Bug 1464792

Summary:	Allow to migrate a paused VM with Post-copy policy
Product:	[Community] Virtualization Tools	Reporter:	Shmuel Melamud <smelamud>
Component:	libvirt	Assignee:	Libvirt Maintainers <libvirt-maint>
Status:	CLOSED DEFERRED	QA Contact:
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Version:	unspecified	CC:	berrange, dzheng, fjin, libvirt-maint
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Last Closed:	2024-12-17 12:21:23 UTC	Type:	Bug
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Description Shmuel Melamud 2017-06-25 19:55:55 UTC

Description of problem:

Libvirt does not allow to migrate a paused VM with post-copy migration. If migration policy of a oVirt cluster contains a "postcopy" action, every attempt to migrate a paused VM fails because libvirt returns an error, not allowing migration to start:

libvirtError: argument unsupported: post-copy migration is not supported with n
on-live or paused migration

On the other side, the current oVirt Post-copy migration policy uses post-copy only as last resort, when a regular migration stalls. It is unlikely that migration will be stalling when migrating a paused VM, so it is safe to use a Post-copy policy with it.

I propose to allow migration to start with such migration policy. If migration stalls and "postcopy" action is reached, libvirt may cancel the migration at that moment.

Comment 1 Daniel Berrangé 2017-06-26 08:16:55 UTC

The benefit of post-copy, over traditional pre-copy mode, is that it can guarantee migration completion no matter how heavily the guest is dirtying RAM. If the VM is paused, then obviously the guest isn't dirtying RAM at all, at which point using post-copy instead of pre-copy is pointless.  That said, I don't see a particular reason why libvirt must forbid post-copy in this case.

Comment 2 Daniel Berrangé 2024-12-17 12:21:23 UTC

Thank you for reporting this issue to the libvirt project. Unfortunately we have been unable to resolve this issue due to insufficient maintainer capacity and it will now be closed. This is not a reflection on the possible validity of the issue, merely the lack of resources to investigate and address it, for which we apologise. If you none the less feel the issue is still important, you may choose to report it again at the new project issue tracker https://gitlab.com/libvirt/libvirt/-/issues The project also welcomes contribution from anyone who believes they can provide a solution.