Re: [Qemu-devel] Can the backing file of qcow2 points to a snapshot of base file?

[Max Reitz]

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