Re: [RFC V8 2/3] libxl domain snapshot API design

[Wei Liu <wei.liu2@citrix.com>]

On Mon, Dec 08, 2014 at 11:12:14AM +0000, Wei Liu wrote:
> On Mon, Dec 08, 2014 at 12:34:47AM -0700, Chun Yan Liu wrote:
> > 
> > 
> > >>> On 12/6/2014 at 12:06 AM, in message
> > <20141205160615.GA24938@zion.uk.xensource.com>, Wei Liu <wei.liu2@citrix.com>
> > wrote: 
> > > I have to admit I'm confused by the back and forth discussion. It's hard 
> > > to justify the design of new API without knowing what the constraints 
> > > and requirements are from your PoV. 
> > >  
> > > Here are my two cents, not about details, but about general constraints. 
> > >  
> > > There are two layers, one is user of libxl (clients -- xl, libvirt etc) 
> > > and libxl (the library itself). 
> > >  
> > > 1. it's better to *not* have storage management in libxl. 
> > >  
> > > It's likely that clients can have their own management functionality 
> > > already.  I'm told that libvirt has that as well as XAPI. Having this 
> > > functionality in libxl is a bit redundant and requires lots of work 
> > > (enlighten libxl on what a disk looks like and call out to various 
> > > utilities). 
> > 
> > Thanks Wei and Ian for your reply. We did have much discussion around
> > can/cannot (e.g. xl can finish disk snapshot?) and should/shouldnot
> > (e.g. disk snapshot process should not in xl? domain_snapshot_delete
> > should not in libxl?), and confusing because have different ideas. So,
> > settling it down is helpful.
> > 
> > Talking about libvirt, it does provide storage management but through
> > storage pools and volumes. But usually, we don't use storage pool/vol
> > but directly use backend files, then libvirt storage driver can not
> > manage them. And for libvirt vol, functionality in storage driver is
> > limited, at least 'snapshot' cannot be done. So, libvirt side also
> > needs to add codes to handle disk snapshot, quite like xl does.
> > 
> 
> OK, so I take it that libvirt can be completely out the picture? I mean,
> it's not a requirement for you to integrate with libvirt?
> 
> I was thinking a stack like this when I replied:
> 
>   libvirt: manages snapshot (including storage snapshot)
>   libxl
>   [other lower level stuffs]
> 
> While I read from your reply, libvirt doesn't have that functionality
> (or very limited), so you would like to do things like:
> 
>   libvirt (or your homebrew toolstack)
>   xl      (xl or libxl manages domain snapshots)
>   libxl
>   [other lower level stuffs]
> 

Note, I'm in no way endorsing this approach. This is my understanding
(or misunderstanding) of what you intended to do. I started drawing
pictures so that we can understand each other better.

Wei.