On 11.09.19 12:31, Kevin Wolf wrote: > 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 > block.c". > >> 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.
Yep. >>> 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 Yes, it probably is. > 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? That’s a good point. > 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. Possible. Maybe it turns out that at least for this series I don’t need bdrv_storage_child() at all. >>> 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. Hm. It feels like something that should be done before this series, actually. So I think we should add at least a child role per child access function so that they match? And then maybe in bdrv_attach_child() assert that a BDS never has more than one primary or filtered child (a filtered child acts as a primary child, too), or more than one COW child. (And that these are always in bs->file or bs->backing so the child access functions do work.) Max
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