[dm-devel] blkio cgroups controller doesn't work with LVM?

[Tejun Heo]

Hello,

On Thu, Feb 25, 2016 at 09:53:14AM -0500, Mike Snitzer wrote:
> Right, LVM created devices are bio-based DM devices in the kernel.
> bio-based block devices do _not_ have an IO scheduler.  Their underlying
> request-based device does.

dm devices are not the actual resource source, so I don't think it'd
work too well to put io controllers on them (can't really do things
like proportional control without owning the queue).

> I'm not well-versed on the top-level cgroup interface and how it maps to
> associated resources that are established in the kernel.  But it could
> be that the configuration of blkio cgroup against a bio-based LVM device
> needs to be passed through to the underlying request-based device
> (e.g. /dev/sda4 in Chris's case)?
> 
> I'm also wondering whether the latest cgroup work that Tejun has just
> finished (afaik to support buffered IO in the IO controller) will afford
> us a more meaningful reason to work to make cgroups' blkio controller
> actually work with bio-based devices like LVM's DM devices?
> 
> I'm very much open to advice on how to proceed with investigating this
> integration work.  Tejun, Vivek, anyone else: if you have advice on next
> steps for DM on this front _please_ yell, thanks!

I think the only thing necessary is dm transferring bio cgroup tags to
the bio's that it ends up passing down the stack.  Please take a look
at fs/btrfs/extent_io.c::btrfs_bio_clone() for an example.  We
probably should introduce a wrapper for this so that each site doesn't
need to ifdef it.

Thanks.

-- 
tejun