Re: [libvirt] [RFC]: Snapshot API v3

On 03/30/2010 08:14 PM, Matthias Bolte wrote: > 2010/3/30 Chris Lalancette <clalance(a)redhat.com&gt;: >> Hello, >>     After our discussions about the snapshot API last week, I went ahead and implemented >> quite a bit of the API.  I also went back to the ESX, Virtualbox, and QEMU API's to >> try and make sure our API's matched up.  What's below is my revised API based on >> that survey.  Following my revised API are notes that I took regarding how the >> libvirt API matches up to the various API's, and some questions about semantics that >> I had while doing the survey.  More comments and questions are welcome.

>> Mapping of our interface to various hypervisors: >> +-------------------------------+-----------------+-------------------+------------------------------+ >> | Libvirt                       | Qemu            | Virtualbox        | ESX                          | >> +-------------------------------+-----------------+-------------------+------------------------------+ >> | virDomainSnapshotCreateXML    | monitor command | takeSnapshot      | CreateSnapshot_task          | >> |                               | "savevm"; if    | Snapshots can     | takes a name, description,   | >> |                               | snapshot name   | be taken on       | memory (true/false) and      | >> |                               | is already in   | powered off,      | quiesce (true/false).        | >> |                               | use, replaces   | saved, running,   | What does "memory" mean?     | > > If memory is true, ESX snapshots the memory of the domain too, > otherwise only a disk snapshot is created. > > Creating a disk-only snapshot is nearly instant, while creating a > memory snapshot also requires a notable amount of time to write the > memory image to disk. Sorry, I misread the documentation yesterday.  That's fairly clear. What's less clear to me is what happens when you take a disk-only snapshot, and then try to RevertToSnapshot from a running VM.  What happens in that case?

> >> |                               | the previous    | or paused VMs.    | Should we model "quiesce"    | > > The vSphere API docs give a good description what the quiesce option does: > > "If TRUE and the virtual machine is powered on when the snapshot is > taken, VMware Tools is used to quiesce the file system in the virtual > machine. This assures that a disk snapshot represents a consistent > state of the guest file systems. If the virtual machine is powered off > or VMware Tools are not available, the quiesce flag is ignored." > > I assume "quiesce the file system" means to flush write caches and > stuff like that. > > This option is important if you want to create a disk-only snapshot of > a running domain. Exactly.  I'm not sure this is going to be possible in general (and I guess it's not even really possible in ESX unless you install VMware Tools inside the guest).  I'm inclined not to model it at the moment, although I could be convinced otherwise.