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Re: [Qemu-block] [PATCH] blockdev-backup: enable non-root nodes for back

From: Max Reitz
Subject: Re: [Qemu-block] [PATCH] blockdev-backup: enable non-root nodes for backup
Date: Fri, 8 Dec 2017 15:30:49 +0100
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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.


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