Can someone please invest few cycles here ?

Thanks, -- Prasanna On Wed, Aug 17, 2016 at 2:32 PM, Prasanna Kalever <pkalever(a)redhat.com&gt; 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. > > As may releases passed from then in libvirt, I hope now there exist a > fallback mechanism for port conflicts in libvirt. > > 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 ? > > [1] http://review.gluster.org/#/c/6210/ > [2] https://bugzilla.redhat.com/show_bug.cgi?id=1018178 > > Sincere Thanks, > -- > Prasanna -- libvir-list mailing list libvir-list(a)redhat.com https://www.redhat.com/mailman/listinfo/libvir-list

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?

Michal

On 23.08.2016 08:57, Prasanna Kalever wrote: > On Tue, Aug 23, 2016 at 4:10 AM, Michal Privoznik <mprivozn(a)redhat.com&gt; wrote: >> On 17.08.2016 11:02, Prasanna Kalever wrote: >>> [ oops! apologies, my previous draft miss the links ] >>> >>> Hello, >>> > 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?) Yes. there's this possibility. But then again - these applications you're using should be configured to use different sets of ports. For instance libvirt can use 49152 - 49182, while gluster could use 49183 - 49213.

From my/our experiences it is better not to take control over

The window is not that long. Maybe my description in the previous e-mail wasn't that clear. At the destination, where the port allocation takes place: 1) libvirt uses its internal module to find a free port 2) libvirt spawns the qemu and tell it to bind to that port 3) libvirt reports back to the migration source host with the port number. So the window is not that big - probably just a couple of milisecs as it's just that part between steps 1 and 2. But there always be some window - even if we implemented what I suggested earlier. That's one of the limitations of kernel API. One simply doesn't detect port free and bind to it atomically.

> > 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 ? You should fix the application then. Or its configuration. We may end up in fallback loop if two applications with the fallback start fighting over the ports. I mean, if it was libvirt who allocates the migration port(socket) for qemu and then just merely pass the FD to it, then libvirt could do a couple of tries to allocate the port (bind socket to it). However, if it is still qemu doing that, we would need to spawn qemu each time we want to try next port. That's not very efficient.

Frankly, I have no idea why we don't allocate the port ourselves. I'll ask offline our migration guru and let you know :)

Michal