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[dm-devel] Re: RFC: I/O bandwidth controller



Hi, Fernando,

> >   - Implement a block layer resource controller. dm-ioband is a working
> > solution and feature rich but its dependency on the dm infrastructure is
> > likely to find opposition (the dm layer does not handle barriers
> > properly and the maximum size of I/O requests can be limited in some
> > cases). In such a case, we could either try to build a standalone
> > resource controller based on dm-ioband (which would probably hook into
> > generic_make_request) or try to come up with something new.
> 
> I doubt about the maximum size of I/O requests problem. You can't avoid
> this problem as far as you use device mapper modules with such a bad
> manner, even if the controller is implemented as a stand-alone controller.
> There is no limitation if you only use dm-ioband without any other device
> mapper modules.

Ryo told me this isn't true anymore. The dm infrastructure introduced
a new feature to support multiple page-sized I/O requests, that was
just merged to the current linux tree. So you and me don't need to
worry about this stuff anymore.

Ryo said he was going to make dm-ioband support this new feature and
post the patches soon.

> And I think the device mapper team just started designing barriers support.
> I guess it won't take long. Right, Alasdair?
> We should know it is logically impossible to support barriers on some
> types of device mapper modules such as LVM. You can't avoid the barrier
> problem when you use this kind of multiple devices even if you implement
> the controller in the block layer.
> 
> But I think a stand-alone implementation will have a merit that it
> makes it easier to setup the configuration rather than dm-ioband.
> From this point of view, it would be good that you move the algorithm
> of dm-ioband into the block layer.
> On the other hand, we should know it will make it impossible to use
> the dm infrastructure from the controller, though it isn't so rich.
> 
> >   - If the I/O tracking patches make it into the kernel we could move on
> > and try to get the Cgroup extensions to CFQ and AS mentioned before (see
> > (1), (2), and (3) above for details) merged.
> >   - Delegate the task of controlling the rate at which a task can
> > generate dirty pages to the memory controller.
> > 
> > This RFC is somewhat vague but my feeling is that we build some
> > consensus on the goals and basic design aspects before delving into
> > implementation details.
> > 
> > I would appreciate your comments and feedback.
> > 
> > - Fernando

Thanks,
Hirokazu Takahashi.


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