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Re: [Qemu-block] [PATCH v6 22/42] block: Fix bdrv_get_allocated_file_siz

From: Kevin Wolf
Subject: Re: [Qemu-block] [PATCH v6 22/42] block: Fix bdrv_get_allocated_file_size's fallback
Date: Wed, 11 Sep 2019 12:31:23 +0200
User-agent: Mutt/1.12.1 (2019-06-15)

Am 11.09.2019 um 12:00 hat Max Reitz geschrieben:
> On 11.09.19 10:27, Kevin Wolf wrote:
> > Am 11.09.2019 um 09:37 hat Max Reitz geschrieben:
> >> On 11.09.19 08:55, Kevin Wolf wrote:
> >>> Well, by default the primary child, which should cover like 90% of the
> >>> drivers?
> >>
> >> Hm, yes.
> >>
> >> But I still think that the drivers that do not want to count every
> >> single non-COW child are the exception.
> > 
> > They are, but drivers that want to count more than their primary node
> > are exceptions, too. And I think you're more likely to remember adding
> > the callback when you want to have a certain feature, not when you don't
> > want to have it.
> > 
> > I really think we're likely to forget adding the callback where we need
> > to disable the feature.
> Well, I mean, we did forget adding it for qcow2.

I'm afraid I have to agree. So the conclusion is that we won't get it
right anyway?

> > I can see two options that should address both of our views:
> > 
> > 1. Just don't have a fallback at all, make the callback mandatory and
> >    provide implementations in block.c that can be referred to in
> >    BlockDriver. Not specifying the callback causes an assertion failure,
> >    so we'd hopefully notice it quite early (assuming that we run either
> >    'qemu-img info' or 'query-block' on a configuration with the block
> >    driver, but I think that's faily safe to assume).
> Hm.  Seems a bit much, but if we can’t agree on what’s a good general
> implementation that works for everything, this is probably the only
> thing that would actually keep us from forgetting to add special cases.
> Though I actually don’t know.  I’d probably add two globally available
> helpers, one that returns the sum of everything but the backing node,
> and one that just returns the primary node.

Yes, I think this is the same as I meant by "provide implementations in

> Now if I were to make qcow2 use the primary node helper function, would
> we have remembered changing it once we added a data file?
> Hmm.  Maybe not, but it should be OK to just make everything use the sum
> helper, except the drivers that want the primary node.  That should work
> for all cases.  (I think that whenever a format driver suddenly gains
> more child nodes, we probably will want to count them.  OTOH, everything
> that has nodes that shouldn’t be counted probably always wants to use
> the primary node helper function from the start.)

The job filter nodes have only one child currently, which should be
counted. We'll add other children that shouldn't be counted only later.

But we already have an idea of what possible extensions look like, so we
can probably choose the right function from the start.

> > 2. Make the 90% solution a 100% solution: Allow drivers to have multiple
> >    storage children (for vmdk) and then have the fallback add up the
> >    primary child plus all storage children. This is what I suggested as
> >    the documented semantics in my initial reply to this patch (that you
> >    chose not to answer).
> I didn’t answer that because I didn’t disagree.
> >    Adding the size of storage children covers qcow2 and vmdk.
> That’s of course exactly what we’re trying to do, but the question is,
> how do we figure out that storage children?  Make it a per-BdrvChild
> attribute?  That seems rather heavy-handed, because I think we’d need it
> only here.

Well, you added bdrv_storage_child(). I'd argue this interface is wrong
because it assumes that only one storage child exists. You just didn't
implement it for vmdk so that the problem didn't become apparent. It
would have to return a list rather than a single child. So fixing the
interface and then using it is what I was thinking.

Now that you mention a per-BdrvChild attribute, however, I start to
wonder if the distinction between COW children, filter children, storage
children, metadata children, etc. isn't really what BdrvChildRole was
supposed to represent?

Maybe we want to split off child_storage from child_file, though it's
not strictly necessary for this specific case because we want to treat
both metadata and storage nodes the same. But it could be useful for
other users of bdrv_storage_child(), if there are any.

> >    As the job filter won't declare the target or any other involved
> >    nodes their storage nodes (I hope), this will do the right thing for
> >    them, too.
> > 
> >    For quorum and blkverify both ways could be justifiable. I think they
> >    probably shouldn't declare their children as storage nodes. They are
> >    more like filters that don't have a single filtered node. So some
> >    kind of almost-filters.
> I don’t think quorum is a filter, and blkverify can only be justified to
> be a filter because it quits qemu when there is a mismatch.
> The better example is replication, but that has a clear filtered child
> (the primary node).
> So all in all I think it’s best to make the callback mandatory and add
> two global helper functions.  That’s simple enough and should prevent
> us from making mistakes by forgetting to adjust something in the
> future.

Yes, that should work.

We should probably still figure out what the relationship between the
child access functions and child roles is, even if we don't need it for
this solution. But it feels like an important part of the design.


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