Re: [Qemu-block] [Qemu-devel] Combining -loadvm and -snapshot

[Drew DeVault]

On 2019-02-13 11:46 AM, Kevin Wolf wrote:
> Reading from the backing file would be correct in your special case
> (because the overlay was only just created and doesn't contain data
> yet), but generally speaking, this would make the disk content
> inconsistent because it would mix newer data from the overlay with old
> data from the snapshot in the backing file.
> 
> But before I make suggestions how this could be addressed (it's probably
> not trivial), let me ask what your real goal is?
> 
> I'm asking because when you start a VM with '-loadvm foo', this means
> that you already got a snapshot at this exact point. So the guest will
> write to the qcow2 file, but when you start the VM the next time, and
> again with '-loadvm foo', the newly written data will be discarded and
> QEMU will revert to the snapshot 'foo' again. So you already got the
> same functionality that -snapshot is supposed to provide, even without
> using it.
> 
> Do you have other reasons why you want to use -snapshot?

The issue is that if I start with -loadvm foo, it will start from the
foo snapshot, but any changes will be written to disk. Since qcow2 is a
sparse format, a mean guest could fill up all of the sectors and balloon
the underlying disk. The next guest starts from the snapshot, too - but
the host is wasting disk space. I'm also not able to run several guests
from the same qcow2 at once - which is a non-starter.