[ovs-discuss] OVS/OVN troubleshooting: where's my packet?

Ben Pfaff blp at ovn.org
Thu Mar 14 16:55:00 UTC 2019

On Thu, Mar 14, 2019 at 04:55:56PM +0100, Daniel Alvarez Sanchez wrote:
> Hi folks,
> Lately I'm getting the question in the subject line more and more
> frequently and facing it myself, especially in the context of
> OpenStack.
> The shift to OVN in OpenStack involves a totally different approach
> when it comes to tracing packet drops. Before OVN, there were a bunch
> of network namespaces and devices where you could hook a tcpdump on
> and inspect the traffic. People are used to those troubleshooting
> techniques and OVS was merely used for normal action switches.
> It's clear that there's tools and techniques to analyze this (trace
> tool, port mirroring, etc.), but often times requires quite high
> knowledge and understanding of the pipeline and OVS itself to
> effectively trace where a packet got dropped. Furthermore, there could
> be some scenarios where the packet can be silently dropped.
> I came across this patch [0] and presentation about it [1] which aims
> to tackle partly the problem described here (focusing in the DPDK
> datapath).
> The intent of this email is to gather some feedback as how to provide
> efficient tools and techniques to troubleshoot OVS/OVN issues and what
> do you think is immediately missing in this context.

I guess that there are multiple things to do here:

- Better document the tools that are available.

- Implement improvements, especially UX-wise, to the existing tools.

- Identify gaps in the available tools (and then fill them).

Do you have any good anecdotes about user/admin frustration?  They might
be helpful for figuring out how to help.  A lot of us here designed and
built this stuff and so the gaps are not always obvious to us.

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