Re: [libvirt] RFC: Exposing backing chains in <domain> XML
On Wed, Mar 12, 2014 at 02:21:46PM -0600, Eric Blake wrote:
A backing chain of 3 files (base <- mid <- top) in the local
file system:
<disk type='file' device='disk'>
<driver name='qemu' type='qcow2'/>
<source file='/var/lib/libvirt/images/top.qcow2'/>
<backingStore type='file'>
<driver name='qemu' type='qcow2'/>
<source file='/var/lib/libvirt/images/mid.qcow2'/>
<backingStore type='file'>
<driver name='qemu' type='qcow2'/>
<source file='/var/lib/libvirt/images/base.qcow2'/>
<backingStore/>
</backingStore>
</backingStore>
<target dev='vda' bus='virtio'/>
</disk>
Libguestfs gets a disk image that we usually know nothing about.
Then we _either_:
(1) Add it to libvirt XML directly.
(2) Create an overlay on top of the disk image, and add the overlay
to the libvirt XML.
It seems like for (1) we don't need to change anything. For (2)
we might add:
<disk type='file' device='disk'>
<driver name='qemu' type='qcow2'/>
<source file='overlay.qcow2'/>
<backingStore type='file'>
<source file='disk.img'/>
</backingStore>
</disk>
Note not specifying disk.img's backing store (we don't know it).
Also, non-disk sources: nbd, iscsi, gluster, ceph etc. It's
especially hard to discover what is in these since it may involve
multiple opens [breaks nbd sometimes], from multiple processes
[security context issues]; and network connections are slower than
opening a local file.
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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