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Re: [Qemu-block] Throttling groups vs filter nodes

From: Kevin Wolf
Subject: Re: [Qemu-block] Throttling groups vs filter nodes
Date: Tue, 30 May 2017 11:37:00 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

[ Cc: qemu-block - noticed only now that it's missing ]

Am 29.05.2017 um 22:57 hat Manos Pitsidianakis geschrieben:
> On Mon, May 29, 2017 at 05:05:17PM +0200, Alberto Garcia wrote:
> >On Sat 27 May 2017 09:56:03 AM CEST, Stefan Hajnoczi wrote:
> >>A quirk in the current implementation is that the throttling limits
> >>for the group are overwritten by each -drive throttling.group=group0.
> >>Limits for all but the last -drive in a group are ignored.
> >  - bps or iops != 0   -> set the I/O limits of a throttling group. The
> >                          selected device is moved to that group if it
> >                          wasn't there yet.
> >
> >  - bps and iops == 0  -> remove a device from a throttling group
> >                          without touching that group's I/O limits.
> These are very unintuitive.

I agree, this is not an interface to extend, but one to get rid of. (Of
course, we'll have to keep it around for a while because compatibility,
but we should try to offer something better.)

> However, even without considering backwards compatibility, I think
> that using -object notation (eg "object-add
> throttle-group,id=foo,iops=...) is intuitive in the case of groups,
> but not when you need individual limits for each device as the syntax
> would be too verbose.  Of course the old interface covers that.
> In any case, is having multiple interfaces a problem or not? And, is
> using QOM straightforward implementation-wise?

We can have an interface for the throttling node that requires that you
specify either a throttle group object name or the limits, but never
both. If you specify the limits, this would just be a convenience
function that creates the right QOM object internally.

As for the implementation, QOM tends to be a bit heavy on boilerplate
code, but I think it's not too bad otherwise.


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