Re: [Qemu-devel] [PATCH 4/7] throttle: Add throttle group support

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Fri, Apr 10, 2015 at 09:58:34AM +0200, Alberto Garcia wrote:
> On Thu, Apr 09, 2015 at 03:22:57PM +0100, Stefan Hajnoczi wrote:
> 
> > > @@ -1941,9 +1951,11 @@ void qmp_block_set_io_throttle(const char *device, int64_t bps, int64_t bps_rd,
> > >      aio_context_acquire(aio_context);
> > >  
> > >      if (!bs->io_limits_enabled && throttle_enabled(&cfg)) {
> > > -        bdrv_io_limits_enable(bs);
> > > +        bdrv_io_limits_enable(bs, has_group ? group : device);
> > >      } else if (bs->io_limits_enabled && !throttle_enabled(&cfg)) {
> > >          bdrv_io_limits_disable(bs);
> > > +    } else if (bs->io_limits_enabled && throttle_enabled(&cfg)) {
> > > +        bdrv_io_limits_update_group(bs, has_group ? group : device);
> > >      }
> > >  
> > >      if (bs->io_limits_enabled) {
> > 
> > The semantics are inconsistent:
> > 
> > 1. Create drive0 with throttle group "mygroup".
> > 2. Issue block-set-io-throttle device="drive0"
> > 
> > The result is that a new throttle group called "drive0" is created.
> > I expected to modify the throttling configuration for drive0 (i.e.
> > "mygroup").
> 
> That's right. What we can do is choose the group to update using the
> following criteria, in order of precedence:
> 
> 1) The 'group' parameter from block-set-io-throttle.
> 2) The group the device is already member of.
> 3) A new group ("drive0" in this example).
> 
> Currently we're not doing 2).

Great, it makes sense to add 2).

> > Now let's disable the throttle group:
> > 
> > 3. Issue block-set-io-throttle with 0 values for device="drive0"
> > 
> > The result is that "mygroup" is changed to all 0s.
> 
> No, that simply removes the device from the group without touching its
> configuration:
> 
> > >      } else if (bs->io_limits_enabled && !throttle_enabled(&cfg)) {
> > >          bdrv_io_limits_disable(bs);
> 
> The group name passed by the user is actually not used in this case.

You are right.

Stefan

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