[virt-tools-list] [libvirt] Project for profiles and defaults for libvirt domains

Wed Mar 21 18:39:52 UTC 2018

On Wed, Mar 21, 2018 at 03:00:41PM -0300, Eduardo Habkost wrote:
> On Tue, Mar 20, 2018 at 03:10:12PM +0000, Daniel P. Berrangé wrote:
> > On Tue, Mar 20, 2018 at 03:20:31PM +0100, Martin Kletzander wrote:
> > > 1) Default devices/values
> > > 
> > > Libvirt itself must default to whatever values there were before any
> > > particular element was introduced due to the fact that it strives to
> > > keep the guest ABI stable.  That means, for example, that it can't just
> > > add -vmcoreinfo option (for KASLR support) or magically add the pvpanic
> > > device to all QEMU machines, even though it would be useful, as that
> > > would change the guest ABI.
> > > 
> > > For default values this is even more obvious.  Let's say someone figures
> > > out some "pretty good" default values for various HyperV enlightenment
> > > feature tunables.  Libvirt can't magically change them, but each one of
> > > the projects building on top of it doesn't want to keep that list
> > > updated and take care of setting them in every new XML.  Some projects
> > > don't even expose those to the end user as a knob, while others might.
> > 
> > This gets very tricky, very fast.
> > 
> > Lets say that you have an initial good set of hyperv config
> > tunables. Now sometime passes and it is decided that there is a
> > different, better set of config tunables. If the module that is
> > providing this policy to apps like OpenStack just updates itself
> > to provide this new policy, this can cause problems with the
> > existing deployed applications in a number of ways.
> > 
> > First the new config probably depends on specific versions of
> > libvirt and QEMU,  and you can't mandate to consuming apps which
> > versions they must be using.  [...]
> 
> This is true.
> 
> >                       [...]  So you need a matrix of libvirt +
> > QEMU + config option settings.
> 
> But this is not.  If config options need support on the lower
> levels of the stack (libvirt and/or QEMU and/or KVM and/or host
> hardware), it already has to be represented by libvirt host
> capabilities somehow, so management layers know it's available.
> 
> This means any new config generation system can (and must) use
> host(s) capabilities as input before generating the
> configuration.

I don't think it is that simple. The capabilities reflect what the
current host is capable of only, not whether it is desirable to
actually use them. Just because a host reports that it has q35-2.11.0
machine type doesn't mean that it should be used. The mgmt app may
only wish to use that if it is available on all hosts in a particular
grouping. The config generation library can't query every host directly
to determine this. The mgmt app may have a way to collate capabilities
info from hosts, but it is probably then stored in a app specific
format and data source, or it may just ends up being a global config
parameter to the mgmt app per host.

There have been a number of times where a feature is available in
libvirt and/or QEMU, and the mgmt app still doesn't yet may still
not wish to use it because it is known broken / incompatible with
certain usage patterns. So the mgmt app would require an arbitrarily
newer libvirt/qemu before considering using it, regardless of
whether host capabilities report it is available.

> > Even if you have the matching libvirt & QEMU versions, it is not
> > safe to assume the application will want to use the new policy.
> > An application may need live migration compatibility with older
> > versions. Or it may need to retain guaranteed ABI compatibility
> > with the way the VM was previously launched and be using transient
> > guests, generating the XML fresh each time.
> 
> Why is that a problem?  If you want live migration or ABI
> guarantees, you simply don't use this system to generate a new
> configuration.  The same way you don't use the "pc" machine-type
> if you want to ensure compatibility with existing VMs.

In many mgmt apps, every VM potentially needs live migration, so
unless I'm misunderstanding, you're effectively saying don't ever
use this config generator in these apps.

> > The application will have knowledge about when it wants to use new
> > vs old hyperv tunable policy, but exposing that to your policy module
> > is very tricky because it is inherantly application specific logic
> > largely determined by the way the application code is written.
> 
> We have a huge set of features where this is simply not a
> problem.  For most virtual hardware features, enabling them is
> not even a policy decision: it's just about telling the guest
> that the feature is now available.  QEMU have been enabling new
> features in the "pc" machine-type for years.
> 
> Now, why can't higher layers in the stack do something similar?
> 
> The proposal is equivalent to what already happens when people
> use the "pc" machine-type in their configurations, but:
> 1) the new defaults/features wouldn't be hidden behind a opaque
>    machine-type name, and would appear in the domain XML
>    explicitly;
> 2) the higher layers won't depend on QEMU introducing a new
>    machine-type just to have new features enabled by default;
> 3) features that depend on host capabilities but are available on
>    all hosts in a cluster can now be enabled automatically if
>    desired (which is something QEMU can't do because it doesn't
>    have enough information about the other hosts).
> 
> Choosing reasonable defaults might not be a trivial problem, but
> the current approach of pushing the responsibility to management
> layers doesn't improve the situation.

The simple cases have been added to the "pc" machine type, but
more complex cases have not been dealt with as they often require
contextual knowledge of either the host setup or the guest OS
choice.

We had a long debate over the best aio=threads,native setting for
OpenStack. Understanding the right defaults required knowledge about
the various different ways that Nova would setup its storage stack.
We certainly know enough now to be able to provide good recommendations
for the choice, with perf data to back it up, but interpreting those
recommendations still requires the app specific knowledge about its
storage mgmt approach, so ends up being code dev work.

Another case is the pvpanic device - while in theory that could
have been enabled by default for all guests, by QEMU or a config
generator library, doing so is not useful on its own. The hard
bit of the work is adding code to the mgmt app to choose the
action for when pvpanic triggers, and code to handle the results
of that action.

Regards,
Daniel
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