Re: [RFC] writeback and cgroup

[Vivek Goyal <vgoyal-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Wed, Apr 11, 2012 at 09:22:31PM +0200, Jan Kara wrote:

[..]
> > >   Well, btrfs plays tricks with bdi's but there is a special bdi called
> > > "btrfs" which backs the whole filesystem and that is what's put in
> > > sb->s_bdi or in each inode's i_mapping->backing_dev_info. So we have a
> > > global bdi to work with.
> > 
> > Ok, that's good to know. How would we configure this special bdi? I am
> > assuming there is no backing device visible in /sys/block/<device>/queue/?
> > Same is true for network file systems.
>   Where should be the backing device visible? Now it's me who is lost :)

I mean how are we supposed to put cgroup throttling rules using cgroup
interface for network filesystems and for btrfs global bdi. Using "dev_t"
associated with bdi? I see that all the bdi's are showing up in
/sys/class/bdi, but how do I know which one I am intereste in or which
one belongs to filesystem I am interestd in putting throttling rule on.

For block devices, we simply use "major:min limit" format to write to
a cgroup file and this configuration will sit in one of the per queue
per cgroup data structure.

I am assuming that when you say throttling should happen at bdi, you
are thinking of maintaining per cgroup per bdi data structures and user
is somehow supposed to pass "bdi_maj:bdi_min  limit" through cgroup files?
If yes, how does one map a filesystem's bdi we want to put rules on?

Also, at request queue level we have bios and we throttle bios. At bdi
level, I think there are no bios yet. So somehow we got to deal with
pages. Not sure how exactly will throttling happen.

Thanks
Vivek