Re: [Qemu-devel] Correct syntax for named snapshots

[Stefan Hajnoczi]

On Wed, Oct 26, 2011 at 05:24:01PM +0100, Ottavio wrote:
> According to the latest doc file you create a named snapshot like this:
> 
> qemu-img snapshot [-l | -a snapshot | -c snapshot | -d snapshot] 
> base-image.img
> 
> Is this other following syntax still supported or deprecated:
> 
> "qemu-img create -f qcow2 -b base-image.img snapshot.img"  ?
> 
> The latter is on the qemu wiki but not on the qemu-doc.html file.
> 
> Can you cc: me please? Thanks

These two commands do different things.  There are two different
snapshot concepts: internal and external snapshot.

Internal snapshots can be created with qcow2 and can be manipulated with
the qemu-img snapshot command.  They are called "internal" because all
snapshot data is part of the same image file; you can have a single
qcow2 file that contains 10 different snapshots.

External snapshots can be created with any format that supports backing
files (qcow2, qed, vmdk, ...) and is created with qemu-img create -o
backing_file=base-image.img -f qcow2 vm001.qcow2 (or with the older -b
option).  They are called "external" snapshots because they involve
multiple files and data is stored separately.  In the command-line I
gave you could access base-image.img in read-only fashion - the base
image is the "snapshot" and the vm001.qcow2 image file contains data
written since creating the new image file.

The word "snapshot" is overloaded in QEMU, we use it for several
different things, but I hope this explanation helps.

Stefan