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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