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

[John Snow <jsnow@redhat.com>]

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.