Re: [Qemu-devel] dropped pkts with Qemu on tap interace (RX)

[Stefan Priebe - Profihost AG]

Hi,
Am 02.01.2018 um 15:20 schrieb Wei Xu:
> On Tue, Jan 02, 2018 at 12:17:29PM +0100, Stefan Priebe - Profihost AG wrote:
>> Hello,
>>
>> currently i'm trying to fix a problem where we have "random" missing
>> packets.
>>
>> We're doing an ssh connect from machine a to machine b every 5 minutes
>> via rsync and ssh.
>>
>> Sometimes it happens that we get this cron message:
>> "Connection to 192.168.0.2 closed by remote host.
>> rsync: connection unexpectedly closed (0 bytes received so far) [sender]
>> rsync error: unexplained error (code 255) at io.c(226) [sender=3.1.2]
>> ssh: connect to host 192.168.0.2 port 22: Connection refused"
> 
> Hi Stefan,
> What kind of virtio-net backend are you using? Can you paste your qemu
> command line here?

Sure netdev part:
-netdev
type=tap,id=net0,ifname=tap317i0,script=/var/lib/qemu-server/pve-bridge,downscript=/var/lib/qemu-server/pve-bridgedown,vhost=on
-device
virtio-net-pci,mac=EA:37:42:5C:F3:33,netdev=net0,bus=pci.0,addr=0x12,id=net0,bootindex=300
-netdev
type=tap,id=net1,ifname=tap317i1,script=/var/lib/qemu-server/pve-bridge,downscript=/var/lib/qemu-server/pve-bridgedown,vhost=on,queues=4
-device
virtio-net-pci,mac=6A:8E:74:45:1A:0B,nedev=net1,bus=pci.0,addr=0x13,id=net1,vectors=10,mq=on,bootindex=301


> 'Connection refused' usually means that the client gets a TCP Reset rather
> than losing packets, so this might not be a relevant issue.

Mhm so you mean these might be two seperate ones?

> Also you can do a tcpdump on both guests and see what happened to SSH packets
> (tcpdump -i tapXXX port 22).

Sadly not as there's too much traffic on that part as rsync is syncing
every 5 minutes through ssh.

>> The tap devices on the target vm shows dropped RX packages on BOTH tap
>> interfaces - strangely with the same amount of pkts?
>>
>> # ifconfig tap317i0; ifconfig tap317i1
>> tap317i0  Link encap:Ethernet  HWaddr 6e:cb:65:94:bb:bf
>>           UP BROADCAST RUNNING PROMISC MULTICAST  MTU:1500  Metric:1
>>           RX packets:2238445 errors:0 dropped:13159 overruns:0 frame:0
>>           TX packets:9655853 errors:0 dropped:0 overruns:0 carrier:0
>>           collisions:0 txqueuelen:1000
>>           RX bytes:177991267 (169.7 MiB)  TX bytes:910412749 (868.2 MiB)
>>
>> tap317i1  Link encap:Ethernet  HWaddr 96:f8:b5:d0:9a:07
>>           UP BROADCAST RUNNING PROMISC MULTICAST  MTU:1500  Metric:1
>>           RX packets:1516085 errors:0 dropped:13159 overruns:0 frame:0
>>           TX packets:1446964 errors:0 dropped:0 overruns:0 carrier:0
>>           collisions:0 txqueuelen:1000
>>           RX bytes:1597564313 (1.4 GiB)  TX bytes:3517734365 (3.2 GiB)
>>
>> Any ideas how to inspect this issue?
> 
> It seems both tap interfaces lose RX pkts, dropping pkts of RX means the
> host(backend) cann't receive packets from the guest as fast as the guest 
> sends.

Inside the guest i see no dropped packets at all. It's only on the host
and strangely on both taps at the same value? And both are connected to
absolutely different networks.

> Are you running some symmetrical test on both guests? 

No.

Stefan


> Wei
> 
>>
>> Greets,
>> Stefan
>>