On 2017-12-05 01:48, John Snow wrote: > > > On 12/04/2017 05:21 PM, Max Reitz wrote: >> On 2017-12-04 23:15, John Snow wrote: >>> >>> >>> On 12/01/2017 02:41 PM, Max Reitz wrote: >>>> ((By the way, I don't suppose that's how it should work... But I don't >>>> suppose that we want propagation of dirtying towards the BDS roots, do >>>> we? :-/)) >>> >>> I have never really satisfactorily explained to myself what bitmaps on >>> intermediate notes truly represent or mean. >>> >>> The simple case is "This layer itself serviced a write request." >>> >>> If that information is not necessarily meaningful, I'm not sure that's a >>> problem except in configuration. >>> >>> >>> ...Now, if you wanted to talk about bitmaps that associate with a >>> Backend instead of a Node... >> >> But it's not about bitmaps on intermediate nodes, quite the opposite. >> It's about bitmaps on roots but write requests happening on intermediate >> nodes. >> > > Oh, I see what you're saying. It magically doesn't really change my > opinion, by coincidence! > >> Say you have a node I and two filter nodes A and B using it (and they >> are OK with shared writers). There is a dirty bitmap on A. >> >> Now when a write request goes through B, I will obviously have changed, >> and because A and B are filters, so will A. But the dirty bitmap on A >> will still be clean. >> >> My example was that when you run a mirror over A, you won't see dirtying >> from B. So you can't e.g. add a throttle driver between a mirror job >> and the node you want to mirror, because the dirty bitmap on the >> throttle driver will not be affected by accesses to the actual node. >> >> Max >> > > Well, in this case I would say that a root BDS is not really any > different from an intermediate one and can't really know what's going on > in the world outside. > > At least, I think that's how we model it right now -- we pretend that we > can record the activity of an entire drive graph by putting the bitmap > on the root-most node we can get a hold of and assuming that all writes > are going to go through us.
Well, yeah, I know we do. But I consider this counter-intuitive and if something is counter-intuitive it's often a bug. > Clearly this is increasingly false the more we modularise the block graph. > > > *uhm* > > > I would say that a bitmap attached to a BlockBackend should behave in > the way you say: writes to any children should change the bitmap here. > > bitmaps attached to nodes shouldn't worry about such things. Do we have bitmaps attached to BlockBackends? I sure hope not. We should not have any interface that requires the use of BlockBackends by now. If we do, that's something that has to be fixed. Max
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