[libvirt] RFC: API additions for enhanced snapshot support

[Eric Blake]

On 06/20/2011 04:52 PM, Eric Blake wrote:
>> LVM snapshot may become invalid in the case of running out the volume size,
>> So, adding an  API for checking whether snapshot is valid will be needed.
> 
> I'm not quite sure I follow the scenario you are envisioning here.
> Would you mind stepping through an example (mapping [proposed] libvirt
> API calls to lvm commands would be helpful), on how an original lvm
> partition is created, then a snapshot partition, then how you run out of
> volume size in that snapshot?  It sounds like you are saying that there
> is a way to create an lvm snapshot that is valid at the time that is
> created, but later on, subsequent actions cause the snapshot to run out
> of space and no longer be valid.  But my understanding is that a
> snapshot is a constant size (it represents a known state of the disk at
> a fixed point in time), only the deltas to that snapshot (aka the live
> disk) ever have the potential to grow beyond the amount of storage used
> by the snapshot.  Or are you worried about creating an lvm snapshot by
> libvirt, but then a third party program changes the property of the lvm
> snapshot volume to change its size?

Or is it something like the following scenario:

start with 10G lvm partition
create snapshot A to a 10GB partition
make 2G worth of changes
create snapshot B to a 2GB partition
decide to delete snapshot A (merging it back into B)

at that point, the 10G partition for the live file is still adequate,
but now the effort of merging the 10G of data from snapshot into the 2G
of storage reserved for snapshot B will fail, because the partition for
B is not large enough.

But I still don't see how that needs a new API; it seems like this would
be a case of making the 'delete A' operation fail rather than a case of
silently making snapshot B invalid.  If the only way to make a snapshot
lvm partition invalid is by making changes to that lvm partition outside
the knowledge of libvirt, then I don't know that libvirt can protect
itself against such 3rd party actions; and if libvirt's own actions can
never cause an invalidation, then what is the point of adding an API to
detect an invalid snapshot?  But I'm relatively inexperienced with lvm,
so an example of what you mean will go a long way to help me understand
the scenario you are envisioning.

-- 
Eric Blake   eblake at redhat.com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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