>The reason for that was in the other half of the thread - it's not possible to magically discover these things from within Openstack's own code because the relevant settings span more than just one server
IMO it's better to have a default of 1500 rather than let VMs automatically default to 1500 because at least we will deduct the encap header length when necessary in the dhcp/ra advertised value so overlays work on standard 1500 MTU networks. In other words, our current empty default is realistically a terrible default of 1500 that doesn't account for network segmentation overhead. On Jan 24, 2016 23:00, "Ian Wells" <ijw.ubu...@cack.org.uk> wrote: > On 24 January 2016 at 20:18, Kevin Benton <blak...@gmail.com> wrote: > >> I believe the issue is that the default is unspecified, which leads to >> nothing being advertised to VMs via dhcp/ra. So VMs end up using 1500, >> which leads to a catastrophe when running on an overlay on a 1500 underlay. >> > That's not quite the point I was making here, but to answer that: looks to > me like (for the LB or OVS drivers to appropriately set the network MTU for > the virtual network, at which point it will be advertised because > advertise_mtu defaults to True in the code) you *must* set one or more of > path_mtu (for L3 overlays), segment_mtu (for L2 overlays) or physnet_mtu > (for L2 overlays with differing MTUs on different physical networks). > That's a statement of faith - I suspect if we try it we'll find a few > niggling problems - but I can find the code, at least. > > The reason for that was in the other half of the thread - it's not > possible to magically discover these things from within Openstack's own > code because the relevant settings span more than just one server. They > have to line up with both your MTU settings for the interfaces in use, and > the MTU settings for the other equipment within and neighbouring the cloud > - switches, routers, nexthops. So they have to be provided by the operator > - then everything you want should kick in. > > If all of that is true, it really is just a documentation problem - we > have the idea in place, we're just not telling people how to make use of > it. We can also include a checklist or a check script with that > documentation - you might not be able to deduce the MTU values, but you can > certainly run some checks to see if the values you have been given are > obviously wrong. > > In the meantime, Matt K, you said you hadn't set path_mtu in your tests, > but [1] says you have to ([1] is far from end-user consumable > documentation, which again illustrates our problem). > > Can you set both path_mtu and segment_mtu to whatever value your switch > MTU is (1500 or 9000), confirm your outbound interface MTU is the same > (1500 or 9000), and see if that changes things? At this point, you should > find that your networks get appropriate 1500/9000 MTUs on VLAN based > networks and 1450/8950 MTUs on VXLAN networks, that they're advertised to > your VMs via DHCP and RA, and that your routers even know that different > interfaces have different MTUs in a mixed environment, at least if > everything is working as intended. > -- > Ian. > > [1] > https://github.com/openstack/neutron/blob/544ff57bcac00720f54a75eb34916218cb248213/releasenotes/notes/advertise_mtu_by_default-d8b0b056a74517b8.yaml#L5 > > >> On Jan 24, 2016 20:48, "Ian Wells" <ijw.ubu...@cack.org.uk> wrote: >> >>> On 23 January 2016 at 11:27, Adam Lawson <alaw...@aqorn.com> wrote: >>> >>>> For the sake of over-simplification, is there ever a reason to NOT >>>> enable jumbo frames in a cloud/SDN context where most of the traffic is >>>> between virtual elements that all support it? I understand that some >>>> switches do not support it and traffic form the web doesn't support it >>>> either but besides that, seems like a default "jumboframes = 1" concept >>>> would work just fine to me. >>>> >>> >>> Offhand: >>> >>> 1. you don't want the latency increase that comes with 9000 byte >>> packets, even if it's tiny (bearing in mind that in a link shared between >>> tenants it affects everyone when one packet holds the line for 6 times >>> longer) >>> 2. not every switch in the world is going to (a) be configurable or (b) >>> pass 9000 byte packets >>> 3. not every VM has a configurable MTU that you can set on boot, or >>> supports jumbo frames, and someone somewhere will try and run one of those >>> VMs >>> 4. when you're using provider networks, not every device attached to the >>> cloud has a 9000 MTU (and this one's interesting, in fact, because it >>> points to the other element the MTU spec was addressing, that *not all >>> networks, even in Neutron, will have the same MTU*). >>> 5. similarly, if you have an external network in Openstack, and you're >>> using VXLAN, the MTU of the external network is almost certainly 50 bytes >>> bigger than that of the inside of the VXLAN overlays, so no one number can >>> ever be right for every network in Neutron. >>> >>> Also, I say 9000, but why is 9000 even the right number? We need a >>> number... and 'jumbo' is not a number. I know devices that will let you >>> transmit 9200 byte packets. Conversely, if the native L2 is 9000 bytes, >>> then the MTU in a Neutron virtual network is less than 9000 - so what MTU >>> do you want to offer your applications? If your apps don't care, why not >>> tell them what MTU they're getting (e.g. 1450) and be done with it? >>> (Memory says that the old problem with that was that github had problems >>> with PMTUD in that circumstance, but I don't know if that's still true, and >>> even if it is it's not technically our problem.) >>> >>> Per the spec, I would like to see us do the remaining fixes to make that >>> work as intended - largely 'tell the VMs what they're getting' - and then, >>> as others have said, lay out simple options for deployments, be they jumbo >>> frame or otherwise. >>> >>> If you're seeing MTU related problems at this point, can you file bugs >>> on them and/or report back the bugs here, so that we can see what we're >>> actually facing? >>> -- >>> Ian. >>> >>> >>> __________________________________________________________________________ >>> OpenStack Development Mailing List (not for usage questions) >>> Unsubscribe: >>> openstack-dev-requ...@lists.openstack.org?subject:unsubscribe >>> http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev >>> >>> >> __________________________________________________________________________ >> OpenStack Development Mailing List (not for usage questions) >> Unsubscribe: >> openstack-dev-requ...@lists.openstack.org?subject:unsubscribe >> http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev >> >> > > __________________________________________________________________________ > OpenStack Development Mailing List (not for usage questions) > Unsubscribe: openstack-dev-requ...@lists.openstack.org?subject:unsubscribe > http://lists.openstack.org/cgi-bin/mailman/listinfo/openstack-dev > >
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