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Re: [Qemu-block] Throttling groups vs filter nodes

From: Kevin Wolf
Subject: Re: [Qemu-block] Throttling groups vs filter nodes
Date: Tue, 30 May 2017 17:32:17 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

Am 30.05.2017 um 16:29 hat Alberto Garcia geschrieben:
> On Sat 27 May 2017 09:56:03 AM CEST, Stefan Hajnoczi wrote:
> > Throttling groups allow multiple drives to share the same throttling
> > state (i.e. budget) between them.  Manos is working on moving the
> > throttling code into a block filter driver so it is no longer
> > hardcoded into the I/O code path.
> Now that we're discussing changes to the throttling code, I'll take the
> opportunity to describe a feature that I'd like to work on at some point
> in the future.
> This is of course out of the scope of the work that Manos is going to
> do, but I'd like to discuss it briefly to see what people think of this
> idea and whether it's compatible with the changes that we are now
> proposing.
> Currently each block device can have either zero or one set of I/O
> limits, and that set of limits can be shared among several drives using
> throttling groups.
> I have the following use case that cannot be handled with the current
> code:
>    - The provider offers the user different storage types, and each one
>      of them can have a different storage backend and a different set of
>      I/O limits.
>    - Inside each storage type, the user can have several volumes, each
>      one with its own set of I/O limits.
>    - So the user could have drive A, B and C, each one of them with its
>      own separate limits, and in addition to that the combination of all
>      three would have a limit specific to the storage type.
> I believe that this could be achieved by simply having two filter nodes
> per drive, one of them would be attached to a throttling object specific
> to that drive, and the other one would be attached to a throttling
> object that would be shared by all drives (i.e. it would be a 3-drive
> throttling group).
> I think this should be possible with the -object approach that we are
> discussing.

Yes, that should be possible. In fact, I don't think it's out of scope
for Manos' work, but it should just natually fall out of it.

That you can put throttle filter nodes anywhere in the graph (and
therefore also stack multiple throttle filter nodes on top of each
other) is the whole point of doing this work, so I would consider this a
very basic requirement for any configuration interface that we could
come up wih.


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