libvirt users - Apr 2014 - Help with understanding and solving snapshot problem

Hello

Fairly new to libvirt.  I’m hoping to both solve a problem with this question as
well as learn more detail about how libvirt works.

Using RHEL 6.4 and libvirt version is 0.10.2 and qemu-img version is 0.12.1.2

Using virt-manager I created a VM.  Nothing unusual as far as I can see.  I then
added a disk. So I have a second virtio based volume which the guest then mounts
as a separate filesystem.  At some stage after that I created a snapshot of the
system.

I can’t delete the snapshot.   The end result is I’d like to create a snapshot
that excludes the second (much larger) disk - that is backed up via conventional
backup systems.  But the issue is first deleting the snapshot and my
understanding of what its saying.

So here is some basic info:

# virsh snapshot-list host1
 Name                 Creation Time             State
------------------------------------------------------------
 snap1-host1           2014-01-19 16:59:10 +1100 shutoff

# 
# virsh snapshot-info host1 --current
Name:           snap1-host1
Domain:         host1
Current:        yes
State:          shutoff
Location:       internal
Parent:         -
Children:       0
Descendants:    0
Metadata:       yes

So I try and delete it:

# virsh snapshot-delete host1 --current

error: Failed to delete snapshot snap1-host1
error: unsupported configuration: deletion of 1 external disk snapshots not
supported yet

Why does it say “external” in the above error, when the snapshot-info says its
Location is internal? Are they not related ideas?

The img file /var/lib/libvirt/images/host1.img  is not being used by KVM.  the
snapshot files are in use instead:

# virsh domblklist host1
Target     Source
------------------------------------------------
vda        /var/lib/libvirt/snap1-host1.qcow2
vdb        /var/lib/libvirt/snap1-host1-db.qcow2
hdc        -

Querying them:

# qemu-img info /var/lib/libvirt/snap1-host1.qcow2 
image: /var/lib/libvirt/snap1-host1.qcow2
file format: qcow2
virtual size: 24G (25769803776 bytes)
disk size: 3.5G
cluster_size: 65536

[root@cocoa libvirt]# qemu-img info /var/lib/libvirt/snap1-host1-db.qcow2 
image: /var/lib/libvirt/snap1-host1-db.qcow2
file format: qcow2
virtual size: 1.6T (1800279089664 bytes)
disk size: 325G
cluster_size: 65536

What have I done wrong and how can I correct this so I can discard the existing
snapshot and have a snapshot of the system excluding the second disk (and
optionally which I can re-take at intervals and discard old ones)?

Many thanks,

rolf.

