Re: [PATCH RFC 0/4] dm thin: support blk-throttle on data and metadata device

[Mike Snitzer <snitzer@redhat.com>]

On Fri, Jan 20 2017 at 10:19am -0500,
Jeff Moyer <jmoyer@redhat.com> wrote:

> Hou Tao <houtao1@huawei.com> writes:
> 
> > Hi all,
> >
> > We need to throttle the O_DIRECT IO on data and metadata device
> > of a dm-thin pool and encounter some problems. If we set the
> > limitation on the root blkcg, the throttle works. If we set the
> > limitation on a child blkcg, the throttle doesn't work well.
> >
> > The reason why the throttle doesn't work is that dm-thin defers
> > the process of bio when the physical block of bio has not been
> > allocated. The bio will be submitted by the pool worker, and the
> > blkcg of the bio will be the blkcg of the pool worker, namely,
> > the root blkcg instead of the blkcg of the original IO thread.
> > We only set a limitation on the blkcg of the original IO thread,
> > so the blk-throttle doesn't work well.
> >
> > In order to handle the situation, we add a "keep_bio_blkcg" feature
> > to dm-thin. If the feature is enabled, the original blkcg of bio
> > will be saved at thin_map() and will be used during blk-throttle.
> 
> Why is this even an option?  I would think that you would always want
> this behavior.

Right, shouldn't be an optional feature.

Also, this implementation is very dm-thin specific.  I still have this
line of work on my TODO because there should be a more generic way to
wire up these associations in either block core or DM core.

Now that there is both dm-crypt and dm-thin specific RFC patches to fix
this I'll see about finding a solution that works for both but that is
more generic.

Not sure how quickly I'll get to this but I'll do my best.