Hi Daniel, I have simulated a slow network by linux bridge on source host machine, but I found that I hardly connected guest by serial or vnc, it is pretty slow, I can't run other big application in the guest, so I change my idea, as long as I have a slow link, whether the network is in source or target machine, finally, I created slow network on target machine, the attachment is a shell script about the network creation process. Using 'tc' to throttle network traffic on the target host # tc qdisc show qdisc tbf 8004: dev eth0 root refcnt 2 rate 128000bit burst 128Kb lat 50.0ms qdisc pfifo_fast 0: dev eth1 root refcnt 2 bands 3 priomap 1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1 WRT big apps, I use stress tool to consume guest memory(current size is 4G), cmd line argument as following: stress --cpu 2 --io 2 --vm 50 --vm-bytes 128M --timeout 480s And then running migration, and open another terminal set maximum migration downtime 60000 milliseconds, the following error information is raised by libvirt: 1.open one terminal run migrate # virsh migrate rhel6 qemu+ssh://10.66.70.152/system root(a)10.66.70.152's password: 2.open another terminal set maximum migration downtime # virsh migrate-setmaxdowntime --domain rhel6 --downtime 60000 error: Requested operation is not valid: domain is not being migrated In fact, step 1 will never be finished. BTW, the guest can be successfully migrated if I don't use the above environment configuration. Best Regards, Alex