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. 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
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