On 04/10/2014 12:00 AM, rolf wrote:
> Hello
> 
> Fairly new to libvirt.  I’m hoping to both solve a problem with this
question as well as learn more detail about how libvirt works.
[Can you convince your mailer to wrap long lines?  It makes it easier
for other readers]
> 
> Using RHEL 6.4 and libvirt version is 0.10.2 and qemu-img version is
0.12.1.2
Have you considered raising this as a support request with Red Hat?
From the upstream perspective, 0.10.2 is quite old, and Red Hat may be
better equipped to answer questions about what snapshot support they
have backported to that version of RHEL.  In particular, the fact that
you are not using RHEL 6.5 is a bit worrisome, and I also understand
that RHEL 6.x tends to not support internal snapshots.
> 
> Using virt-manager I created a VM.  Nothing unusual as far as I can see.  I
then added a disk. So I have a second virtio based volume which the guest then
mounts as a separate filesystem.  At some stage after that I created a snapshot
of the system.
> 
> I can’t delete the snapshot.   The end result is I’d like to create a
snapshot that excludes the second (much larger) disk - that is backed up via
conventional backup systems.  But the issue is first deleting the snapshot and
my understanding of what its saying.
> 
> So here is some basic info:
> 
> # virsh snapshot-list host1
>  Name                 Creation Time             State
> ------------------------------------------------------------
>  snap1-host1           2014-01-19 16:59:10 +1100 shutoff
> 
> # 
> # virsh snapshot-info host1 --current
> Name:           snap1-host1
> Domain:         host1
> Current:        yes
> State:          shutoff
> Location:       internal
So the fact that you created an internal snapshot may have already put
you in unsupported territory for the versions of software that you are
using.  That said, I can still try to help, and I hope that upstream
behaves nicer in this regards, although you have certainly given us
enough steps to try and reproduce if this is still a bug in upstream.
Or maybe the bug is here, and you really did create an external snapshot
but the code is reporting it incorrectly.  Can you post the actual
command that you used to create the snapshot?
> Parent:         -
> Children:       0
> Descendants:    0
> Metadata:       yes
> 
> So I try and delete it:
> 
> # virsh snapshot-delete host1 --current
> 
> error: Failed to delete snapshot snap1-host1
> error: unsupported configuration: deletion of 1 external disk snapshots not
supported yet
> 
> Why does it say “external” in the above error, when the snapshot-info says
its Location is internal? Are they not related ideas?
It's very likely that you have tripped up on a bug, and perhaps on a bug
that has been fixed in the meantime in newer libvirt, which either needs
backporting to the RHEL version, or which Red Hat has deemed not worth
backporting because of their level of limited snapshot support in RHEL
6.  But again, going through a Red Hat support ticket will get faster
results than asking upstream.
> 
> The img file /var/lib/libvirt/images/host1.img  is not being used by KVM. 
the snapshot files are in use instead:
> 
> # virsh domblklist host1
> Target     Source
> ------------------------------------------------
> vda        /var/lib/libvirt/snap1-host1.qcow2
> vdb        /var/lib/libvirt/snap1-host1-db.qcow2
> hdc        -
> 
> Querying them:
> 
> # qemu-img info /var/lib/libvirt/snap1-host1.qcow2 
> image: /var/lib/libvirt/snap1-host1.qcow2
> file format: qcow2
> virtual size: 24G (25769803776 bytes)
> disk size: 3.5G
> cluster_size: 65536
No external backing file listed; but I'm not sure how this relates to
the original snapshot.  Maybe you also want to show 'virsh
snapshot-dumpxml host1 snap1-host1' to make it more obvious what the
domain was using at the time you took the snapshot?
> 
> [root@cocoa libvirt]# qemu-img info /var/lib/libvirt/snap1-host1-db.qcow2 
> image: /var/lib/libvirt/snap1-host1-db.qcow2
> file format: qcow2
> virtual size: 1.6T (1800279089664 bytes)
> disk size: 325G
> cluster_size: 65536
> 
> What have I done wrong and how can I correct this so I can discard the
existing snapshot and have a snapshot of the system excluding the second disk
(and optionally which I can re-take at intervals and discard old ones)?
I don't quite have a full picture of how you got into the situation.  If
you are trying to just get rid of the snapshot, you could always try
'virsh snapshot-delete --metadata host1 snap1-host1', to make libvirt
forget about the snapshot without cleaning up any actual data (leaving
any external backing chains intact, and not removing internal snapshots
from qcow2 files).

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

Hello

Thanks heaps for your suggestions.  responses inline.

On 10 Apr 2014, at 10:34 pm, Eric Blake <eblake@rehost1hat.com>
wrote:
> 
> [ … ]
> [Can you convince your mailer to wrap long lines?  It makes it easier
> for other readers]
I’ll try and keep the lines short. I don’t see any obvious setting to change the
wrap.
>> 
>> Using RHEL 6.4 and libvirt version is 0.10.2 and qemu-img version is
0.12.1.2
> 
> Have you considered raising this as a support request with Red Hat?
My error. I am using RHEL 6.5. Sorry about the wrong info.

