[Kimchi-devel] [RFC] Live migration backend + API

[Walter Niklaus]

On 15.10.2015 21:18, Daniel Henrique Barboza wrote:
> Hi,
>
> Live migration will be one of the new features for Kimchi 2.0 and I've 
> been investigating it for a week or so. This is how I'm planning to 
> implement it:
>
> - API:
>
> The API will be similar to the API of the 'cloneGuest', but with extra 
> parameters:
>
> url : 'plugins/kimchi/vms/' +'vm_name' + "/migrate"
> type: 'POST'
> parameters:
> * destination: the IP address of the destination host
> * (optional, not sure if applicable) password: the password to log in 
> the destination via ssh tunnel
>
> In my opinion we can start with a 'migrate' button in the options of 
> the VM, which will open up a new window to insert the destination IP 
> address. This of course thinking about the old UI. I believe we can do 
> whatever it is done with the 'Clone' function in the case of the new 
> UI, with this extra window.
>
> As the migration might take a while, we'll need a progress bar or 
> something to demonstrate that the progress is still ongoing. Again, 
> the same thing as clone does :)
This progress bar may be a little bit tricky because depending on the 
workload of the VM the amount of dirty pages could increase again at a 
certain point. But from a user point of view I really would like to get 
the reality, meaning to see regression as well in order to give me the 
chance to react.
>
> - backend and core functionality:
>
> This is where it gets tricky. Libvirt has *a lot* of ways to do a live 
> migration between hosts. My approach, at least for this first attempt, 
> will be focusing in a single way, implement it, stabilize it and then 
> extend it to the other migration types, if necessary.
>
Makes a lot of sense.
> Same thing goes for the preset up. The only way I managed to do a live 
> migration was with a NFS storage pool in both hosts, same name and 
> same NFS server. This shared storage is necessary because the live 
> migration will *not* copy the disks of the VM, just the RAM memory. I 
> had to share ssh keys between the hosts to allow for password less ssh 
> sessions.
>
 From what I remember there is a validation step before starting 
migration where libvirt is checking if the resources of the VM are 
available on the remote host as well. I suspect the check is that simple 
to define and start the domain in listen mode on the remote host by 
using the domain xml from the target. And I guess this one would already 
fail if the artifacts specified in the domain xml are not present.
Live migration has an option to perform volume migration as well ( it's 
"copy-storage-all" in virsh I think), but I have never tried it out.
Are you planning to support also FCP attached storage ? I guess this one 
could work:
   - if both Hosts have access to the LUN
   - the disk-id used in the domain definition is the same on source and 
target:
         - for example by using the multipathing id like 
36005076802810d504800000000002c1d
         - because sda on source could point to a different disk as sda 
on target.
> At first, I want to relieve this first implementation from this pre 
> config, which will have to be done entirely by the user. If everything 
> goes well, I can add a few automation in this process inside the 
> backend before the 2.0 launch.
>
> Kimchi internals will be implemented by using async task facilities.
>
>
>
> Any questions? Comments?
>
>
Are you planning to offer a "cancel live migration" API and task as well 
? I'm thinking of the situation where migration will not converge due to 
high workload on the VM and the user may decide to stop it ?
Is this migration going to be supported only from one Kimchi managed 
host to another or could the target host be also a non Kimchi managed host ?
Are we planning to transfer also Kimchi-template data to the target host 
in case this one is managed by Kimchi ?
>
> Daniel
>
>
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