Re: [Qemu-devel] [PATCH for-2.6] block: Forbid I/O throttling on nodes with multiple parents for 2.6

[Alberto Garcia]

On Wed 06 Apr 2016 11:59:47 AM CEST, Kevin Wolf wrote:
> Let me draw some ASCII art. I'm drawing I/O throttling like a filter
> to make the difference clear; in reality it's part of the BDS now or
> of the BB in 2.7, so don't let this confuse you.
>
> Let's assume these command line options:
>
> -drive file=backimg.img,if=none,id=backing,iops=1024
> -drive file=overlay.qcow2,id=overlay,backing=backing
>
> If we didn't catch this configuration and error out, this is what we
> would end up with in 2.6:
>
>     [BB] overlay
>          |
>          |
>     [BDS] qcow2         [BB] backing
>          |                    |
>          +--------------------+
>          .                    |
>          .             I/O throttling
>          .                    |
>                         [BDS] raw
>                               .
>                               .
>                               .

I see, thanks for the explanation!

But what's the use of having a BlockBackend on a node that is used as a
backing image? Is it because "we allow this so let's prevent the user
from doing weird things", or is there anything useful we can do with it?

Berto