Re: [Qemu-devel] Minutes from the "Stuttgart block Gipfele"

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Dec 18, 2015 at 02:15:38PM +0100, Markus Armbruster wrote:
> What should happen when the user asks for a mutation at a place where we
> have implicit filter(s)?

Please suspend your disbelief for a second:

In principle it's simplest not having implicit filters.  The client
needs to set up throttling nodes or the backup filter explicitly.

Okay, now it's time to tear this apart:

For backwards compatibility it's necessary to support throttling,
copy-on-read, backup notifier, etc.  It may be possible to tag implicit
filter nodes so that mutation operations that wouldn't be possible today
are rejected.  The client must use the explicit syntax to do mutations
on implicit filters.  This is easier said than done, I'm not sure it can
be implemented cleanly.

Another problem is that the backup block job and other operations that
require a single command today could require sequences of low-level
setup commands to create filter nodes.  The QMP client would need to
first create a write notifier filter and then start the backup block job
with the write notifier node name.  It's clumsy.

Stefan

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