Re: Exclusive VM lock

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On Tue, Dec 15, 2015 at 03:26:44PM +0300, Dmitry Andreev wrote:


On 15.12.2015 11:47, Martin Kletzander wrote:
On Mon, Dec 14, 2015 at 06:50:56PM +0300, Dmitry Andreev wrote:
On 14.12.2015 18:10, Peter Krempa wrote:
On Mon, Dec 14, 2015 at 17:18:22 +0300, Dmitry Andreev wrote:
Hi,

I'm looking for a mechanism to do a sequence of API calls as atomic
operation. Is there any way for libvirt's API client to acquire an
exclusive VM lock to prevent other client from changing VM state
through libvirt?

No there isn't anything. Libvirt is locking only on internal events that
need protection.

Doing transaction locking should be relatively easy to implement on
application layer though.

Sure we can implement a lock manager on the API client side, but it
can't lock VM for other clients. And that’s the problem for us.

Are maintainers interested in VM lock manager on the server side?
Can I move in this direction or you have something against it?


Well, I think that if you want to lock it in the layer above, you should
lock it in the layer above.  And you can disable other clients, restrict
them, etc.  Would you mind describing the use-case more thoroughly?
Maybe we have a solution for that already.

We allow users to manage VMs with virt-manager but when we updates VM
configuration using API we want the read-change-write operation to be
atomic.

In few other cases we don't want to allow user run or stop vm while we
doing post creation or guest configuration job.


Oh, for that PolicyKit might be a bit overkill, but it's currently the
only thing I can think of.  You might modify the rules so that users can
do what you allow only if you want them to.  Maybe the best way to
handle this is to do an automatic update of polkit rules.


Peter


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