[libvirt] RFCv2: virDomainSnapshotCreateXML enhancements

[Eric Blake]

On 08/10/2011 04:08 PM, Eric Blake wrote:
> Next, we need a way to control which subset of disks is involved in a
> snapshot command. Previous mail has documented that for ESX, the
> decision can only be made at boot time - a disk can be persistent
> (involved in snapshots, and saves changes across domain boots);
> independent-persistent (is not involved in snapshots, but saves changes
> across domain boots); or independent-nonpersistent (is not involved in
> snapshots, and all changes during a domain run are discarded when the
> domain quits). In <domain> xml, I will represent this by two new
> optional attributes:
>
> <disk snapshot='no|external|internal' persistent='yes|no'>...</disk>

As I'm starting to code this, it looks more and more like persistent 
will always be a binary property.  Also, it doesn't appear in 
<domainsnapshot>, just in <domain>.  In that case, it would be nicer to 
represent it similar to other binary properties (readonly, sharable) - 
that is, as a sub-element that is omitted for the default, and present 
as <transient/> when overriding the default.

But making snapshot an attribute was definitely the right thing - since 
discussion certainly pointed out that we may eventually want more than 
one type of external snapshot ('external' means the original name 
becomes the snapshot while the new name is active, but we may come up 
with a good term and implementation where the original name remains 
active and creates a new file as the snapshot).  So I'm revising this 
implementation to be:

<disk snapshot='no|external|internal'>
   [<transient/>]
   ...
</disk>

-- 
Eric Blake   eblake at redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org