Re: [Qemu-devel] blockdev-commit design

[Max Reitz <mreitz@redhat.com>]

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On 2017-09-26 19:59, Kevin Wolf wrote:

[...]

>   * The old block-commit command decides between an "actual" commit job
>     and the mirror-based active commit based on whether top is the
>     active layer.
> 
>     We don't get an option for the active layer any more now, so this
>     isn't how things can work for blockdev-commit. We could probably
>     check whether top has a BlockBackend parent, but that's not really
>     what we're interested in anyway. Maybe the best we could do to
>     decide this automatically is to check whether there is any parent of
>     top that requires write permissions. If there is, we need active
>     commit, otherwise the "normal" one is good enough.
> 
>     However, who says that the intentions of the user stay as we deduce
>     them at the start of the block job? Who says that the user doesn't
>     want to add a writable disk as a user of the node while the block
>     job is running?
> 
>     The optimal solution to this would be that the commit filter node
>     responds to permission requests and switches between active and
>     "normal" commit mode. I'm not sure how hard this would be to
>     implement.
> 
>     As long as we don't have the automatic switch, do we have to allow
>     the user to specify explicitly which mode they want instead of
>     automatically choosing one?

Probably a stupid question: What advantage does the commit job have over
the mirror job? I.e. would it be possible to just always do a mirror?

Max


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