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

[Max Reitz]

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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