Hi , I hit a bug with libvirt+ qemu+kvm when doing follow steps: 1.initial status :vm = base+z(z is a snapshot) 2. call snapshotCreateXML( ): vm = base+z +z' 3. call blockRebase( ): vm = base+z''(z rebase to z') 4 .(after rebasing work done) vm shutdown and then start an error accord like this: unable to set user and group to '0:0' on '/tvm/tvm/405b9ea5-10a7-4e56-946d-58e93e2688d6/405b9ea5-10a7-4e56-946d-58e93e2688d6.vda_20141101044033': No such file or directory A second start may sucess. I debugged with gdb and found that the error message was cauesed by virSecurityDACSetSecurityDiskLabel misuse the disk chain(that is base+z +z', after rebase), and z' was deleted yet. why virSecurityDACSetSecurityDiskLabel used old disk chain after rebasing ? I finally find that In qemuProcessHandleBlockJob when rebase complete, disk chain in vm->def is updated but vm->newDef is not(when vm->newDef is not null). In shutdown( actrully in qemuProcessStop ), if vm->newDef is not null , vm->def will be replaced by vm->newDef, which contains a wrong disk chain!!! the next start will failed with above message.