Re: [libvirt] Disk snapshot mode proposal: patch for storing the snapshot mode from .vmx to .xml

Anyway, you decided to add an snapshot_mode attribute to the disk element and exposed the VMX values there. I'm not sure that this is a good idea as scsi0:0.mode affects two aspects. scsi0:0.mode can basically have three different modes - persistent, the default, a disk with this mode will take part in snapshots and changes to the disk's content persist domain power cycles and snapshot restoring. - independent-persistent, a disk with this mode will not take part in snapshots, but changes to the disk's content persist domain power cycles and snapshot restoring. - independent-nonpersistent, a disk with this mode will be not take part in snapshots and changes to the disk's content don't persist domain power cycles and snapshot restoring. This is realized by writing all changes into an additional .vmdk instead of the original .vmdk. This additional .vmdk is automatically deleted on domain power cycles and snapshot restoring. There are two additional but outdated modes undoable and nonpersistent that aren't supported anymore. So the two aspects scsi0:0.mode affects is snapshot and the persistence of changes. I think it makes more sense to use two attributes for the disk element to expose this. <disk ... snapshot='yes|no' persistent='yes|no'> - snapshot=yes persistent=yes maps to scsi0:0.mode=persistent - snapshot=yes persistent=no is unsupported for ESX - snapshot=no persistent=yes maps to scsi0:0.mode=independent-persistent - snapshot=no persistent=no maps to scsi0:0.mode=independent-nonpersistent

The more important question is: Is this an general concept or is it too ESX specific?

Eric is currently discussing/designing an extension to libvirt's snapshot/checkpoint capabilities. At the moment libvirt supports checkpointing a complete domain including RAM image and all storage volumes, but it doesn't support snapshotting of single storage volumes or a subset of the storage volumes of a domain. https://www.redhat.com/archives/libvir-list/2011-June/msg00761.html Eric suggest to extend the <domainsnapshot> and virStorageVol* APIs to allow to include only a subset of the domain's storage volumes in a checkpoint. This approach allows to specifiy for each checkpoint which storage volumes to include. ESX allows something similar with the independent modes, but you cannot define this on a per snapshot basis, but have to decided this before. Eric's approach is more flexible but doesn't work for ESX. I wonder if we could add the snapshot attribute/subelement to the disk element. This allows to set the independent mode for ESX and allows to define a preset for other hypervisors like QEMU that will support Eric's more flexible approach. So when you don't explicitly define which disk to include in a checkpoint in the <domainsnapshot> XML then the snapshot setting from the the <domain> XML apply. If there are no presets for snapshot it defaults to yes. For QEMU you could override the snapshot setting from the <domain> XML in the <domainsnapshot> XML, for ESX you either don't specify this in the <domainsnapshot> XML or have to match the settings from the <domain> XML due to the way ESX works.

The persistence setting might be more ESX specific, but I think libvirt could realize this for QEMU too, when the domain is using qcow2 images with a base image. In that case libvirt could clear the qcow2 image when the domain is restarted to realize persistent=no. I might be incorrect here as I'm not the QEMU expert here.

On a second thought we might want to use negative word so we don't add subelements for the defaults, for example <disk type='file' device='disk'> <source file='[datastore] test1/test1.vmdk'/> <target dev='sda' bus='scsi'/> <independent/> <nonpersistent/> <address type='drive' controller='0' bus='0' unit='0'/> </disk>