Re: [PATCH v0 0/4] background snapshot

[Peter Xu]

On Thu, Jul 23, 2020 at 11:03:55AM +0300, Denis Plotnikov wrote:
> 
> 
> On 22.07.2020 19:30, Peter Xu wrote:
> > On Wed, Jul 22, 2020 at 06:47:44PM +0300, Denis Plotnikov wrote:
> > > 
> > > On 22.07.2020 18:42, Denis Plotnikov wrote:
> > > > 
> > > > On 22.07.2020 17:50, Peter Xu wrote:
> > > > > Hi, Denis,
> > > > Hi, Peter
> > > > > ...
> > > > > > How to use:
> > > > > > 1. enable background snapshot capability
> > > > > >      virsh qemu-monitor-command vm --hmp migrate_set_capability
> > > > > > background-snapshot on
> > > > > > 
> > > > > > 2. stop the vm
> > > > > >      virsh qemu-monitor-command vm --hmp stop
> > > > > > 
> > > > > > 3. Start the external migration to a file
> > > > > >      virsh qemu-monitor-command cent78-bs --hmp migrate exec:'cat
> > > > > > > ./vm_state'
> > > > > > 4. Wait for the migration finish and check that the migration
> > > > > > has completed state.
> > > > > Thanks for continued working on this project! I have two high level
> > > > > questions
> > > > > before dig into the patches.
> > > > > 
> > > > > Firstly, is step 2 required?  Can we use a single QMP command to
> > > > > take snapshots
> > > > > (which can still be a "migrate" command)?
> > > > With this series it is required, but steps 2 and 3 should be merged into
> > > > a single one.
> > I'm not sure whether you're talking about the disk snapshot operations, 
> > anyway
> > yeah it'll be definitely good if we merge them into one in the next version.
> 
> After thinking for a while, I remembered why I split these two steps.
> The vm snapshot consists of two parts: disk(s) snapshot(s) and vmstate.
> With migrate command we save the vmstate only. So, the steps to save
> the whole vm snapshot is the following:
> 
> 2. stop the vm
>     virsh qemu-monitor-command vm --hmp stop
> 
> 2.1. Make a disk snapshot, something like
>     virsh qemu-monitor-command vm --hmp snapshot_blkdev drive-scsi0-0-0-0 
> ./new_data
> 3. Start the external migration to a file
>     virsh qemu-monitor-command vm --hmp migrate exec:'cat ./vm_state'
> 
> In this example, vm snapshot consists of two files: vm_state and the disk 
> file. new_data will contain all new disk data written since [2.1.] executing.

But that's slightly different to the current interface of savevm and loadvm
which only requires a snapshot name, am I right?  Now we need both a snapshot
name (of the vmstate) and the name of the new snapshot image.

I'm not familiar with qemu image snapshots... my understanding is that current
snapshot (save_snapshot) used internal image snapshots, while in this proposal
you want the live snapshot to use extrenal snapshots.  Is there any criteria on
making this decision/change?

Thanks,

-- 
Peter Xu