Re: [Xen-devel] [RFC V9 1/4] domain snapshot terms

["Chun Yan Liu" <cyliu@xxxxxxxx>]

>>> On 12/18/2014 at 11:05 PM, in message 
>>> <1418915119.11882.79.camel@xxxxxxxxxx>,
Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote: 
> On Tue, 2014-12-16 at 14:32 +0800, Chunyan Liu wrote: 
> > Changes to V8: 
> >   * add a document for domain snapshot related terms, they will be 
> >     referred in later documents. 
> >  
> > ===================================================================== 
> > Terms 
> >  
> > * Active domain: domain created and started 
> >  
> > * Inactive domain: domain created but not started 
>  
> As Wei says I think you mean "defined" here, since created and started 
> are (essentially) synonyms for some toolstacks. 
>  
> You'll probably want to define "defined" too for clarity. 

OK. I'll update.

>  
> >  
> > * Domain snapshot: 
> >  
> >   Domain snapshot is a system checkpoint of a domain. It contains 
> >   the memory status at the checkpoint and the disk status. 
> >  
> > * Disk-only snapshot: 
> >  
> >   Disk-only snapshot only keeps the status of disk, not saving 
> >   memory status. 
> >  
> >   Contents of disks (whether a subset or all disks associated with 
> >   the domain) are saved at a given point of time, and can be restored 
> >   back to that state. On a running guest, a disk-only snapshot is 
> >   likely to be only crash-consistent rather than clean (that is, it 
> >   represents the state of the disk on a sudden power outage); on an 
> >   inactive guest, a disk-only snapshot is clean if the disks were 
> >   clean when the guest was last shut down. 
>  
> There is the possibility of doing clean snapshots if a guest agent is 
> involved to quiesce the disks at the right moment (e.g. I believe qemu 
> has such a thing, or at least I've seen talks about it being developed 
> at conferences).

Right. Qemu has it. Libvirt qemu driver supports that. 

> Are you including this possibility or explicitly ruling 
> it out of scope? 

Just libxl has no mechanism to quiesce the disks even when domain
is paused, I didn't mention this scenario. I can add it.

Chunyan

> 
> > * Live Snapshot: 
> >  
> >   Like live migration, it will increase size of the memory dump file, 
> >   but reducess downtime of the guest. 
> >  
> > * Internal Disk Snapshot 
> >  
> >   File formats such as qcow2 track both the snapshot and changes 
> >   since the snapshot in a single file. 
> >  
> > * External Disk Snapshot 
> >  
> >   The snapshot is one file, and the changes since the snapshot 
> >   are in another file. 
>  
>  
>  
>  



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