Re: [Qemu-block] [PATCH v3 4/7] block: convert ThrottleGroup to object with QOM

[Kevin Wolf]

Am 03.08.2017 um 12:53 hat Stefan Hajnoczi geschrieben:
> On Thu, Aug 03, 2017 at 10:08:01AM +0200, Kevin Wolf wrote:
> > Am 02.08.2017 um 12:57 hat Manos Pitsidianakis geschrieben:
> > > On Wed, Aug 02, 2017 at 11:39:22AM +0100, Stefan Hajnoczi wrote:
> > > > On Tue, Aug 01, 2017 at 07:49:33PM +0300, Manos Pitsidianakis wrote:
> > > > > On Tue, Aug 01, 2017 at 04:47:03PM +0100, Stefan Hajnoczi wrote:
> > > > > > On Mon, Jul 31, 2017 at 12:54:40PM +0300, Manos Pitsidianakis wrote:
> > > > > > > ThrottleGroup is converted to an object. This will allow the 
> > > > > > > future
> > > > > > > throttle block filter drive easy creation and configuration of 
> > > > > > > throttle
> > > > > > > groups in QMP and cli.
> > > > > > >
> > > > > > > A new QAPI struct, ThrottleLimits, is introduced to provide a 
> > > > > > > shared
> > > > > > > struct for all throttle configuration needs in QMP.
> > > > > > >
> > > > > > > ThrottleGroups can be created via CLI as
> > > > > > >     -object throttle-group,id=foo,x-iops-total=100,x-..
> > > > > > > where x-* are individual limit properties. Since we can't add 
> > > > > > > non-scalar
> > > > > > > properties in -object this interface must be used instead. 
> > > > > > > However,
> > > > > > > setting these properties must be disabled after initialization 
> > > > > > > because
> > > > > > > certain combinations of limits are forbidden and thus 
> > > > > > > configuration
> > > > > > > changes should be done in one transaction. The individual 
> > > > > > > properties
> > > > > > > will go away when support for non-scalar values in CLI is 
> > > > > > > implemented
> > > > > > > and thus are marked as experimental.
> > > > > > >
> > > > > > > ThrottleGroup also has a `limits` property that uses the 
> > > > > > > ThrottleLimits
> > > > > > > struct.  It can be used to create ThrottleGroups or set the
> > > > > > > configuration in existing groups as follows:
> > > > > > >
> > > > > > > { "execute": "object-add",
> > > > > > >   "arguments": {
> > > > > > >     "qom-type": "throttle-group",
> > > > > > >     "id": "foo",
> > > > > > >     "props" : {
> > > > > > >       "limits": {
> > > > > > >           "iops-total": 100
> > > > > > >       }
> > > > > > >     }
> > > > > > >   }
> > > > > > > }
> > > > > > > { "execute" : "qom-set",
> > > > > > >     "arguments" : {
> > > > > > >         "path" : "foo",
> > > > > > >         "property" : "limits",
> > > > > > >         "value" : {
> > > > > > >             "iops-total" : 99
> > > > > > >         }
> > > > > > >     }
> > > > > > > }
> > > > > > >
> > > > > > > This also means a group's configuration can be fetched with 
> > > > > > > qom-get.
> > > > > > >
> > > > > > > ThrottleGroups can be anonymous which means they can't get 
> > > > > > > accessed by
> > > > > > > other users ie they will always be units instead of group 
> > > > > > > (Because they
> > > > > > > have one ThrottleGroupMember).
> > > > > >
> > > > > > blockdev.c automatically assigns -drive id= to the group name if
> > > > > > throttling.group= wasn't given.  So who will use anonymous 
> > > > > > single-drive
> > > > > > mode?
> > > > > 
> > > > > Manual filter nodes. It's possible to not pass a group name value and 
> > > > > the
> > > > > resulting group will be anonymous. Are you suggesting to move this 
> > > > > change to
> > > > > the throttle filter patch?
> > > > 
> > > > What is the use case?  Human users will stick to the legacy syntax
> > > > because it's convenient.  Management tools will use the filter
> > > > explicitly in the future, and it's easy for them to choose a name.
> > > > 
> > > > Unless there is a need for this case I'd prefer to make the group name
> > > > mandatory.  That way there are less code paths to worry about.
> > > 
> > > I think Kevin requested this though I don't really remember the use case.
> > 
> > There is no legacy syntax for putting a throttle node anywhere but at
> > the root of a BlockBackend. If you want to throttle e.g. only a specific
> > backing file, you need to manually create a throttle node.
> > 
> > (We tend to forget this occasionally, but the work you're doing is not
> > only cleanup just for fun, but it's actually new features that enable
> > new use cases by making everything more flexible.)
> 
> It's not clear to me from your reply whether you support anonymous
> throttle groups or not.  It is possible to throttle arbitrary nodes in
> the graph either way.

I think it would be nice for human users to have them, but on second
thought you're right that it's just syntactic sugar for an explicit
-object, so if you think there is any difficulty with supporting
anonymous groups, feel free to drop them.

Hm, actually... Does this mean that then the whole 'limits' option in
the throttle driver could go away, with all of the parsing code, and the
group name becomes mandatory? That certainly does look tempting.

Kevin

signature.asc
Description: PGP signature