Re: [libvirt] RFC: Exposing backing chains in <domain> XML

On 03/12/2014 02:21 PM, Eric Blake wrote: > tl;dr: > I am working on a series of patches to expose backing chain information > in <domain> XML. Comments are welcome, to make sure my XML design is on > the right track. > > > 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> Now that I've been working on virStorageSource for a while, I think I want to modify this slightly. Most attributes of <driver> are per-device (things you only set once, such as cache='writethrough'); furthermore, the <volume> XML for storage volumes uses a dedicated <format type='qcow2'/> rather than a <driver> element. For back-compat, we can't drop the old spelling, but I'm thinking the backing chain should prefer <format>. So if we use nested format, it would look something like: <disk type='file' device='disk'> <format type='qcow2'/> <source file='/var/lib/libvirt/images/top.qcow2'/> <backingStore type='file'> <format type='qcow2'/> <source file='/var/lib/libvirt/images/mid.qcow2'/> <backingStore type='file'> <format type='qcow2'/> <source file='/var/lib/libvirt/images/base.qcow2'/> <backingStore/> </backingStore> </backingStore> <driver name='qemu' type='qcow2' cache='writethrough'/> <target dev='vda' bus='virtio'/> </disk> Or with Dan's proposal to prefer a flat listing, something like: <disk type='file' device='disk'> <id index='1'/> <format type='qcow2'/> <source file='/var/lib/libvirt/images/top.qcow2'/> <backingStore type='file'> <id index='2' parent='1'/> <format type='qcow2'/> <source file='/var/lib/libvirt/images/mid.qcow2'/> </backingStore> <backingStore type='file'> <id index='3' parent='2'/> <format type='qcow2'/> <source file='/var/lib/libvirt/images/base.qcow2'/> </backingStore> <backingStore/> <driver name='qemu' type='qcow2' cache='writethrough'/> <target dev='vda' bus='virtio'/> </disk> As I look at it, I'm also worried that a flat listing makes it hard to tell the difference between a partial chain (for example, we know 'top' has a backing of 'mid', but haven't followed the chain to see if 'mid' also has a backing file) vs. an explicit end of the chain. At least with the nested listing, a <backingStore/> marker fits in better.