Re: [Qemu-devel] [RFC][PATCH v2] block: add write threshold reporting for block devices

[Stefan Hajnoczi]

On Thu, Nov 20, 2014 at 11:30:53AM +0100, Kevin Wolf wrote:
> Am 17.11.2014 um 17:49 hat Stefan Hajnoczi geschrieben:
> > On Fri, Nov 07, 2014 at 02:12:13PM +0100, Francesco Romani wrote:
> > > +void bdrv_set_usage_threshold(BlockDriverState *bs, int64_t 
> > > threshold_bytes)
> > > +{
> > > +    BlockDriverState *target_bs = bs;
> > > +    if (bs->file) {
> > > +        target_bs = bs->file;
> > > +    }
> > 
> > Hmm...I think now I understand why you are trying to use bs->file.  This
> > is an attempt to make image formats work with the threshold.
> > 
> > Unfortunately the BlockDriverState topology can be more complicated than
> > just 1 level.
> > 
> > If we hardcode a strategy to traverse bs->file then it will work in most
> > cases:
> > 
> >   while (bs->file) {
> >       bs = bs->file;
> >   }
> > 
> > But there are cases like VMDK extent files where a BlockDriverState
> > actually has multiple children.
> > 
> > One way to solve this is to require that the management tool tells QEMU
> > which exact BlockDriverState node the threshold applies to.  Then QEMU
> > doesn't need any hardcoded policy.  But I'm not sure how realistic that
> > it at the moment (whether management tools are uses node names for each
> > node yet), so it may be best to hardcode the bs->file traversal that
> > I've suggested.
> > 
> > Kevin: Do you agree?
> 
> I have a feeling that we would regret this in the long run because it
> would allow only one special case of a general problem (watching a BDS).
> This means that we'll get inconsistent APIs.
> 
> We're "only" talking about an optimisation here, even though a very
> useful one, so I wouldn't easily make compromises here. We should
> probably insist on using the node-name. Management tools need new code
> anyway to make use of the new functionality, so they can implement
> node-name support as well while they're at it.

Using node-name is the best thing to do.

My concern is just whether libvirt and other management tools are
actually using node-name yet.

Stefan

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