Re: [Qemu-block] [PATCH v3 5/7] block: add throttle block filter driver

[Manos Pitsidianakis]

On Tue, Aug 08, 2017 at 04:53:08PM +0200, Alberto Garcia wrote:
> On Tue 08 Aug 2017 03:45:44 PM CEST, Manos Pitsidianakis wrote:
> > On Tue, Aug 08, 2017 at 03:13:36PM +0200, Alberto Garcia wrote:
> > > On Mon 31 Jul 2017 11:54:41 AM CEST, Manos Pitsidianakis wrote:
> > > > block/throttle.c uses existing I/O throttle infrastructure inside a
> > > > block filter driver. I/O operations are intercepted in the filter's
> > > > read/write coroutines, and referred to block/throttle-groups.c
> > > >
> > > > The driver can be used with the syntax
> > > > -drive driver=throttle,file.filename=foo.qcow2, \
> > > >         limits.iops-total=...,throttle-group=bar
> > >
> > > Sorry for not having noticed this earlier, but can't you define the
> > > throttling group (and its limits) using -object throttle-group ... as
> > > shown in the previous patch, and simply reference it here? Or would we
> > > have two alternative ways of setting the throttling limits?
> > >
> > > What happens if you have many -drive lines each one with a different set
> > > of limits but with the same throttling group?
> >
> > The limits of the last one to be processed will win.
>
> That's what the current throttling API does, and I tend to agree that
> it's not completely straightforward (a few people have asked me the same
> question since this feature is available).
>
> If we're going to add a new API we could eliminate this ambiguity. After
> all the previous -drive throttling.iops-total=... would still be
> available, wouldn't it?

Indeed, it already is.

> > So basically if we have anonymous groups, we accept limits in the
> > driver options but only without a group-name.
>
> In the commit message you do however have limits and a group name, is
> that a mistake?
>
>    -drive driver=throttle,file.filename=foo.qcow2, \
>           limits.iops-total=...,throttle-group=bar

Sorry this wasn't clear, I'm actually proposing to remove limits from 
the throttle driver options and only create/config throttle groups via 
-object/object-add.

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