[ovs-discuss] millions of packets going to a flow?

[Gabe Black]

I saw that FAQ prior to posting.  I probably should have mentioned that.  Unfortunately the ovs-appctl does not work and complains of not finding /usr/var/run/openvswitch/ovs-vswitchd.pid.  I tried both coping the pidfile and symlinking it to where ovs-appctl is looking, but it still failed with the same error message.

Running strace when the pid file is there shows that it does finde the file, but it performs a fcntl( ..., F_GETLK, {type=F_UNLCK,...} ...) which to me makes me believe it might be seeing if the file/process is locked and erroring out because it finds it can't unlock it.

The error message ovs-appctl displays is the same for both cases though: _cannot read pidfile "/usr/var/run/openvswitch/ovs-vswitchd.pid"_.
  
I just now out of curiousity tried adding the --pidfile to the ovs-dpdk init script (where it invokes vswitchd daemon, and found that now ovs-appctl works!  

I'll follow that faq again now armed with a working ovs-appctl. I wonder if a bug should be opened against this to add the --pidfile to the ovs-dpdk-init script.

Thanks for taking your time to help me out!   I'll post back with anything else I find that might be an defect.
Gabe



> -----Original Message-----
> From: Ben Pfaff [mailto:blp at nicira.com]
> Sent: Friday, August 28, 2015 9:08 AM
> To: Gabe Black
> Cc: Mooney, Sean K; bugs at openvswitch.org
> Subject: Re: [ovs-discuss] millions of packets going to a flow?
> 
> On Fri, Aug 28, 2015 at 03:01:52PM +0000, Gabe Black wrote:
> > However, all the commands I show in this inquiry are ovs-xyz commands.
> > My questions are around how to debug/troubleshoot where the packets
> > are going.  I am new to all of this, but to me this did seem like the
> > most relevant list to post that question.
> 
> Maybe you want this FAQ.
> 
> ### Q: I have a sophisticated network setup involving Open vSwitch, VMs or
>    multiple hosts, and other components.  The behavior isn't what I
>    expect.  Help!
> 
> A: To debug network behavior problems, trace the path of a packet,
>    hop-by-hop, from its origin in one host to a remote host.  If
>    that's correct, then trace the path of the response packet back to
>    the origin.
> 
>    Usually a simple ICMP echo request and reply ("ping") packet is
>    good enough.  Start by initiating an ongoing "ping" from the origin
>    host to a remote host.  If you are tracking down a connectivity
>    problem, the "ping" will not display any successful output, but
>    packets are still being sent.  (In this case the packets being sent
>    are likely ARP rather than ICMP.)
> 
>    Tools available for tracing include the following:
> 
>    - "tcpdump" and "wireshark" for observing hops across network
>      devices, such as Open vSwitch internal devices and physical
>      wires.
> 
>    - "ovs-appctl dpif/dump-flows <br>" in Open vSwitch 1.10 and
>      later or "ovs-dpctl dump-flows <br>" in earlier versions.
>      These tools allow one to observe the actions being taken on
>      packets in ongoing flows.
> 
>      See ovs-vswitchd(8) for "ovs-appctl dpif/dump-flows"
>      documentation, ovs-dpctl(8) for "ovs-dpctl dump-flows"
>      documentation, and "Why are there so many different ways to
>      dump flows?" above for some background.
> 
>    - "ovs-appctl ofproto/trace" to observe the logic behind how
>      ovs-vswitchd treats packets.  See ovs-vswitchd(8) for
>      documentation.  You can out more details about a given flow
>      that "ovs-dpctl dump-flows" displays, by cutting and pasting
>      a flow from the output into an "ovs-appctl ofproto/trace"
>      command.
> 
>    - SPAN, RSPAN, and ERSPAN features of physical switches, to
>      observe what goes on at these physical hops.
> 
>    Starting at the origin of a given packet, observe the packet at
>    each hop in turn.  For example, in one plausible scenario, you
>    might:
> 
>    1. "tcpdump" the "eth" interface through which an ARP egresses
>       a VM, from inside the VM.
> 
>    2. "tcpdump" the "vif" or "tap" interface through which the ARP
>       ingresses the host machine.
> 
>    3. Use "ovs-dpctl dump-flows" to spot the ARP flow and observe
>       the host interface through which the ARP egresses the
>       physical machine.  You may need to use "ovs-dpctl show" to
>       interpret the port numbers.  If the output seems surprising,
>       you can use "ovs-appctl ofproto/trace" to observe details of
>       how ovs-vswitchd determined the actions in the "ovs-dpctl
>       dump-flows" output.
> 
>    4. "tcpdump" the "eth" interface through which the ARP egresses
>       the physical machine.
> 
>    5. "tcpdump" the "eth" interface through which the ARP
>       ingresses the physical machine, at the remote host that
>       receives the ARP.
> 
>    6. Use "ovs-dpctl dump-flows" to spot the ARP flow on the
>       remote host that receives the ARP and observe the VM "vif"
>       or "tap" interface to which the flow is directed.  Again,
>       "ovs-dpctl show" and "ovs-appctl ofproto/trace" might help.
> 
>    7. "tcpdump" the "vif" or "tap" interface to which the ARP is
>       directed.
> 
>    8. "tcpdump" the "eth" interface through which the ARP
>       ingresses a VM, from inside the VM.
> 
>    It is likely that during one of these steps you will figure out the
>    problem.  If not, then follow the ARP reply back to the origin, in
>    reverse.