libvirt users - Jul 2012 - Questions related to the snapshot

Hi,

I faced two problems with the snapshot:

First issue, I can use the snapshot functions via the virsh correctly such
as snapshot-list. And when I removed the snapshot XML files from
/var/lib/libvirt/qemu/snapshot/**/, I don't see the snapshot of the Virtual
Machine any more. That's correct.
But after I copying  the XML files back, I can't see the the snapshot as
well. Only if I restart my laptop can I see them. However, restarting is
what I want to avoid. Because I wish I could utilize the functions in a
server in the future.
Does anyone have ideas about this, Did I do something wrong or  it's a
limitation.
So far, I am using the qemu-img command to check the snapshot and do the
revert without restarting my laptop. Because the snapshots are already
embedded in the img file.

Second issue, I find if I create the XML file of snapshot by using
getXMLDesc and output to a new file in python, there is a slightly
different. But, the new XML file can't be recognized even if I restart my
laptop. The new XML file miss the attribute <active>  as far as I can see,
is it so important?
Here is one example:
The original XML in the default folder:
<domainsnapshot>
  <name>Ex</name>
  <description>Snapshot of OS updates</description>
  <state>running</state>
  <parent>
    <name>1342427860</name>
  </parent>
  <creationTime>1342428835</creationTime>
  <domain>
    <uuid>2c853de2-6bba-ad26-bf05-0045e5f1beb9</uuid>
  </domain>
  <active>0</active>
</domainsnapshot>

The new XML file I created:
<domainsnapshot>
  <name>Ex</name>
  <description>Snapshot of OS updates</description>
  <state>running</state>
  <parent>
    <name>1342427860</name>
  </parent>
  <creationTime>1342428835</creationTime>
  <domain>
    <uuid>2c853de2-6bba-ad26-bf05-0045e5f1beb9</uuid>
  </domain>
</domainsnapshot>

Thank you so much.

Best Regards

Tan
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On 07/19/2012 05:38 AM, lin tan wrote:
> Hi,
> 
> I faced two problems with the snapshot:
> 
> First issue, I can use the snapshot functions via the virsh correctly such
> as snapshot-list.
So far, so good.
> And when I removed the snapshot XML files from
> /var/lib/libvirt/qemu/snapshot/**/,
Ouch.  You should never modify files in /var/lib/libvirt - they are
internal only.  If you want to remove a snapshot, you should do so
through the public API (virsh snapshot-delete, in this case).
> I don't see the snapshot of the Virtual
> Machine any more. That's correct.
No, it's sheer dumb luck.  You mucked with libvirt's internal storage,
so all bets are off.
> But after I copying  the XML files back, I can't see the the snapshot
as
> well. Only if I restart my laptop can I see them.
You might also get lucky and see them by restarting just libvirtd,
rather than your entire computer.
> However, restarting is
> what I want to avoid. Because I wish I could utilize the functions in a
> server in the future.
> Does anyone have ideas about this, Did I do something wrong or  it's a
> limitation.
What are you really trying to do?  Then we can tell you how to do it
using the APIs instead of mucking around with files behind libvirt's back.
> So far, I am using the qemu-img command to check the snapshot and do the
> revert without restarting my laptop. Because the snapshots are already
> embedded in the img file.
> 
> Second issue, I find if I create the XML file of snapshot by using
> getXMLDesc and output to a new file in python, there is a slightly
> different. But, the new XML file can't be recognized even if I restart
my
> laptop. The new XML file miss the attribute <active>  as far as I can
see,
> is it so important?
The attribute <active> is an internal-only detail, it only shows up in
the /var/lib/libvirt locations because that is where libvirt stores
internal-only information.  You should never directly manipulate it.
Instead, if you want to reinstate XML for a snapshot description after
having previously erased that metadata, the correct approach is:

virsh snapshot-create $dom $file --redefine [--current]

where the optional use of --current will control whether
/var/lib/libvirt sets the <active> internal element to 0 or 1.

-- 
Eric Blake   eblake at redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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