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From William Henry <>
Subject Re: Stateful Vs Stateless Instances Findings
Date Fri, 25 Feb 2011 02:51:32 GMT

What is meant by "Condor must have "suspend"/stop/restart support."?

I think Condor supports these semantics so I want to understand it more.

Sent from my iPad

On Feb 24, 2011, at 9:30 AM, Ian Main <> wrote:

> We had a meeting this morning and we managed to hash some things out on
> this subject.  Here is the result.  I am posting this to both
> aeolus-devel and deltacloud-devel as it impacts both.
> Stateful vs Stateless instances and the cloud
> ---------------------------------------------
> In an effort to support private cloud, we had to sort out what to do about
> 'stateful' instances.  We're defining a stateful instance as one where
> the image for the instance does not get destroyed when the instance is
> stopped and it is possible to restart it with the same state.
> Also, the model that Aeolus Conductor is using regarding images is the
> same as ec2, where a given image can be used to launch as many instances
> as needed.  This is not the same as the model used by many private cloud
> providers where an image is used directly by a instance/VM and it is
> not possible for it to be used by multiple running instances.
> We want to tackle these questions in two different phases.  Stateless
> cloud on supported providers is presently the most important requirement,
> but we will need both soon.
> ---------------
> The way that some providers (such as rhevm and vmware) launch instances
> now are not what we need.  Cloud launch must start an instance in
> such a way that it can perform multiple launches from the same image.
> To support that what really needs to happen is that prior to launch the
> image needs to be either cloned or snapshotted.
> Below are a couple of ways we thought of to do this:
> - Deltacloud API provider driver start call must clone disk on startup
> if the provider does not already do that.
> - On destroy, API must clean up cloned disk image if the provider does
> not already do it.
> - On shutdown, the API leaves the disk alone.
> OR
> - Deltacloud API provides instrospection into the driver and specifies
> whether or not an instance start will clone the disk for us.
> - It also needs to specify if the storage needs to be destroyed on
> instance shutdown (some clouds do, some don't).
> - Deltacloud API provides APIs for performing the clone and then cleanup
> of cloned images.
> - It is then up to the client to perform the right sequence of actions
> to get the desired behavior.
> Either of these will work and allow us to make the Conductor model
> consistent with existing public cloud providers.
> ------------------------
> Stateful instances have the ability to be stopped without destroying the
> disk image and then resumed from the same state.  In order to support
> this we need:
> Condor:
> - Condor must have "suspend"/stop/restart support.
> - Image must be built to target stateful - we need a toggle at image
> build time and at launch time.
> Deltacloud API:
> - Deltacloud API must support stop that does not cleanup disk image but
> only if available.
> - Introspection as to whether or not a stop is valid.  Currently
> stop/destroy is the same op on ec2, but 'stop' should be invalid.  Stop is
> only valid for stateful instances.  Destroy cleans up disk image.  This is
> to act as introspection as to whether the instance is stateful or not.
> This may not be needed depending on how we support disk clone/cleanup as
> defined above.
> Conductor: 
> - If an instance on a stateful-only cloud is designated stateless, we must
> track that extra condition in the conductor (because stateful/stateless
> will act the same).  This lets us know if we need to clean up disk
> images etc.
> - We could provide a 'stateful/stateless' toggle at launch time that
> warned the user there is no match and they must build a stateful instance
> for it to work.
> Thanks!
>    Ian

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