[libvirt] snapshot-create-as doesn't always work across libvirtd restarts

[Shanzhi Yu]

On 12/06/2014 03:33 PM, Eric Blake wrote:
> I just found out the hard way that we have a bug in snapshot-create-as,
> when I corrupted a guest.  I was testing with an offline domain:
>
> # virsh dumpxml win64 |grep -C1 driver
>      <disk type='block' device='disk'>
>        <driver name='qemu' type='raw'/>
>        <source dev='/dev/sda4'/>
> # virsh snapshot-create-as win64 --disk-only --diskspec
> hda,file=/var/lib/libvirt/images/win64.qcow2 --no-metadata
> Domain snapshot 1417850911 created
> # virsh dumpxml win64 |grep -C1 driver
>      <disk type='block' device='disk'>
>        <driver name='qemu' type='qcow2'/>
>        <source dev='/var/lib/libvirt/images/win64.qcow2'/>
> # systemctl restart libvirtd
> # virsh dumpxml win64 |grep -C1 driver
>      <disk type='block' device='disk'>
>        <driver name='qemu' type='raw'/>
>        <source dev='/dev/sda4'/>
>
> Even though we don't want to save the snapshot metadata, we DO need to
> save the XML change.  Otherwise, the restarted libvirtd sees the wrong
> disk as the source; worse, if the user started the guest before
> restarting libvirtd, they run the risk of accidentally reverting to the
> pre-snapshot state and losing all changes they made in the meantime,
> perhaps even getting their guest filesystem into an inconsistent state.
>
> I'm out of time to get to a root cause or fix before next week, but
> wanted to report it now.  I'm not sure if --no-metadata is essential to
> reproducing the bug, or if it happens in all cases.

Hi Eric,
I can reproduce it without "--no-metadata" option. Can you give the bug 
number if you have file it so that QE can track it
I post a patch 
http://www.redhat.com/archives/libvir-list/2014-December/msg00369.html. 
It works for me after a simple test


>
>
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-- 
Regards
shyu

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