[Xen-devel] Strange (???) xl behavior for save, migrate and migrate-receive

[Daniel Kiper <dkiper@xxxxxxxxxxxx>]

Hi,

During work on memory hotplug for Xen I have received some notices
that it breaks machine migration. I had some time and I done some
tests a few days ago. It looks that source of this problem is
xl command itself. I discovered that generic save/restore mechanism
is used for machine migration. xl save store machine config which
was used at machine startup with current machine state. It means
that it does not take into account any config changes which were made
during machine run. This behavior does not allow migrating domain,
on which memory hotplug was used, to restore on destination host
because current size of memory allocated for machine is larger than
size of memory allocated at startup by memory option. Yes, it is
memory option not maxmem option. However, it is not important here
because I think that generic behavior of xl save, migrate and migrate-receive
should be changed (fix for memory hotplug case is workaround for the
generic problem which will return sooner or later). I think that xl save,
migrate and migrate-receive should use current machine state and __CURRENT__
config (from xenstore ???) to do their tasks. However, I am aware that
this change could have large impact on current users. That is why I decided
to ask you about your opinion and suggested solutions in that case
(in general not memory hotplug only).

Currently, these problems could be workaround by passing
path to config file with current config to xl command.

I have done tests on Xen Ver. 4.1.2-rc3. I have not done tests
on xm command, however, I suppose that it has similar behavior.

Daniel

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