Hi,
I am seeing a lot of (almost 50%) over-mtu packets on the tap interfaces
used by KVM guests.
I just don't know if this is an issue with Open vSwitch, the tap driver,
KVM or the guest OS?
Any feedback on how I could further troubleshoot/deal with this issue would
be appreciated.
OS on the hypervisor is Debian 8, latest stable. With Open vSwitch 2.3.0
# dmesg | tail
[48548.208021] openvswitch: tap0: dropped over-mtu packet: 1501 > 1500
[48548.208029] openvswitch: tap0: dropped over-mtu packet: 1501 > 1500
[48548.208035] openvswitch: tap0: dropped over-mtu packet: 1501 > 1500
[48548.208094] openvswitch: tap0: dropped over-mtu packet: 1501 > 1500
[48548.243494] openvswitch: tap0: dropped over-mtu packet: 1501 > 1500
[48548.243512] openvswitch: tap0: dropped over-mtu packet: 1501 > 1500
[48548.243532] openvswitch: tap0: dropped over-mtu packet: 1501 > 1500
[48548.243538] openvswitch: tap0: dropped over-mtu packet: 1501 > 1500
[48548.243544] openvswitch: tap0: dropped over-mtu packet: 1501 > 1500
[48548.243550] openvswitch: tap0: dropped over-mtu packet: 1501 > 1500
Note: If I increase the MTU of the tap0 interface to 1501, I start seeing
MTU 1502 packets
# ovs-vsctl get interface tap0 statistics
{collisions=0, rx_bytes=7244, rx_crc_err=0, rx_dropped=3, rx_errors=0,
rx_frame_err=0, rx_over_err=0, rx_packets=35, tx_bytes=24616649,
tx_dropped=16732, tx_errors=0, tx_packets=33296}
# ovs-vsctl show
99411ad2-a91e-49ec-88ce-1afe9093c469
Bridge vmbrext
Port "vbond0"
Interface "eth0"
Interface "eth1"
Port "vlan92"
tag: 92
Interface "vlan92"
type: internal
Port "patchext-0"
Interface "patchext-0"
type: patch
options: {peer="patch0-ext"}
Port vmbrext
Interface vmbrext
type: internal
Bridge "vmbr0"
Port "vmbr0"
Interface "vmbr0"
type: internal
Port "vlan57"
tag: 57
Interface "vlan57"
type: internal
Port "tap0"
tag: 57
Interface "tap0"
Port "patch0-ext"
Interface "patch0-ext"
type: patch
options: {peer="patchext-0"}
ovs_version: "2.3.0"
# ip addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group
default
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9000 qdisc mq master
ovs-system state UP group default qlen 1000
link/ether 0c:c4:7a:14:25:48 brd ff:ff:ff:ff:ff:ff
inet6 fe80::ec4:7aff:fe14:2548/64 scope link
valid_lft forever preferred_lft forever
3: eth1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9000 qdisc mq master
ovs-system state UP group default qlen 1000
link/ether 0c:c4:7a:14:25:49 brd ff:ff:ff:ff:ff:ff
inet6 fe80::ec4:7aff:fe14:2549/64 scope link
valid_lft forever preferred_lft forever
4: ovs-system: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group
default
link/ether 0a:bc:10:fb:ae:e6 brd ff:ff:ff:ff:ff:ff
5: vmbrext: <BROADCAST,MULTICAST> mtu 9000 qdisc noop state DOWN group
default
link/ether 0c:c4:7a:14:25:48 brd ff:ff:ff:ff:ff:ff
6: vbond0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9000 qdisc noqueue state
UNKNOWN group default
link/ether d2:f1:5a:92:ed:a6 brd ff:ff:ff:ff:ff:ff
inet6 fe80::d0f1:5aff:fe92:eda6/64 scope link
valid_lft forever preferred_lft forever
7: vlan92: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9000 qdisc noqueue state
UNKNOWN group default
link/ether ae:47:99:40:9e:ab brd ff:ff:ff:ff:ff:ff
inet 10.62.16.101/21 brd 10.62.23.255 scope global vlan92
valid_lft forever preferred_lft forever
inet6 fe80::ac47:99ff:fe40:9eab/64 scope link
valid_lft forever preferred_lft forever
8: vmbr0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN group
default
link/ether 36:93:5d:b7:d1:40 brd ff:ff:ff:ff:ff:ff
9: vlan57: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state
UNKNOWN group default
link/ether 56:da:39:00:85:13 brd ff:ff:ff:ff:ff:ff
inet 10.69.48.91/24 brd 10.69.48.255 scope global vlan57
valid_lft forever preferred_lft forever
inet6 fe80::54da:39ff:fe00:8513/64 scope link
valid_lft forever preferred_lft forever
10: tap0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast
master ovs-system state UNKNOWN group default qlen 500
link/ether 9e:1e:7c:da:5e:77 brd ff:ff:ff:ff:ff:ff
inet6 fe80::9c1e:7cff:feda:5e77/64 scope link
valid_lft forever preferred_lft forever
Regards,
Emil Flink
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