I could put this is to Red Hat but thought the mailing list might be a better
initial place.
> From the upstream perspective, 0.10.2 is quite old, and Red Hat may be
> better equipped to answer questions about what snapshot support they
> have backported to that version of RHEL.  In particular, the fact that
> you are not using RHEL 6.5 is a bit worrisome, and I also understand
> that RHEL 6.x tends to not support internal snapshots.
[ … ]
>> # virsh snapshot-list host1
>> Name                 Creation Time             State
>> ------------------------------------------------------------
>> snap1-host1           2014-01-19 16:59:10 +1100 shutoff
>> 
>> # 
>> # virsh snapshot-info host1 --current
>> Name:           snap1-host1
>> Domain:         host1
>> Current:        yes
>> State:          shutoff
>> Location:       internal
> 
> So the fact that you created an internal snapshot may have already put
> you in unsupported territory for the versions of software that you are
> using.  That said, I can still try to help, and I hope that upstream
> behaves nicer in this regards, although you have certainly given us
> enough steps to try and reproduce if this is still a bug in upstream.
> Or maybe the bug is here, and you really did create an external snapshot
> but the code is reporting it incorrectly.  Can you post the actual
> command that you used to create the snapshot?
> 
Unfortunately no as I cannot remember it.  I have a feeling that it was a menu
item
in the virt-manager gui, but that said I can’t find it.  It is most likely to
have been the simplest
form of the virsh snapshot-create command.

>> Parent:         -
>> Children:       0
>> Descendants:    0
>> Metadata:       yes
>> 
>> So I try and delete it:
>> 
>> # virsh snapshot-delete host1 --current
>> 
>> error: Failed to delete snapshot snap1-host1
>> error: unsupported configuration: deletion of 1 external disk snapshots
not supported yet
>> 
>> Why does it say “external” in the above error, when the snapshot-info
says its Location is internal? Are they not related ideas?
> 
> It's very likely that you have tripped up on a bug, and perhaps on a
bug
> that has been fixed in the meantime in newer libvirt, which either needs
> backporting to the RHEL version, or which Red Hat has deemed not worth
> backporting because of their level of limited snapshot support in RHEL
> 6.  But again, going through a Red Hat support ticket will get faster
> results than asking upstream.
Thanks.  I just did a yum update to libvirt and got
libvirt-0.10.2-29.el6_5.7.x86_64.rpm
Which is not a big change.

In any event the error persists unchanged when I try to delete the snapshot.
>> 
>> The img file /var/lib/libvirt/images/host1.img  is not being used by
KVM.  the snapshot files are in use instead:
>> 
>> # virsh domblklist host1
>> Target     Source
>> ------------------------------------------------
>> vda        /var/lib/libvirt/snap1-host1.qcow2
>> vdb        /var/lib/libvirt/snap1-host1-db.qcow2
>> hdc        -
>> 
>> Querying them:
>> 
>> # qemu-img info /var/lib/libvirt/snap1-host1.qcow2 
>> image: /var/lib/libvirt/snap1-host1.qcow2
>> file format: qcow2
>> virtual size: 24G (25769803776 bytes)
>> disk size: 3.5G
>> cluster_size: 65536
> 
> No external backing file listed; but I'm not sure how this relates to
> the original snapshot.  Maybe you also want to show 'virsh
> snapshot-dumpxml host1 snap1-host1' to make it more obvious what the
> domain was using at the time you took the snapshot?
ok. That output shows:

