Open Stack

IMO, the desired behavior of 'resize' is:
- user should be able to influence the expected 'downtime', i.e., whether 
it should be done dynamically on the same host (zero downtime), using 
'live' migration (close to zero downtime), or non-live. Ideally, there 
should be also an API to determine which modes are supported.
- user should be able to influence the placement, similarly to instance 
provisioning, meaning that either scheduler hints should be persisted and 
used during 'resize', or the user should be able to specify scheduler 
hints when applying resize (or both). In particular, it might make sense 
to have a dedicated weight function preferring to keep the instance on the 
same host, if possible.
- optionally, it might make sense to have a filter (to be specified by the 
admin) that would prevent migration of instances with certain 
characteristics (which would apply during resize)

The combination of the above would determine whether it can or will be on 
the same host (transparently to the user).

Having said that, as a short-term measure, making "resize_to_same_host" 
more flexible certainly sounds like a step in the right direction.

Regards,
Alex




From:   Michael J Fork <mjfork at us.ibm.com>
To:     openstack-dev at lists.openstack.org, 
Date:   15/02/2013 06:08 PM
Subject:        [openstack-dev] Optionally force instances to "stay put" 
on resize



The patch for the configurable-resize-placement blueprint (
https://blueprints.launchpad.net/nova/+spec/configurable-resize-placement) 
has generated a discussion on the review boards and needed to be brought 
to the mailing list for broader feedback.

tl;dr would others find useful the addition of a new config option 
"resize_to_same_host" with values "allow", "require", "forbid" that 
deprecates "allow_resize_to_same_host" (functionality equivalent to 
"allow" and "forbid" in "resize_to_same_host")?  Existing use cases and 
default behaviors are retained unchanged.  The new use case is 
"resize_to_same_host = require" retains the exact same external API 
sematics and would make it such that no user actions can cause a VM 
migration (and the network traffic with it).  An administrator can still 
perform a manual migration that would allow a subsequent resize to 
succeed.  This patch would be most useful in environments with 1GbE or 
with large ephemeral disks. 

Blueprint  Description

> Currently OpenStack has a boolean "allow_resize_to_same_host" config 
option that constrains
> placement during resize. When this value is false, the ignore_hosts 
option is passed to the scheduler. 
> When this value is true, no options are passed to the scheduler and the 
current host can be
> considered. In some use cases - e.g. PowerVM - a third option of 
"require same host' is desirable.

> This blueprint will deprecate the "allow_resize_to_same_host" config 
option and replace it with 
> "resize_to_same_host" that supports 3 values - allow, forbid, require. 
Allow is equivalent to true in the
> current use case (i.e. not scheduler hint, current host is considered), 
forbid to false in current use case 
> (i.e. the ignore_hosts scheduler hint is set), and require forces the 
same host through the use of the
> force_hosts scheduler hint.

To avoid incorrectly paraphrasing others, the review comments against the 
change are below in their entirety followed by my comments to those 
concerns.  The question we are looking to answer - would others find this 
function useful and / or believe that OpenStack should have this option?

Comments from https://review.openstack.org/#/c/21139/:

> I still think this is a bad idea. The only reason the flag was there in 
the first place was so we could 
> run tempest on devstack in the gate and test resize. Semantically this 
changes the meaning of resize
> in a way that I don't think should be done.

> I understand what the patch does, and I even think it appears to be 
functionally correct based on
> what the intention appears to be. However, I'm not convinced that the 
option is a useful addition.
>
> First, it really just doesn't seem in the spirit of OpenStack or "cloud" 
to care this much about where 
> the instance goes like this. The existing option was only a hack for 
testing, not something expected 
> for admins to care about.
>
> If this really *is* something admins need to care about, I'd like to 
better understand why. Further, if 
> that's the case, I'm not sure a global config option is the right way to 
go about it. I think it may make 
> more sense to have this be API driven. I'd like to see some thoughts 
from others on this point."

> "I completely agree with the "spirit of cloud" argument. I further think 
that exposing anything via the 
> API that would support this (i.e. giving the users control or even 
indication of where their instance lands) 
> is a dangerous precedent to set.
>
> I tend to think that this use case is so small and specialized, that it 
belongs in some other sort of policy 
> implementation, and definitely not as yet-another-config-option to be 
exposed to the admins. That, or in 
> some other project entirely :)"

and my response to those concerns:

> I agree this is not an 80% use case, or probably even that popular in 
the other 20%, but resize today 
> is the only user facing API that can trigger the migration of a VM to a 
new machine. In some environments, 
> this network traffic is undesirable - especially 1GBe - and may want to 
be explicitly controlled by an 
> Administrator. In this implementation, an Admin can still invoke a 
migration manually to allow the resize to 
> succeed. I would point to the Island work by Sina as an example, they 
wrote an entire Cinder driver 
> designed to minimize network traffic.
>
> I agree with the point above that exposing this on an end-user API is 
not correct, users should not know 
> or care where this goes. However, as the cloud operator, I should be 
able to have that level of control 
> and this puts it in their hands.
>
> Obviously this option would need documented to allow administrators to 
decide if they need to change it,
> but it certainly wouldn't be default. Expectation is that it would of 
use in smaller installations or enterprise 
> uses cases more often than service providers.
>
> Additionally, it continues to honor the existing resize API contract.

An additional use case - beyond 1GbE - is if an environment uses large 
ephemeral disks.

Would others find this function useful and / or believe that OpenStack 
should have this option?  Again, the API contract is unchanged and it 
gives a cloud operator an additional level of control over the movement of 
instances.  It would not be the default behavior, but rather enabled by an 
administrator depending on their specific use cases and requirements and 
the environment they are in.

Thanks.

Michael

-------------------------------------------------
Michael Fork
OpenStack Architect, Cloud Solutions and OpenStack Development
IBM Systems & Technology Group
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