Hi We need jumbo frames for containers since they need access to an internal network which has jumbo frames enabled and is used solely for NFS traffic.
I figured it is not possible to have jumbo frames enabled inside a CT. Even if the CT internally configures its interface to mtu 9000 and the host bridge is configured to mtu 9000, after starting the CT everything falls back to mtu 1500. It seems that bridges in general use the smallest mtu of all members. It also seems that the CT's host interface defaults to mtu 1500, no matter what the CT uses internally and no matter what is used on the bridge. I found the solution to be a small change in the skript /usr/sbin/vznetaddbr. The attached patch changes vznetaddr so that it sets an mtu of 9000 when it configures a host interface. This works fine for all situations, even for CTs and host bridges that actually use mtu 1500, since the host interface never creates any traffic (with possibly wrong mtu) but only transports it. Could this change be considered for future versions of vzctl? Or is there another|better way to get jumbo frames for containers? Cheers Roman
--- vznetaddbr 2014-11-25 15:40:56.994982738 +0100 +++ vznetaddbr.new 2014-11-25 16:51:40.140253183 +0100 @@ -32,6 +32,7 @@ echo "Adding interface $host_ifname to bridge $bridge on CT0 for CT$VEID" ip link set dev "$host_ifname" up ip addr add 0.0.0.0/0 dev "$host_ifname" + ifconfig "$host_ifname" mtu 9000 echo 1 >"/proc/sys/net/ipv4/conf/$host_ifname/proxy_arp" echo 1 >"/proc/sys/net/ipv4/conf/$host_ifname/forwarding" brctl addif "$bridge" "$host_ifname"
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