[pve-devel] Snapshot questions

Dietmar Maurer dietmar at proxmox.com
Mon Sep 7 10:44:23 CEST 2015

> > we
> > want to use it with LXC containers:
> > 
> > https://git.proxmox.com/?p=pve-container.git;a=summary
> > 
> > Above container management toolkit 'pct' already supports drbd, and it would
> > be really cool to have snapshots, so that we can make snapshot and 
> > snapshot backups ;-)
> The same should be easily possible for KVM, too.

Yes, but we do not need that for KVM backup purposes, because we have 
a better method with KVM.

> > But that code is quite new, so I would start with qemu, and implement those
> > advanced features later.
> Hmmm, I'm not sure where qemu comes in, when implementing snapshots for 
> > > While the Thin LVM 
> > > snapshots that DRBDmanage uses might be addressable locally[1], accessing 
> > > them is not recommended. Is that just for information purposes in the GUI?
> > 
> > We want to access them when making container backups. I think this should
> > work with dm-thin - why not? 
> Because _directly_ accessing them might change the data within them (at 
> least replay the filesystem journal).
> The use case we thought of is
>   1) install a VM
>   2) make a snapshot
>   3) use that snapshot as template for new VMs
> That's similar to what docker does.
> Now, if after (2) the snapshot data gets modified, the VMs created via (3) 
> might be different from each other (in unexpected ways, ie. the hostname, 
> ip, etc. _should_ be different - but other data not), which might then 
> cause support overhead.

We do not want to modify the snapshot. We only want read-only access.

> In Cinder the workflow is to create a new volume from a snapshot, even if 
> it's just being looked at ... so all the machines in (3) would have the 
> same (unmodified) snapshot as base.
> > We can extend the storage API with 'activate_snapshot_volume()' and 
> > 'deactivate_snapshot_volume()' if required?
> Well, perhaps.
> Would that make the snapshot itself available for access? 

there must be a way toö access snapshot data -  else we cannot 
(obviously) make a snapshot backup. It is up to the storage driver to
implement that.

> What if the 
> snapshot data is accessible only on another node and not on the current 
> one?

Well, the storage driver needs to solve that problem...

> >> - but using a snapshot for a new ("clean") VM isn't being done, right?

Yes, right.

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