libvirt users - Jan 2014 - Re: how to detect if qemu supports live disk snapshot

Hi everyone,

Using the QEMU hypervisor, when a live disk snapshot is requested through
libvirt,
the request can fail if the underyling qemu binary lacks the snapshotting
support.

In python, we have something like

libvirtError: Operation not supported: live disk snapshot not supported with
this QEMU binary

I'd like to detect ahead of time if the underlying QEMU can or cannot do
snapshotting,
and disable right from the start the snapshotting support in my application,
avoiding to throw
errors to the user.

After reading from the docs, it seems that libvirt doesn't reports yet this
information about
the hypervisor in the capabilities XML: http://libvirt.org/formatcaps.html

a quick test against libvirt 1.1.3 (qemu 1.6.1) on Fedora 20 seems to confirm
(see XML dump below[1]).

So, if I'm not mistaken, looks like the only way to test for the support in
QEMU is to actually
request a snapshot and see what happens.
Is this right?

If this is correct, I would like to craft and propose a patch to libvirt to
export this information,
maybe into a form of some kind of warning list/blacklist: disk snapshotting it
is supposed to be
available, but this particular QEMU doesn't support it.

What would be the best form to export such information? the options I see are
* enhance the capabilities XML
* add a new API: something like (very rough first draft)

  int virDomainIsSnapshotSupported(virConnectPtr conn, unsigned int flags);

Thoughts? Any suggestion is welcome.

Best regards,

+++

[1]

# virsh -c qemu:///system
Welcome to virsh, the virtualization interactive terminal.

Type:  'help' for help with commands
       'quit' to quit

virsh # capabilities
<capabilities>

  <host>
    <uuid>01c4dc83-b652-cb11-9e6d-f3bf03b35d26</uuid>
    <cpu>
      <arch>x86_64</arch>
      <model>SandyBridge</model>
      <vendor>Intel</vendor>
      <topology sockets='1' cores='2'
threads='2'/>
      <feature name='erms'/>
      <feature name='smep'/>
      <feature name='fsgsbase'/>
      <feature name='rdrand'/>
      <feature name='f16c'/>
      <feature name='osxsave'/>
      <feature name='pcid'/>
      <feature name='pdcm'/>
      <feature name='xtpr'/>
      <feature name='tm2'/>
      <feature name='est'/>
      <feature name='smx'/>
      <feature name='vmx'/>
      <feature name='ds_cpl'/>
      <feature name='monitor'/>
      <feature name='dtes64'/>
      <feature name='pbe'/>
      <feature name='tm'/>
      <feature name='ht'/>
      <feature name='ss'/>
      <feature name='acpi'/>
      <feature name='ds'/>
      <feature name='vme'/>
    </cpu>
    <power_management>
      <suspend_mem/>
      <suspend_disk/>
      <suspend_hybrid/>
    </power_management>
    <migration_features>
      <live/>
      <uri_transports>
        <uri_transport>tcp</uri_transport>
      </uri_transports>
    </migration_features>
    <topology>
      <cells num='1'>
        <cell id='0'>
          <memory unit='KiB'>7871524</memory>
          <cpus num='4'>
            <cpu id='0' socket_id='0' core_id='0'
siblings='0-1'/>
            <cpu id='1' socket_id='0' core_id='0'
siblings='0-1'/>
            <cpu id='2' socket_id='0' core_id='1'
siblings='2-3'/>
            <cpu id='3' socket_id='0' core_id='1'
siblings='2-3'/>
          </cpus>
        </cell>
      </cells>
    </topology>
    <secmodel>
      <model>selinux</model>
      <doi>0</doi>
    </secmodel>
    <secmodel>
      <model>dac</model>
      <doi>0</doi>
    </secmodel>
  </host>

  <guest>
    <os_type>hvm</os_type>
    <arch name='i686'>
      <wordsize>32</wordsize>
      <emulator>/usr/bin/qemu-system-i386</emulator>
      <machine canonical='pc-i440fx-1.6'
maxCpus='255'>pc</machine>
      <machine maxCpus='255'>pc-q35-1.4</machine>
      <machine maxCpus='255'>pc-q35-1.5</machine>
      <machine canonical='pc-q35-1.6'
maxCpus='255'>q35</machine>
      <machine maxCpus='1'>isapc</machine>
      <machine maxCpus='255'>pc-0.10</machine>
      <machine maxCpus='255'>pc-0.11</machine>
      <machine maxCpus='255'>pc-0.12</machine>
      <machine maxCpus='255'>pc-0.13</machine>
      <machine maxCpus='255'>pc-0.14</machine>
      <machine maxCpus='255'>pc-0.15</machine>
      <machine maxCpus='255'>pc-1.0</machine>
      <machine maxCpus='255'>pc-1.1</machine>
      <machine maxCpus='255'>pc-1.2</machine>
      <machine maxCpus='255'>pc-1.3</machine>
      <machine maxCpus='255'>pc-i440fx-1.4</machine>
      <machine maxCpus='255'>pc-i440fx-1.5</machine>
      <machine maxCpus='1'>none</machine>
      <domain type='qemu'>
      </domain>
      <domain type='kvm'>
        <emulator>/usr/bin/qemu-kvm</emulator>
        <machine canonical='pc-i440fx-1.6'
maxCpus='255'>pc</machine>
        <machine maxCpus='255'>pc-q35-1.4</machine>
        <machine maxCpus='255'>pc-q35-1.5</machine>
        <machine canonical='pc-q35-1.6'
maxCpus='255'>q35</machine>
        <machine maxCpus='1'>isapc</machine>
        <machine maxCpus='255'>pc-0.10</machine>
        <machine maxCpus='255'>pc-0.11</machine>
        <machine maxCpus='255'>pc-0.12</machine>
        <machine maxCpus='255'>pc-0.13</machine>
        <machine maxCpus='255'>pc-0.14</machine>
        <machine maxCpus='255'>pc-0.15</machine>
        <machine maxCpus='255'>pc-1.0</machine>
        <machine maxCpus='255'>pc-1.1</machine>
        <machine maxCpus='255'>pc-1.2</machine>
        <machine maxCpus='255'>pc-1.3</machine>
        <machine maxCpus='255'>pc-i440fx-1.4</machine>
        <machine maxCpus='255'>pc-i440fx-1.5</machine>
        <machine maxCpus='1'>none</machine>
      </domain>
    </arch>
    <features>
      <cpuselection/>
      <deviceboot/>
      <acpi default='on' toggle='yes'/>
      <apic default='on' toggle='no'/>
      <pae/>
      <nonpae/>
    </features>
  </guest>

