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From Yiping Zhang <>
Subject Re: why instance must be stopped in order to update its affinity groups?
Date Thu, 11 Jan 2018 19:16:02 GMT
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:
   let the update succeed
   if auto-migration is true && there is a suitable host to migrate to
      live migrate instance to new host and update instance’s affinity group membership
      raise an exception

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?


On 1/11/18, 12:19 AM, "Marc-Aurèle Brothier" <> 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
    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 <>
    > 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
    > 53 Chandos Place, Covent Garden, London  WC2N 4HSUK
    > @shapeblue
    > -----Original Message-----
    > From: Yiping Zhang []
    > Sent: 10 January 2018 19:51
    > To:
    > 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 (
    > 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

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