On Fri, Jan 12, 2018 at 10:18:42AM -0800, Ben Pfaff wrote: > On Fri, Jan 12, 2018 at 10:51:11AM -0500, Eric Garver wrote: > > On Fri, Jan 12, 2018 at 12:33:23PM +0000, James Page wrote: > > > Hi > > > > > > We're currently trying to debug an issue we're seeing when using OpenStack > > > with OVS 2.8.1 (see [0]). > > > > > > When using GRE tunnels, we're seeing broken/slow performance due to what > > > appears to be an incorrect MTU setting on the gre_sys device; for previous > > > OVS releases (2.6.1 specifically but I think this applies back to 2.5.x as > > > well) this device was created with an MTU of 65490; however under 2.8.1 it > > > gets created with an MTU of 1472, which when used with GRE networks with > > > larger MTU settings causes problems with fragmentation and broken path MTU > > > discovery. > > > > > > I've tested under Linux 4.4 and 4.13 and see the problem with both > > > kernels. > > > > > > Its possible to workaround the issue by setting the MTU on the gre_sys > > > device directly, but this change will get reset back to 1472 on a restart > > > of openvswitch. > > > > > > I know the mtu_request feature was introduced around 2.7.0 - groking the > > > code I can't see where the MTU of this device is actually configured... > > > > > > Any help much appreciated > > > > > > James > > > > > > [0] https://bugs.launchpad.net/ubuntu/+source/openvswitch/+bug/1742505 > > > > Most likely you are seeing this bug: > > > > https://bugzilla.redhat.com/show_bug.cgi?id=1488484 > > > > It's actually an issue with the kernel GRE driver ignoring IFLA_MTU when > > the device is created. > > Can OVS work around this by following up its RTM_NEWLINK by an > (otherwise redundant) RTM_SETLINK?
Yes. > (And should it do so?) I don't see any negative effect other than another round trip to the kernel. Worth noting that the other tunnels (vxlan, geneve) are not affected. _______________________________________________ dev mailing list d...@openvswitch.org https://mail.openvswitch.org/mailman/listinfo/ovs-dev