Paul, Marc:
Thanks for clarifying.
As cloud admin/operator, I do care about the instance’s placement and that is
why I’d like to apply affinity groups to all instances whenever possible.
It sounds like there is no fundamental technical reasons that a running
instance’s affinity group membership can’t be updated. Then why not allow this
operation? The logic could be as simple as follows:
If current host placement is compatible with new affinity group’s placement:
then
let the update succeed
else
if auto-migration is true && there is a suitable host to migrate to
then
live migrate instance to new host and update instance’s affinity group
membership
else
raise an exception
end
end
Here “auto-migrate” is controlled by a new global setting parameter, and it is
for migrating VM to another host in the same cluster. IOW, it does not involve
storage migration. If for some technical reasons that live migration can’t be
done here, then that inner “if ... else ... end” block can be reduced to just
“raise an exception”.
Is this reasonable?
Yiping
On 1/11/18, 12:19 AM, "Marc-Aurèle Brothier" <[email protected]> wrote:
Hi Yiping,
To add to Paul's comment, you also need to understand the goal of the
anti-affinity groups. If they don't care, you should simply block the
command so that your users don't use it (you should list the
createAffinityGroup command as a root admin call in the commands.properties
file by changing it's flag value).
The goal is to spread a group of VMs, a cluster of a service, so that in
case of a hardware failure on one hyperisor, the cluster can be sure that
only 1 of its instances will go down and the srvice can keep running.
On Thu, Jan 11, 2018 at 9:01 AM, Paul Angus <[email protected]>
wrote:
> Hi Yiping,
>
> Anti-affinity groups deal with the placement of VMs when they are started,
> but doesn't/can't 'move' running VMs (it isn't like vSphere DRS). If you
> change a VM's anti-affinity group, it's current placement on a host may
> suddenly become invalid. As the Anti-Affinity group code isn't designed
to
> move VMs, the safest option is to ensure that the VM is stopped when its
> group is changed so that when it is started again, CloudStack can then
> properly decide where it can/should go.
>
>
>
> Kind regards,
>
> Paul Angus
>
> [email protected]
> www.shapeblue.com
> 53 Chandos Place, Covent Garden, London WC2N 4HSUK
> @shapeblue
>
>
>
>
> -----Original Message-----
> From: Yiping Zhang [mailto:[email protected]]
> Sent: 10 January 2018 19:51
> To: [email protected]
> Subject: why instance must be stopped in order to update its affinity
> groups?
>
> Hi, List:
>
> Can someone please explain why a VM instance must be in stopped state when
> updating its affinity group memberships? This requirement is in “Feature
> assumptions” section of the original 4.2 design document (
> https://cwiki.apache.org/confluence/display/CLOUDSTACK/
> FS+-+Affinity-Anti-affinity+groups).
>
> My users either don’t understand or don’t care about affinity groups and I
> see a large number of instances with sub-optimal host placement (from
> anti-host affinity group point of view). But it is too much trouble for
me
> to coordinate with so many users to shut them down in order to fix their
> host placement. What bad things would happen if a running instance’s
> affinity group is changed?
>
> Thanks,
>
> Yiping
>
>