  <guest>
    <os_type>hvm</os_type>
    <arch name='x86_64'>
      <wordsize>64</wordsize>
      <emulator>/usr/bin/qemu-system-x86_64</emulator>
      <machine canonical='pc-i440fx-1.6'
maxCpus='255'>pc</machine>
      <machine maxCpus='255'>pc-q35-1.4</machine>
      <machine maxCpus='255'>pc-q35-1.5</machine>
      <machine canonical='pc-q35-1.6'
maxCpus='255'>q35</machine>
      <machine maxCpus='1'>isapc</machine>
      <machine maxCpus='255'>pc-0.10</machine>
      <machine maxCpus='255'>pc-0.11</machine>
      <machine maxCpus='255'>pc-0.12</machine>
      <machine maxCpus='255'>pc-0.13</machine>
      <machine maxCpus='255'>pc-0.14</machine>
      <machine maxCpus='255'>pc-0.15</machine>
      <machine maxCpus='255'>pc-1.0</machine>
      <machine maxCpus='255'>pc-1.1</machine>
      <machine maxCpus='255'>pc-1.2</machine>
      <machine maxCpus='255'>pc-1.3</machine>
      <machine maxCpus='255'>pc-i440fx-1.4</machine>
      <machine maxCpus='255'>pc-i440fx-1.5</machine>
      <machine maxCpus='1'>none</machine>
      <domain type='qemu'>
      </domain>
      <domain type='kvm'>
        <emulator>/usr/bin/qemu-kvm</emulator>
        <machine canonical='pc-i440fx-1.6'
maxCpus='255'>pc</machine>
        <machine maxCpus='255'>pc-q35-1.4</machine>
        <machine maxCpus='255'>pc-q35-1.5</machine>
        <machine canonical='pc-q35-1.6'
maxCpus='255'>q35</machine>
        <machine maxCpus='1'>isapc</machine>
        <machine maxCpus='255'>pc-0.10</machine>
        <machine maxCpus='255'>pc-0.11</machine>
        <machine maxCpus='255'>pc-0.12</machine>
        <machine maxCpus='255'>pc-0.13</machine>
        <machine maxCpus='255'>pc-0.14</machine>
        <machine maxCpus='255'>pc-0.15</machine>
        <machine maxCpus='255'>pc-1.0</machine>
        <machine maxCpus='255'>pc-1.1</machine>
        <machine maxCpus='255'>pc-1.2</machine>
        <machine maxCpus='255'>pc-1.3</machine>
        <machine maxCpus='255'>pc-i440fx-1.4</machine>
        <machine maxCpus='255'>pc-i440fx-1.5</machine>
        <machine maxCpus='1'>none</machine>
      </domain>
    </arch>
    <features>
      <cpuselection/>
      <deviceboot/>
      <acpi default='on' toggle='yes'/>
      <apic default='on' toggle='no'/>
    </features>
  </guest>

</capabilities>


-- 
Francesco Romani

On 01/13/2014 04:57 AM, Francesco Romani wrote:
> 
> After reading from the docs, it seems that libvirt doesn't reports yet
this information about
> the hypervisor in the capabilities XML: http://libvirt.org/formatcaps.html
> 
> a quick test against libvirt 1.1.3 (qemu 1.6.1) on Fedora 20 seems to
confirm (see XML dump below[1]).
Correct.  It might indeed be a useful addition to the capability XML.
> 
> So, if I'm not mistaken, looks like the only way to test for the
support in QEMU is to actually
> request a snapshot and see what happens.
> Is this right?
Unfortunately true for the moment.
> 
> If this is correct, I would like to craft and propose a patch to libvirt to
export this information,
> maybe into a form of some kind of warning list/blacklist: disk snapshotting
it is supposed to be
> available, but this particular QEMU doesn't support it.
Capability XML would be the proper way to expose this.

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

>
>
> >
> > So, if I'm not mistaken, looks like the only way to test for the
support
> in QEMU is to actually
> > request a snapshot and see what happens.
> > Is this right?
>
> Unfortunately true for the moment.
>
Hi, does this command can help with you?
  virsh qemu-monitor-command vmname  --hmp 'help' | grep snapshot_blkdev