On 2015/7/6 20:09, Max Reitz wrote: > On 06.07.2015 10:20, vt wrote: >> Hi. >> If a base qcow2 image snapshot chain like this: >> base.qcow2: [A] -> [B] -> [C] >> [C] is the current image where guest read/write to,usually we create a >> new image base on the base.qcow2 like this >> qemu-img create -f qcow2 -o backing_file=/path/base.qcow2 new.qcow2 >> so the data of new.qcow2 is from [C] of base.qcow2, assuming the >> new.qcow2 has not been write to since created. >> What I want is that the data of new.qcow2 is from [B] of base.qcow2, >> like this: >> qemu-img create -f qcow2 -o >> backing_file=/path/base.qcow2,backing_snapshot=B new.qcow2 >> Can the qemu support it now? >> Thanks >> ------------------------------------------------------------------------ >> vt > > Hi vt, > > I don't think this is possible. For this to work, the snapshot would have to > be loaded temporarily, but the only place I can see in the qemu code base > where this is done is inside qemu-img and qemu-nbd, so qemu itself > appearently does not support this. > > What you can do is export the snapshot via qemu-nbd (-l) and then use that > NBD server as a backing file with qemu. This is not very nice, but I don't > suppose there are many people eager to implement the functionality you'd like > to have (although I don't think it would be too difficult, frankly…). > > Max
Thanks, Max! It seems that I have to implement the functionality by myself... Here is my plan: 1. Pass the snapshot name(id) by -drive 2. Get the snapshot name(id) in the bdrv_open_backing_file() -> qcow2_open() and load the snapshot header 3. Load BDRVQcowState.l1_table with the snapshot's l1 table Does it look ok? Thanks ------------------------------------------------------------------------ vt
