Which I/O controller is the fairest of them all?

The combination of dm-ioband + blkio-cgroup ends up with a really overcomplicated userspace interface for controlling I/O. To quote from the blkio-cgroup announcement, you have todo:

[quote]
make new bio cgroups and put some processes in them.

# mkdir /cgroup/grp1
# mkdir /cgroup/grp2
# echo 1234 > /cgroup/grp1/tasks
# echo 5678 > /cgroup/grp2/tasks

Now, check the ID of each blkio cgroup which is just created.

# cat /cgroup/grp1/blkio.id
2
# cat /cgroup/grp2/blkio.id
3

Finally, attach the cgroups to "ioband1" and assign them weights.

# dmsetup message ioband1 0 type cgroup
# dmsetup message ioband1 0 attach 2
# dmsetup message ioband1 0 attach 3
# dmsetup message ioband1 0 weight 2:30
# dmsetup message ioband1 0 weight 3:60
[/quote]

Now, consider if this was done entirely within cgroups, without any use of dmsetup. You could achieve the same result with

[example]
make new bio cgroups and put some processes in them.

# mkdir /cgroup/grp1
# mkdir /cgroup/grp2
# echo 1234 > /cgroup/grp1/tasks
# echo 5678 > /cgroup/grp2/tasks

Then assign them weights.

# echo "2:30" > /cgroup/grp1/blkio.weight
# echo "3:60" > /cgroup/grp2/blkio.weight
[/example]

This has the added benefit that if you're writing management APIs for this, you only need to be able to create/delete files & directories, and not worry about spawning external processes like dm-setup. So when the time comes to use this capability in libvirt, I'm rather hoping the kernel guys have decided on a pure cgroups userspace interface without device-mapper in the way.