At a minimum I think we should pick a default in devstack and dump a warning in neutron if operators don't specify it.
I would still be preferable to changing the default even though it's a behavior change considering the current behavior is annoying. :) On Jan 24, 2016 23:31, "Ian Wells" <ijw.ubu...@cack.org.uk> wrote: > On 24 January 2016 at 22:12, Kevin Benton <blak...@gmail.com> wrote: > >> >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. >> > It's pretty clear that, while the current setup is precisely the old > behaviour (backward compatibility, y'know?), it's not very useful. Problem > is, anyone using the 1550+hacks and other methods of today will find their > system changes behaviour if we started setting that specific default. > > Regardless, we need to take that documentation and update it. It was a > nasty hack back in the day and not remotely a good idea now. > > > >> 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 >>> >>> >> __________________________________________________________________________ >> 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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