[Openstack-operators] [openstack-dev] [nova][publiccloud-wg] Proposal to shelve on stop/suspend

Tim Bell Tim.Bell at cern.ch
Sat Sep 15 14:51:26 UTC 2018

Found the previous discussion at http://lists.openstack.org/pipermail/openstack-operators/2016-August/011321.html from 2016.


-----Original Message-----
From: Tim Bell <Tim.Bell at cern.ch>
Date: Saturday, 15 September 2018 at 14:38
To: "OpenStack Development Mailing List (not for usage questions)" <openstack-dev at lists.openstack.org>, "openstack-operators at lists.openstack.org" <openstack-operators at lists.openstack.org>, "openstack-sigs at lists.openstack.org" <openstack-sigs at lists.openstack.org>
Subject: Re: [openstack-dev] [nova][publiccloud-wg] Proposal to shelve on stop/suspend

    One extra user motivation that came up during past forums was to have a different quota for shelved instances (or remove them from the project quota all together). Currently, I believe that a shelved instance still counts towards the instances/cores quota thus the reduction of usage by the user is not reflected in the quotas.
    One discussion at the time was that the user is still reserving IPs so it is not zero resource usage and the instances still occupy storage.
    (We disabled shelving for other reasons so I'm not able to check easily)
    -----Original Message-----
    From: Matt Riedemann <mriedemos at gmail.com>
    Reply-To: "OpenStack Development Mailing List (not for usage questions)" <openstack-dev at lists.openstack.org>
    Date: Saturday, 15 September 2018 at 01:27
    To: "OpenStack Development Mailing List (not for usage questions)" <openstack-dev at lists.openstack.org>, "openstack-operators at lists.openstack.org" <openstack-operators at lists.openstack.org>, "openstack-sigs at lists.openstack.org" <openstack-sigs at lists.openstack.org>
    Subject: [openstack-dev] [nova][publiccloud-wg] Proposal to shelve on	stop/suspend
        tl;dr: I'm proposing a new parameter to the server stop (and suspend?) 
        APIs to control if nova shelve offloads the server.
        Long form: This came up during the public cloud WG session this week 
        based on a couple of feature requests [1][2]. When a user stops/suspends 
        a server, the hypervisor frees up resources on the host but nova 
        continues to track those resources as being used on the host so the 
        scheduler can't put more servers there. What operators would like to do 
        is that when a user stops a server, nova actually shelve offloads the 
        server from the host so they can schedule new servers on that host. On 
        start/resume of the server, nova would find a new host for the server. 
        This also came up in Vancouver where operators would like to free up 
        limited expensive resources like GPUs when the server is stopped. This 
        is also the behavior in AWS.
        The problem with shelve is that it's great for operators but users just 
        don't use it, maybe because they don't know what it is and stop works 
        just fine. So how do you get users to opt into shelving their server?
        I've proposed a high-level blueprint [3] where we'd add a new 
        (microversioned) parameter to the stop API with three options:
        * auto
        * offload
        * retain
        Naming is obviously up for debate. The point is we would default to auto 
        and if auto is used, the API checks a config option to determine the 
        behavior - offload or retain. By default we would retain for backward 
        compatibility. For users that don't care, they get auto and it's fine. 
        For users that do care, they either (1) don't opt into the microversion 
        or (2) specify the specific behavior they want. I don't think we need to 
        expose what the cloud's configuration for auto is because again, if you 
        don't care then it doesn't matter and if you do care, you can opt out of 
        "How do we get users to use the new microversion?" I'm glad you asked.
        Well, nova CLI defaults to using the latest available microversion 
        negotiated between the client and the server, so by default, anyone 
        using "nova stop" would get the 'auto' behavior (assuming the client and 
        server are new enough to support it). Long-term, openstack client plans 
        on doing the same version negotiation.
        As for the server status changes, if the server is stopped and shelved, 
        the status would be 'SHELVED_OFFLOADED' rather than 'SHUTDOWN'. I 
        believe this is fine especially if a user is not being specific and 
        doesn't care about the actual backend behavior. On start, the API would 
        allow starting (unshelving) shelved offloaded (rather than just stopped) 
        instances. Trying to hide shelved servers as stopped in the API would be 
        overly complex IMO so I don't want to try and mask that.
        It is possible that a user that stopped and shelved their server could 
        hit a NoValidHost when starting (unshelving) the server, but that really 
        shouldn't happen in a cloud that's configuring nova to shelve by default 
        because if they are doing this, their SLA needs to reflect they have the 
        capacity to unshelve the server. If you can't honor that SLA, don't 
        shelve by default.
        So, what are the general feelings on this before I go off and start 
        writing up a spec?
        [1] https://bugs.launchpad.net/openstack-publiccloud-wg/+bug/1791681
        [2] https://bugs.launchpad.net/openstack-publiccloud-wg/+bug/1791679
        [3] https://blueprints.launchpad.net/nova/+spec/shelve-on-stop
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