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. 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.
