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Date: Fri, 10 Apr 2015 09:58:34 +0200
From: Alberto Garcia <berto@igalia.com>
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Subject: Re: [Qemu-devel] [PATCH 4/7] throttle: Add throttle group support
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>

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).

> 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.

Berto