Jesse, I'm resurrecting this thread after a fairly lengthy discussion of MTU with Ben at the recent OpenStack summit. Have you given the topic any further thought toward implementation in a reasonable way? Can you elaborate on the architectural limitations? At the moment, the OpenStack implementation of OVN doesn't use DPDK.
Matt On Tue, Feb 2, 2016 at 7:39 PM, Jesse Gross <je...@kernel.org> wrote: > On Tue, Feb 2, 2016 at 6:03 PM, Matt Kassawara <mkassaw...@gmail.com> > wrote: > > Jesse, > > > > We can raise the MTU of the underlying network until it reaches the > maximum > > value for the physical equipment, a common situation for 10+ Gbps data > > center networks. For example, if the physical equipment supports a > maximum > > 9000 MTU, we can't further increase it to account for overlay protocol > > overhead. We can easily reduce the VM MTU to account for outbound > traffic, > > but inbound traffic assumes it can use 9000 bytes and therefore requires > > PMTUD... or (grudgingly) reducing the MTU of all hosts on the physical > > network... effectively moving the problem rather than solving it. > > > > I think we need to solve this problem sooner than later to prevent the > > current loathing by operators using conventional OpenStack neutron > drivers > > from seeping into OVN before it has a chance at success. Right now, the > > ability to eliminate (often obscure) problems for any underlying network > MTU > > provides a huge selling point, at least for OpenStack. > > The architectural limitations that I was referring to earlier are > mostly in the Linux kernel and since the DPDK datapath will be the > preferred software gateway for OVN, perhaps they don't really apply. > I'm a little hesitant to introduce actions that can only be handled in > one place but since this would be purely internal to OVS it might not > matter since we could change the implementation in the future if > necessary. That would potentially allow us to handle this in an > cleaner way in the near future. >
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