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

[Vivek Goyal]

On Fri, Jan 20, 2017 at 10:19:22AM -0500, Jeff Moyer wrote:
> Hou Tao  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.

Agreed that this should not be an optional feature.

One thing which always bothers me is that often we have dependencies
on finishing of these bios. So if we assign bios to their original
cgroups and if we create a cgroup with very low limit, then its
possible that everything slows down.

I guess answer to that is its a configuration issue and don't
create groups with too small a limits. And don't allow unprivileged
users to specify group limits.

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

[Mike Snitzer]

On Fri, Jan 20 2017 at 10:19am -0500,
Jeff Moyer  wrote:

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

[Jeff Moyer]

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

[Hou Tao]

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.

Tao

Hou Tao (4):
  dm thin: add a pool feature "keep_bio_blkcg"
  dm thin: parse "keep_bio_blkcg" from userspace tools
  dm thin: show the enabled status of keep_bio_blkcg feature
  dm thin: associate bio with current task if keep_bio_blkcg is enabled

 drivers/md/dm-thin.c | 26 --
 drivers/md/dm-thin.h | 17 +
 2 files changed, 41 insertions(+), 2 deletions(-)
 create mode 100644 drivers/md/dm-thin.h

-- 
2.5.0

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