[dm-devel] blkio cgroups controller doesn't work with LVM?
Mike Snitzer
snitzer at redhat.com
Thu Feb 25 14:53:14 UTC 2016
On Thu, Feb 25 2016 at 2:48am -0500,
Nikolay Borisov <kernel at kyup.com> wrote:
>
>
> On 02/24/2016 08:12 PM, Chris Friesen wrote:
> >
> > Hi,
> >
> > Are there known limitations with the blkio cgroup controller when used
> > with LVM?
> >
> > I'm using Ubuntu 15.10 with the 4.2 kernel. I got the same results with
> > CentOS 7.
> >
> > I set up two groups, /sys/fs/cgroup/blkio/test1 and
> > /sys/fs/cgroup/blkio/test2. I set the weight for test1 to 500, and the
> > weight for test2 to 1000.
>
> The weighed mode of blkio works only with CFQ scheduler. And as far as I
> have seen you cannot set CFQ to be the scheduler of DM devices. In this
> case you can use the BLK io throttling mechanism. That's what I've
> encountered in my practice. Though I'd be happy to be proven wrong by
> someone. I believe the following sentence in the blkio controller states
> that:
> "
> First one is proportional weight time based division of disk policy. It
> is implemented in CFQ. Hence this policy takes effect only on leaf nodes
> when CFQ is being used.
> "
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.
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!
Mike
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