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

[Jeff Moyer <jmoyer@redhat.com>]

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.

-Jeff