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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 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0 |
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.
Max
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- Re: [Qemu-block] [PATCH] blockdev-backup: enable non-root nodes for backup, Max Reitz, 2017/12/01
- Re: [Qemu-block] [PATCH] blockdev-backup: enable non-root nodes for backup, John Snow, 2017/12/04
- Re: [Qemu-block] [PATCH] blockdev-backup: enable non-root nodes for backup, Max Reitz, 2017/12/04
- Re: [Qemu-block] [PATCH] blockdev-backup: enable non-root nodes for backup, John Snow, 2017/12/04
- Re: [Qemu-block] [PATCH] blockdev-backup: enable non-root nodes for backup,
Max Reitz <=
- Re: [Qemu-block] [Qemu-devel] [PATCH] blockdev-backup: enable non-root nodes for backup, John Snow, 2017/12/08
- Re: [Qemu-block] [Qemu-devel] [PATCH] blockdev-backup: enable non-root nodes for backup, Max Reitz, 2017/12/11
- Re: [Qemu-block] [Qemu-devel] [PATCH] blockdev-backup: enable non-root nodes for backup, John Snow, 2017/12/11
- Re: [Qemu-block] [Qemu-devel] [PATCH] blockdev-backup: enable non-root nodes for backup, Max Reitz, 2017/12/11
- Re: [Qemu-block] [Qemu-devel] [PATCH] blockdev-backup: enable non-root nodes for backup, John Snow, 2017/12/11