[Bug 1860759] Re: [REGRESSION] option `-snapshot` ignored with blockdev

[Eric Blake]

On 1/24/20 4:41 AM, Ildar wrote:
> Max, thanks a lot for the explanation.
> Do you mean that snapshot-ing isn't possible totally for blockdev? Then I
> guess some libvirt users are in trouble :((
> Actually I didn't quite caught the reason why a blockdev supports backing
> but not {backing to a file on /tmp then promptly deleted} ? What's the
> technical difference?
> 

On 1/24/20 4:05 AM, Max Reitz wrote:
> 
> 
> I don’t know much about libvirt, but I would have thought that any
> manual modification of the qemu command line isn’t supported and might
> always break.
> 
> Anyway, from a QEMU POV, -snapshot only works with -drive (this includes
> -hda, etc.).  It doesn’t work with -blockdev.  I can see that this isn’t
> documented for -snapshot, but basically whenever -blockdev is used, the
> user assumes full responsibility for the block graph (or at least that
> particular subgraph).  We cannot enable snapshot functionality then.

Libvirt has never produced a qemu command line containing '-snapshot'. 
Part of this is that libvirt wants to control SELinux settings, and 
letting qemu create a temporary overlay in /tmp in order to implement 
-snapshot does not play nicely with libvirt pre-creating all files that 
qemu is allowed to access.

The fact that you were able to manually add -snapshot to your qemu 
command line with older libvirt using -drive (I'm assuming you were also 
not using libvirt's SELinux support, because if you were, qemu would 
have been unable to create/access the temporary wrapper in /tmp), is a 
nice hack.  But since modern qemu has declared -snapshot to be 
unsupported with -blockdev, and modern libvirt has switched to 
-blockdev, I claim that this is not a qemu bug, but a libvirt feature 
request.

That said, libvirt has had a vision for a design for implementing the 
equivalent of -drive -snapshot: the <transient/> sub-element added to 
the domain/disk/source/driver element has been documented for a long time:

https://libvirt.org/formatdomain.html
"transient
     If present, this indicates that changes to the device contents 
should be reverted automatically when the guest exits. With some 
hypervisors, marking a disk transient prevents the domain from 
participating in migration or snapshots. Since 0.9.5 "

However, no one has yet implemented it for libvirt's qemu driver.  Part 
of our reluctance has been that we knew that implementing it would 
require libvirt to precreate the wrapper file on every guest start, and 
it is only very recently that we've even had enough functionality in 
libvirt's qemu driver coupled with new qemu commands to create qcow2 
images using QMP rather than having to shell out to qemu-img.  And part 
of it is that there was no point in implementing something to work with 
-drive, when we knew we had to rework everything for -blockdev anyways. 
But now that the work in libvirt to switch to -blockdev is done, it 
should be a lot easier to implement PROPER support for the <transient/> 
tag, at least for -blockdev usage.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org