[libvirt-users] how to detect if qemu supports live disk snapshot
Francesco Romani
fromani at redhat.com
Mon Jan 13 11:57:26 UTC 2014
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
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