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. Kevin
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