[libvirt] Does libvirt live migration have error handling on port bind failure

[Prasanna Kalever]

On Tue, Aug 23, 2016 at 4:10 AM, Michal Privoznik <mprivozn at redhat.com> wrote:
> On 17.08.2016 11:02, Prasanna Kalever wrote:
>> [ oops! apologies, my previous draft miss the links ]
>>
>> Hello,
>>
>> This was the scenario close to 3 years back, libvirt's live migration
>> tries to use ports in ephemeral port range, but has no fallback to use
>> (an)other port(s) when the one it wants is already in use.
>>
>> If some port say 49152 is already used by some application say gluster
>> in our case (gluster as of today also uses 49152-65535), live
>> migration fails because of lack of fallback mechanism in libvirt,
>> that's where gluster had compromised to go with some hack [1] on bug
>> [2] since getting that addressed in libvirt takes more time than it
>> does with gluster.
>
> I'd say we are pretty quick when it comes to bugfixes but maybe that's
> just my POV.
>
>>
>> As may releases passed from then in libvirt, I hope now there exist a
>> fallback mechanism for port conflicts in libvirt.
>
> Firstly, you can configure whatever port range you want in
> /etc/libvirt/qemu.conf:
>
> #migration_port_min = 49152
> #migration_port_max = 49215
>
> Secondly, since 9c9d4d32 (merged Jan 16 2013) libvirt uses this port
> allocator. It is an internal module that tries to allocate next free
> port within configured range. So the whole migration then works like this:
>
> 1) source client (or daemon in case of p2p migration) contacts remote
> daemon telling it about intended domain migration
> 2) destination daemon uses port allocator module to find a free port
> within configure range
> 3) destination daemon sends back to the source port number (or an error
> if none was found)
> 4) source proceeds with the received port number
>
>>
>> Can someone confirm so ?
>>
>> Also It will be greatly appreciable, if someone can tell how the port
>> binding (mostly defense on clash) works with libvirt live migration
>> today ?
>>
>
> There is none. I mean, there will always be a window between "detecting
> port free" and "starting qemu which binds to it".
> One thing that we could do is to bind to the port immediately (thus
> holding it open without receiving anything on it) and passing it to qemu
> as it starts. However, who would unbind the port if the source decides
> not to migrate after step 4) from my example above?

Thanks Michal that really helps.

So If I understand it right, in the window between destination daemon
passing the port (listening) and source (qemu) binding to it, there
could be some other external process that uses the same port (race?
again I'm not sure how big the window is?)

Don't you think libvirt needs a fall back mechanism here, since the
port numbers could be in ephemeral port range and are free to be used
by any application ?

This is just a suggestion, this is how we had implemented our fall
back mechanism [1]
But now we migrating to a different model something like [2]

Please correct me if I took it wrong.

[1] http://review.gluster.org/#/c/14235/
[2] http://review.gluster.org/#/c/14613/


Cheers,
--
Prasanna


>
> Michal