[libvirt] migration of vnlink VMs

Fri Apr 29 12:45:50 UTC 2011

On 04/28/2011 04:15 AM, Oved Ourfalli wrote:
>> From: "Laine Stump"<lstump at redhat.com
>> On 04/27/2011 09:58 AM, Oved Ourfalli wrote:
>>> Laine, hello.
>>>
>>> We read your proposal for abstraction of guest<-->  host network
>>> connection in libvirt.
>>>
>>> You has an open issue there regarding the vepa/vnlink attributes:
>>> "3) What about the parameters in the<virtualport>  element that are
>>> currently used by vepa/vnlink. Do those belong with the host, or
>>> with the guest?"
>>>
>>> The parameters for the virtualport element should be on the guest,
>>> and not the host, because a specific interface can run multiple
>>> profiles,
>> Are you talking about host interface or guest interface? If you mean
>> that multiple different profiles can be used when connecting to a
>> particular switch - as long as there are only a few different
>> profiles,
>> rather than each guest having its own unique profile, then it still
>> seems better to have the port profile live with the network definition
>> (and just define multiple networks, one for each port profile).
>>
> The profile names can be changed regularly, so it looks like it will be better to put them in the guest level, so that the network host file won't have to be changed on all hosts once something has changed in the profiles.
>
> Also, you will have a duplication of data, writing all the profile name on all the hosts that are connected to the vn-link/vepa switch.

But is it potentially the same for many/all guests, or is it necessarily 
different for every guest? If it's the former, then do you have more 
guests, or more hosts?

>>>    so it will be a mistake to define a profile to be interface
>>>    specific on the host. Moreover, putting it in the guest level will
>>>    enable us in the future (if supported by libvirt/qemu) to migrate
>>>    a vm from a host with vepa/vnlink interfaces, to another host with
>>>    a bridge, for example.
>> It seems to me like doing exactly the opposite would make it easier to
>> migrate to a host that used a different kind of switching (from vepa
>> to
>> vnlink, or from a bridged interface to vepa, etc), since the port
>> profile required for a particular host's network would be at the host
>> waiting to be used.
> You are right, but we would want to have the option to prevent that from happening in case we wouldn't want to allow it.
> We can make the ability to migrate between different network types configurable, and we would like an easy way to tell libvirt - "please allow/don't allow it".

I *think* what you're getting at is this situation:

HostA has a group of interfaces that are connected to a vepa-capable 
switch, HostB has a group of interfaces connected to a vnlink-capable 
switch. Guest1 is allowed to connect either via a vnlink switch or a 
vepa switch, but Guest2 should only use vepa.

In that case, HostA would have a network that had a pool of interfaces 
and type "vepa", while HostB would have a pool of interfaces and a type 
"vnlink". Guest1 could be accommodated by giving both networks the same 
name, or Guest2 could be accommodated by giving each network a different 
name (when migrating, if the dest. host doesn't have the desired 
network, the migration would fail). However, using just the network 
naming, it wouldn't be possible to allow both.

I don't think keeping the virtualport parameters only with the guest 
would help (or hurt) this though. What would be needed would be to have 
the information about network type *optionally* specified in the guest 
interface config (as well as in the network config); if present the 
migration would only succeed if the given network on the dest host 
matched the given type (and parameters, if any) in the guest config.

>>> So, in the networks at the host level you will have:
>>> <network type='direct'>
>>> <name>red-network</name>
>>> <source mode='vepa'>
>>>     <pool>
>>>      <interface>
>>>         <name>eth0</name>
>>>         .....
>>>      </interface>
>>>      <interface>
>>>         <name>eth4</name>
>>>         .....
>>>      </interface>
>>>      <interface>
>>>         <name>eth18</name>
>>>         .....
>>>      </interface>
>>>     </pool>
>>> </source>
>>> </network>
>>>
>>> And in the guest you will have (for vepa):
>>> <interface type='network'>
>>> <source network='red-network'/>
>>> <virtualport type="802.1Qbg">
>>>      <parameters managerid="11" typeid="1193047" typeidversion="2"
>>>      instanceid="09b11c53-8b5c-4eeb-8f00-d84eaa0aaa4f"/>
>>> </virtualport>
>>> </interface>
>>>
>>> Or (for vnlink):
>>> <interface type='network'>
>>> <source network='red-network'/>
>>> <virtualport type="802.1Qbh">
>>>      <parameters profile_name="profile1"/>
>>> </virtualport>
>>> </interface>

What would the interface for a 2nd guest of each type look like? Could 
it be identical? Or might some parameters change for every single guest? 
Perhaps it would be best to have virtualport parameters on both network 
and guest interface XML, and merge the two to arrive at what's used (the 
network definition could contain all the attributes that would be common 
to all guests using that network on that host, and the guest interface 
definition would contain extra parameters specific to that host. In the 
case of a parameter being specified in both places, if they were not 
identical, the migration would fail).

>> This illustrates the problem I was wondering about - in your example
>> it
>> would not be possible for the guest to migrate from the host using a
>> vepa switch to the host using a vnlink switch (and it would be
>> possible
> You are right. When trying to migrate between vepa and vnlink there will be missing attributes in each in case we leave it on the host.

(you mean if we leave the config on the *guest*, I guess...)

>> to migrate to a host using a standard bridge only if the virtualport
>> element was ignored). If the virtualport element lived with the
>> network
>> definition of red-network on each host, it could be migrated without
>> problem.
>>
>> The only problematic thing would be if any of the attributes within
>> <parameters>  was unique for each guest (I don't know anything about
>> the
>> individual attributes, but "instanceid" sounds like it might be
>> different for each guest).
>>> Then, when migrating from a vepa/vnlink host to another vepa/vnlink
>>> host containing red-network, the profile attributes will be
>>> available at the guest domain xml.
>>> In case the target host has a red-network, which isn't vepa/vnlink,
>>> we want to be able to choose whether to make the use of the profile
>>> attributes optional (i.e., libvirt won't fail in case of migrating
>>> to a network of another type), or mandatory (i.e., libvirt will fail
>>> in case of migration to a non-vepa/vnlink network).
>>>
>>> We have something similar in CPU flags:
>>> <cpu match="exact">
>>>                <model>qemu64</model>
>>>                <topology sockets="S" cores="C" threads="T"/>
>>>                <feature policy="require/optional/disable......"
>>>                name="sse2"/>
>>>    </cpu>
>> In this analogy, does "CPU flags" == "mode (vepa/vnlink/bridge)" or
>> does
>> "CPU flags" == "virtualport parameters" ? It seems like what you're
>> wanting can be satisfied by simply not defining "red-network" on the
>> hosts that don't have the proper networking setup available (maybe
>> what
>> you *really* want to call it is "red-vnlink-network").
> What I meant to say in that is that we would like to have the ability to say if an attribute must me used, or not.

Sure, it sounds useful. Would what I outlined above be sufficient? (It 
would allow you to say "this guest must have a vepa network connection" 
or "this guest can have any network connection, as long as it's named 
"red-network". It *won't* allow saying "this guest must have vepa or 
vnlink, bridge is not allowed, even if the network name is the same". 
You could also put most of the config with the host network definition, 
but allow, eg instanceid to be specified in the guest config.

> The issues you mention are indeed interesting. I'm cc-ing libvirt-list to see what other people think.
> Putting it on the guest will indeed make it problematic to migrate between networks that need different parameters (vnlink/vepa for example).