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On Fri, Aug 26, 2016 at 7:37 AM, Andrew Laski <andrew at lascii.com> wrote:

>
>
> On Fri, Aug 26, 2016, at 03:44 AM, Kostiantyn.Volenbovskyi at swisscom.com
> wrote:
> > Hi,
> > option 1 (=that's what patches suggest) sounds totally fine.
> > Option 3 > Allow block device mappings, when present, to mostly determine
> > instance  packing
> > sounds like option 1+additional logic (=keyword 'mostly')
> > I think I miss to understand the part of 'undermining the purpose of the
> > flavor'
> > Why new behavior might require one more parameter to limit number of
> > instances of host?
> > Isn't it that those VMs will be under control of other flavor
> > constraints, such as CPU and RAM anyway and those will be the ones
> > controlling 'instance packing'?
>
> Yes it is possible that CPU and RAM could be controlling instance
> packing. But my understanding is that since those are often
> oversubscribed

​I don't understand why the oversubscription ratio matters here?
​


> while disk is not that it's actually the disk amounts
> that control the packing on some environments.

​Maybe an explanation of what you mean by "packing" here.  Customers that
I've worked with over the years have used CPU and Mem as their levers and
the main thing that they care about in terms of how many Instances go on a
Node.  I'd like to learn more about why that's wrong and that disk space is
the mechanism that deployers use for this.
​


> But that is a sub option
> here, just document that disk amounts should not be used to determine
> flavor packing on hosts and instead CPU and RAM must be used.
>
> > Does option 3 covers In case someone relied on eg. flavor root disk for
> > disk volume booted from volume - and now instance packing will change
> > once patches are implemented?
>
> That's the goal. In a simple case of having hosts with 16 CPUs, 128GB of
> RAM and 2TB of disk and a flavor with VCPU=4, RAM=32GB, root_gb=500GB,
> swap/ephemeral=0 the deployer is stating that they want only 4 instances
> on that host.

​How do you arrive at that logic?  What if they actually wanted a single
VCPU=4,RAM=32GB,root_gb=500 but then they wanted the remaining resources
split among Instances that were all 1 VCPU, 1 G ram and a 1 G root disk?

If there is CPU and RAM oversubscription enabled then by
> using volumes a user could end up with more than 4 instances on that
> host. So a max_instances=4 setting could solve that. However I don't
> like the idea of adding a new config, and I think it's too simplistic to
> cover more complex use cases. But it's an option.
>
​
​I would venture to guess that most Operators would be sad to read that.
So rather than give them an explicit lever that does exactly what they want
clearly and explicitly we should make it as complex as possible and have it
be the result of a 4 or 5 variable equation?  Not to mention it's
completely dynamic (because it seems like lots of clouds have more than one
flavor).

All I know is that the current state is broken.  It's not just the
scheduling problem, I could live with that probably since it's too hard to
fix... but keep in mind that you're reporting the complete wrong
information for the Instance in these cases.  My flavor says it's 5G, but
in reality it's 200 or whatever.  Rather than make it perfect we should
just fix it.  Personally I thought the proposals for a scheduler check and
the addition of the Instances/Node option was a win win for everyone.  What
am I missing?  Would you rather a custom filter scheduler so it wasn't a
config option?
​

>
> >
> > BR,
> > Konstantin
> >
> > > -----Original Message-----
> > > From: Andrew Laski [mailto:andrew at lascii.com]
> > > Sent: Thursday, August 25, 2016 10:20 PM
> > > To: openstack-dev at lists.openstack.org
> > > Cc: openstack-operators at lists.openstack.org
> > > Subject: [Openstack-operators] [Nova] Reconciling flavors and block
> device
> > > mappings
> > >
> > > Cross posting to gather some operator feedback.
> > >
> > > There have been a couple of contentious patches gathering attention
> recently
> > > about how to handle the case where a block device mapping supersedes
> flavor
> > > information. Before moving forward on either of those I think we
> should have a
> > > discussion about how best to handle the general case, and how to
> handle any
> > > changes in behavior that results from that.
> > >
> > > There are two cases presented:
> > >
> > > 1. A user boots an instance using a Cinder volume as a root disk,
> however the
> > > flavor specifies root_gb = x where x > 0. The current behavior in Nova
> is that the
> > > scheduler is given the flavor root_gb info to take into account during
> scheduling.
> > > This may disqualify some hosts from receiving the instance even though
> that disk
> > > space  is not necessary because the root disk is a remote volume.
> > > https://review.openstack.org/#/c/200870/
> > >
> > > 2. A user boots an instance and uses the block device mapping
> parameters to
> > > specify a swap or ephemeral disk size that is less than specified on
> the flavor.
> > > This leads to the same problem as above, the scheduler is provided
> information
> > > that doesn't match the actual disk space to be consumed.
> > > https://review.openstack.org/#/c/352522/
> > >
> > > Now the issue: while it's easy enough to provide proper information to
> the
> > > scheduler on what the actual disk consumption will be when using block
> device
> > > mappings that undermines one of the purposes of flavors which is to
> control
> > > instance packing on hosts. So the outstanding question is to what
> extent should
> > > users have the ability to use block device mappings to bypass flavor
> constraints?
> > >
> > > One other thing to note is that while a flavor constrains how much
> local disk is
> > > used it does not constrain volume size at all. So a user can specify an
> > > ephemeral/swap disk <= to what the flavor provides but can have an
> arbitrary
> > > sized root disk if it's a remote volume.
> > >
> > > Some possibilities:
> > >
> > > Completely allow block device mappings, when present, to determine
> instance
> > > packing. This is what the patches above propose and there's a strong
> desire for
> > > this behavior from some folks. But changes how many instances may fit
> on a
> > > host which could be undesirable to some.
> > >
> > > Keep the status quo. It's clear that is undesirable based on the bug
> reports and
> > > proposed patches above.
> > >
> > > Allow block device mappings, when present, to mostly determine instance
> > > packing. By that I mean that the scheduler only takes into account
> local disk that
> > > would be consumed, but we add additional configuration to Nova which
> limits
> > > the number of instance that can be placed on a host. This is a
> compromise
> > > solution but I fear that a single int value does not meet the needs of
> deployers
> > > wishing to limit instances on a host. They want it to take into
> account cpu
> > > allocations and ram and disk, in short a flavor :)
> > >
> > > And of course there may be some other unconsidered solution. That's
> where
> > > you, dear reader, come in.
> > >
> > > Thoughts?
> > >
> > > -Andrew
> > >
> > >
> > > _______________________________________________
> > > OpenStack-operators mailing list
> > > OpenStack-operators at lists.openstack.org
> > > http://lists.openstack.org/cgi-bin/mailman/listinfo/
> openstack-operators
>
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