Open Stack

Potentially,

Although I think the lack of formalization and visibility (and the ability to easily change its transitions) into the state machine is at this point causing part of this pain.

If the state machine was well defined (and adjustable - to a degree...) then you could imagine only allowing delete transitions from the powered off state while another deployer may prefer a more "experimental" allow delete transition at any time (with associated side effects).

Without the underlying state machine being in a more formalized state it's hard to do either in a manageable (and recoverable & resumable...) manner IMHO.

Sent from my really tiny device...

On Oct 26, 2013, at 9:12 AM, "Abhishek Lahiri" <aviostack at gmail.com<mailto:aviostack at gmail.com>> wrote:

Deletes should only be allowed when the vm is in a power off state. This will allow consistent state transition.

Thanks
Al


On Oct 26, 2013, at 8:55 AM, Joshua Harlow <harlowja at yahoo-inc.com<mailto:harlowja at yahoo-inc.com>> wrote:

I think I will try to have a unconference at the HK summit about ideas the cinder developers (and the taskflow developers, since it's not a concept that is unique /applicable to just cinder) are having about said state machine (and it's potential usage).

So look out for that, be interesting to have some nova folks involved there also :-)

Sent from my really tiny device...

On Oct 26, 2013, at 3:14 AM, "Alex Glikson" <GLIKSON at il.ibm.com<mailto:GLIKSON at il.ibm.com>> wrote:

+1

Regards,
Alex


Joshua Harlow <harlowja at yahoo-inc.com<mailto:harlowja at yahoo-inc.com>> wrote on 26/10/2013 09:29:03 AM:
>
> An idea that others and I are having for a similar use case in
> cinder (or it appears to be similar).
>
> If there was a well defined state machine/s in nova with well
> defined and managed transitions between states then it seems like
> this state machine could resume on failure as well as be interrupted
> when a "dueling" or preemptable operation arrives (a delete while
> being created for example). This way not only would it be very clear
> the set of states and transitions but it would also be clear how
> preemption occurs (and under what cases).
>
> Right now in nova there is a distributed and ad-hoc state machine
> which if it was more formalized it could inherit some if the
> described useful capabilities. It would also be much more resilient
> to these types of locking problems that u described.
>
> IMHO that's the only way these types of problems will be fully be
> fixed, not by more queues or more periodic tasks, but by solidifying
> & formalizing the state machines that compose the work nova does.
>
> Sent from my really tiny device...
>
> > On Oct 25, 2013, at 3:52 AM, "Day, Phil" <philip.day at hp.com<mailto:philip.day at hp.com>> wrote:
> >
> > Hi Folks,
> >
> > We're very occasionally seeing problems where a thread processing
> a create hangs (and we've seen when taking to Cinder and Glance).
> Whilst those issues need to be hunted down in their own rights, they
> do show up what seems to me to be a weakness in the processing of
> delete requests that I'd like to get some feedback on.
> >
> > Delete is the one operation that is allowed regardless of the
> Instance state (since it's a one-way operation, and users should
> always be able to free up their quota).   However when we get a
> create thread hung in one of these states, the delete requests when
> they hit the manager will also block as they are synchronized on the
> uuid.   Because the user making the delete request doesn't see
> anything happen they tend to submit more delete requests.   The
> Service is still up, so these go to the computer manager as well,
> and eventually all of the threads will be waiting for the lock, and
> the compute manager will stop consuming new messages.
> >
> > The problem isn't limited to deletes - although in most cases the
> change of state in the API means that you have to keep making
> different calls to get past the state checker logic to do it with an
> instance stuck in another state.   Users also seem to be more
> impatient with deletes, as they are trying to free up quota for other things.
> >
> > So while I know that we should never get a thread into a hung
> state into the first place, I was wondering about one of the
> following approaches to address just the delete case:
> >
> > i) Change the delete call on the manager so it doesn't wait for
> the uuid lock.  Deletes should be coded so that they work regardless
> of the state of the VM, and other actions should be able to cope
> with a delete being performed from under them.  There is of course
> no guarantee that the delete itself won't block as well.
> >
> > ii) Record in the API server that a delete has been started (maybe
> enough to use the task state being set to DELETEING in the API if
> we're sure this doesn't get cleared), and add a periodic task in the
> compute manager to check for and delete instances that are in a
> "DELETING" state for more than some timeout. Then the API, knowing
> that the delete will be processes eventually can just no-op any
> further delete requests.
> >
> > iii) Add some hook into the ServiceGroup API so that the timer
> could depend on getting a free thread from the compute manager pool
> (ie run some no-op task) - so that of there are no free threads then
> the service becomes down. That would (eventually) stop the scheduler
> from sending new requests to it, and make deleted be processed in
> the API server but won't of course help with commands for other
> instances on the same host.
> >
> > iv) Move away from having a general topic and thread pool for all
> requests, and start a listener on an instance specific topic for
> each running instance on a host (leaving the general topic and pool
> just for creates and other non-instance calls like the hypervisor
> API).   Then a blocked task would only affect request for a specificinstance.
> >
> > I'm tending towards ii) as a simple and pragmatic solution in the
> near term, although I like both iii) and iv) as being both generally
> good enhancments - but iv) in particular feels like a pretty seismic change.
> >
> > Thoughts please,
> >
> > Phil
> >
> > _______________________________________________
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