# virsh snapshot-dumpxml host1 snap1-host1
<domainsnapshot>
  <name>snap1-host1</name>
  <description>After install completed</description>
  <state>shutoff</state>
  <creationTime>1390111150</creationTime>
  <memory snapshot='no'/>
  <disks>
    <disk name='vda' snapshot='external'>
      <driver type='qcow2'/>
      <source file='/var/lib/libvirt/snap1-host1.qcow2'/>
    </disk>
    <disk name='vdb' snapshot='external'>
      <driver type='qcow2'/>
      <source file='/var/lib/libvirt/snap1-host1-db.qcow2'/>
    </disk>
    <disk name='hdc' snapshot='no'/>
  </disks>
  <domain type='kvm'>
    <name>host1</name>
    <uuid>e1a43a89-af8f-95e2-e242-a42a44afc127</uuid>
    <memory unit='KiB'>16777216</memory>
    <currentMemory unit='KiB'>16777216</currentMemory>
    <vcpu placement='static'>6</vcpu>
    <os>
      <type arch='x86_64'
machine='rhel6.5.0'>hvm</type>
      <boot dev='hd'/>
      <bootmenu enable='no'/>
    </os>
    <features>
      <acpi/>
      <apic/>
      <pae/>
    </features>
    <clock offset='utc'/>
    <on_poweroff>destroy</on_poweroff>
    <on_reboot>restart</on_reboot>
    <on_crash>restart</on_crash>
    <devices>
      <emulator>/usr/libexec/qemu-kvm</emulator>
      <disk type='file' device='disk'>
        <driver name='qemu' type='raw'
cache='none'/>
        <source file='/var/lib/libvirt/images/host1.img'/>
        <target dev='vda' bus='virtio'/>
        <address type='pci' domain='0x0000'
bus='0x00' slot='0x05' function='0x0'/>
      </disk>
      <disk type='block' device='disk'>
        <driver name='qemu' type='raw' cache='none'
io='native'/>
        <source dev='/dev/sdd1'/>
        <target dev='vdb' bus='virtio'/>
        <address type='pci' domain='0x0000'
bus='0x00' slot='0x07' function='0x0'/>
      </disk>
      <disk type='file' device='cdrom'>
        <driver name='qemu' type='raw'/>
        <target dev='hdc' bus='ide'/>
        <readonly/>
        <address type='drive' controller='0' bus='1'
target='0' unit='0'/>
      </disk>
      <controller type='ide' index='0'>
        <address type='pci' domain='0x0000'
bus='0x00' slot='0x01' function='0x1'/>
      </controller>
      <controller type='usb' index='0'/>
      <interface type='bridge'>
        <mac address='52:54:00:97:0e:67'/>
        <source bridge='br3'/>
        <model type='virtio'/>
        <address type='pci' domain='0x0000'
bus='0x00' slot='0x03' function='0x0'/>
      </interface>
      <serial type='pty'>
        <target port='0'/>
      </serial>
      <console type='pty'>
        <target type='serial' port='0'/>
      </console>
      <input type='tablet' bus='usb'/>
      <input type='mouse' bus='ps2'/>
      <graphics type='vnc' port='-1'
autoport='yes'/>
      <sound model='ich6'>
        <address type='pci' domain='0x0000'
bus='0x00' slot='0x04' function='0x0'/>
      </sound>
      <video>
        <model type='cirrus' vram='9216'
heads='1'/>
        <address type='pci' domain='0x0000'
bus='0x00' slot='0x02' function='0x0'/>
      </video>
      <memballoon model='virtio'>
        <address type='pci' domain='0x0000'
bus='0x00' slot='0x06' function='0x0'/>
      </memballoon>
    </devices>
  </domain>
</domainsnapshot>

Not completely sure what to derive from that output?
>> [root@cocoa libvirt]# qemu-img info
/var/lib/libvirt/snap1-host1-db.qcow2
>> image: /var/lib/libvirt/snap1-host1-db.qcow2
>> file format: qcow2
>> virtual size: 1.6T (1800279089664 bytes)
>> disk size: 325G
>> cluster_size: 65536
>> 
> 
> I don't quite have a full picture of how you got into the situation. 
If
> you are trying to just get rid of the snapshot, you could always try
> 'virsh snapshot-delete --metadata host1 snap1-host1', to make
libvirt
> forget about the snapshot without cleaning up any actual data (leaving
> any external backing chains intact, and not removing internal snapshots
> from qcow2 files).
ok. What implications does this have long term?
Is there then a related step to remove the data of the snapshot that is no
longer referenced?
Reclaiming the space would be handy.

And as before, given the structure of the VM and its two disks, how is a
snapshot created
excluding the second disk? vdb in the above xml output?

Many thanks for your help so far.

regards

rolf.

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