> >
>> > Currently, we import the QEMU VMs with virt-install.
>> >
>> > Is it possible to automatically discover the distro variant of a QEMU
>> > image in order to use it in the “--os-variant” option?
>> >
>>
>> You might be able to get that info using libguestfs (virt-v2v should be able to
>> do that for you).
>
>Yes, I did it last week. I'm doing the following steps:
>
>1. I mount the disk image using libguestfs
>2. Print the value MOUNT_PATH_DISK/etc/*-release
>3. Take the distro and version
>
>Do you know a straightforward path?
>

  virt-inspector $filename

or

  virt-inspector -d $domain

>> > Here is how we register a VM:
>> > https://github.com/rodrigosiqueira/kworkflow/pull/23/files#diff-70617d452b008bc71362804d54032f24R113
>> >
>> > 2) The requirement of sudo to create a network
>> >
>> > When we register our VM, we want to keep the ssh working well. However,
>> > every time we register a VM we create a new network bridge as a result
>> > it requires sudo. Is it possible to avoid this? Or at least make this a
>> > single action?
>> >
>>
>> You don't need to have a libvirt network for each VM, you can create a bridge
>> and use <interface type='bridge'/> or just use <interface type='direct'/>.  Or
>> you can have one network for multiple VMs.  What did you use with plain QEMU?
>
>When I use QEMU for work, I need two things: ssh and share a directory.
>I'm using samba for sharing a directory and openssh. Again, this is the
>command that I'm using:
>
>qemu-system-x86_64 -enable-kvm -daemonize -m 3G -smp cores=4,cpus=4 -net nic \
>-net user,hostfwd=tcp::2222-:22,smb=/home/USER /home/USER/p/DISK.qcow2
>
>For our network configuration, we have:
>
><network>
>  <name>kworkflow-network</name>
>  <uuid>7d167334-7e36-475b-a0ae-9889b87848d7</uuid>
>  <forward mode='nat'/>
>  <bridge name='virbr-kworkflow' stp='on' delay='0'/>
>  <mac address='52:54:00:8a:da:00'/>
>  <ip address='10.0.1.1' netmask='255.255.255.0'>
>    <dhcp>
>      <range start='10.0.1.2' end='10.0.1.2'/>
>      <host mac='52:54:00:21:23:52' name='kworflow-vm' ip='10.0.1.2'/>
>    </dhcp>
>  </ip>
></network>
>
>This configuration file worked well on Debian, but when we tried it on
>Archlinux things did not work. Actually, we want to keep these things as
>simple as possible. Our requirements are only: share a directory and
>access VM via ssh. In order to try to make things simples and
>'portable', we think about the following approach:
>
>1. Mount the disk image, and set a static IP
>2. Our set of scripts, manage the static IPs
>
>I feel that I'm reinventing the wheel, but this approach sounds easier
>for me due to my lack of knowledge in the libvirt. Do you have any other
>suggestion to make libvirt do the hard work for me?
>

More straightforward might be using libvirt's default network and libvirt_nss to
connect without knowing the IP address.

To share a directory you can use <filesystem/> for the domain:

  https://libvirt.org/formatdomain.html#elementsFilesystems

but to be brutally honest with you, I don't know what that does on the command
line or how to access it in the guest.

As I said, the easiest option for you might be using just virt-v2v for
*everything*.  I'm not sure how that supports just qemu domains and filesystem
passthrough, though.

HTH,
Martin

>> I'm not sure what the trouble is, also what the use cases are, so the answers
>> might not be fit for your needs.
>>
>> > Here is how we handled this:
>> > * The code for register:
>> > https://github.com/rodrigosiqueira/kworkflow/pull/23/files#diff-70617d452b008bc71362804d54032f24R113
>> > * The configuration file that we adopted as a default:
>> > https://github.com/rodrigosiqueira/kworkflow/pull/23/files#diff-d8c16482496875afc0d37b181487ae46R1
>> >
>> > 3) When using libvirt it changes the owner of our image
>> >
>> > If we try to use libvirt, it changes the ownership of our QEMU images
>> > (root). We fixed it by changing the file “/etc/libvirt/qemu.conf”, and
>> > switch the option dynamic_ownership to “0”. What is the impact of that
>> > change? Is it dangerous? There is a way to avoid this change?
>> >
>>
>> dynamic_ownership=0 keeps the owners as they are, but libvirt cannot guarantee
>> that the VM will have access to all its resources.  If you are taking care of
>> that, then keeping that turned off should be OK.
>>
>> > Finally, here is the full code of the libvirt part:
>> > https://github.com/rodrigosiqueira/kworkflow/pull/23/files
>> >
>> > Thanks
>> > Best Regards
>> >
>> > --
>> > Rodrigo Siqueira
>> > http://siqueira.tech
>> > Graduate Student
>> > Department of Computer Science
>> > University of São Paulo
>>
>>
>>
>> > _______________________________________________
>> > libvirt-users mailing list
>> > libvirt-users at redhat.com
>> > https://www.redhat.com/mailman/listinfo/libvirt-users
>>
>
>
>
>-- 
>Rodrigo Siqueira
>https://siqueira.tech
>https://twitter.com/siqueirajordao
>Graduate Student
>Department of Computer Science
>University of São Paulo

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