[libvirt-users] Immutable backing files

[Peter Krempa]

On Thu, Sep 06, 2018 at 18:23:46 +0200, Gionatan Danti wrote:
> Il 06-09-2018 12:54 Peter Krempa ha scritto:
> > You forgot to specify the format of the backing file into the overlay
> > file (qemu-img option -F). Libvirt treats any unspecified format as raw
> > since it's not secure to do probing of the format.
> 
> Hi, the immutable base file *was* a raw image. Then a qcow2 overlay was
> added, but the vm (which accesses the overlay file) only boot if the base
> file is writable.

I suspect this is a permission problem. The backing file still needs to
be accessed by the VM so libvirt should correctly relabel it (if
possible) if the dynamic_ownership feature is enabled.

Since you are seeing problems it's possible that libvirt's treatment was
not enough. Libvirt's labelling e.g. does not ensure that the directory
user/group/label are correct, we only deal with the image. As the
messages you've shared have obfuscated paths I can't tell you if that
might be a problem.

If you are sure that the paths are accessible by the qemu process and it
still does not work, please attach debug logs and the XML file to a bug
report.

> It would surprise me if a read-only raw image + qcow2 overlay is an
> unsupported configuration, so I feel I am missing something...

No, that is a supported configuration and even the security should not
create a problem here as we assume the backing image is raw in this
case. That is only a problem if you have a qcow2 image as the base
image.

> 
> Thanks.
> 
> -- 
> Danti Gionatan
> Supporto Tecnico
> Assyoma S.r.l. - www.assyoma.it
> email: g.danti at assyoma.it - info at assyoma.it
> GPG public key ID: FF5F32A8
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 833 bytes
Desc: not available
URL: <http://listman.redhat.com/archives/libvirt-users/attachments/20180907/83ea6f5b/attachment.sig>