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Re: [Qemu-devel] blockdev-commit design
From: |
Max Reitz |
Subject: |
Re: [Qemu-devel] blockdev-commit design |
Date: |
Wed, 27 Sep 2017 18:29:33 +0200 |
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Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.3.0 |
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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