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

[Kevin Wolf <kwolf@redhat.com>]

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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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