[Xen-devel] [PATCH 0/2] dm-band: The I/O bandwidth controller: Overview

[Ryo Tsuruta <ryov@xxxxxxxxxxxxx>]

Hi everyone,

I'm happy to announce that I've implemented a Block I/O bandwidth controller.
The controller is designed to be of use in a cgroup or virtual machine
environment. The current approach is that the controller is implemented as
a device-mapper driver.

What's dm-band all about?
========================
Dm-band is an I/O bandwidth controller implemented as a device-mapper driver.
Several jobs using the same physical device have to share the bandwidth of
the device. Dm-band gives bandwidth to each job according to its weight, 
which each job can set its own value to.

At this time, a job is a group of processes with the same pid or pgrp or uid.
There is also a plan to make it support cgroup. A job can also be a virtual
machine such as KVM or Xen.

  +------+ +------+ +------+   +------+ +------+ +------+ 
  |cgroup| |cgroup| | the  |   | pid  | | pid  | | the  |  jobs
  |  A   | |  B   | |others|   |  X   | |  Y   | |others| 
  +--|---+ +--|---+ +--|---+   +--|---+ +--|---+ +--|---+   
  +--V----+---V---+----V---+   +--V----+---V---+----V---+   
  | group | group | default|   | group | group | default|  band groups
  |       |       |  group |   |       |       |  group | 
  +-------+-------+--------+   +-------+-------+--------+
  |         band1          |   |         band2          |  band devices
  +-----------|------------+   +-----------|------------+
  +-----------V--------------+-------------V------------+
  |                          |                          |
  |          sdb1            |           sdb2           |  physical devices
  +--------------------------+--------------------------+


How dm-band works.
========================
Every band device has one band group, which by default is called the default
group.

Band devices can also have extra band groups in them. Each band group
has a job to support and a weight. Proportional to the weight, dm-band gives
tokens to the group.

A group passes on I/O requests that its job issues to the underlying
layer so long as it has tokens left, while requests are blocked
if there aren't any tokens left in the group. One token is consumed each
time the group passes on a request. Dm-band will refill groups with tokens
once all of groups that have requests on a given physical device use up their
tokens.

With this approach, a job running on a band group with large weight is
guaranteed to be able to issue a large number of I/O requests.


Getting started
=============
The following is a brief description how to control the I/O bandwidth of
disks. In this description, we'll take one disk with two partitions as an
example target.

You can also check the manual at Document/device-mapper/band.txt of the
linux kernel source tree for more information.


Create and map band devices
---------------------------
Create two band devices "band1" and "band2" and map them to "/dev/sda1"
and "/dev/sda2" respectively.

 # echo "0 `blockdev --getsize /dev/sda1` band /dev/sda1 1" | dmsetup create 
band1
 # echo "0 `blockdev --getsize /dev/sda2` band /dev/sda2 1" | dmsetup create 
band2

If the commands are successful then the device files "/dev/mapper/band1"
and "/dev/mapper/band2" will have been created.


Bandwidth control
----------------
In this example weights of 40 and 10 will be assigned to "band1" and
"band2" respectively. This is done using the following commands:

 # dmsetup message band1 0 weight 40
 # dmsetup message band2 0 weight 10

After these commands, "band1" can use 80% --- 40/(40+10)*100 --- of the
bandwidth of the physical disk "/dev/sda" while "band2" can use 20%.


Additional bandwidth control
---------------------------
In this example two extra band groups are created on "band1".
The first group consists of all the processes with user-id 1000 and the
second group consists of all the processes with user-id 2000. Their
weights are 30 and 20 respectively.

Firstly the band group type of "band1" is set to "user".
Then, the user-id 1000 and 2000 groups are attached to "band1".
Finally, weights are assigned to the user-id 1000 and 2000 groups.

 # dmsetup message band1 0 type user
 # dmsetup message band1 0 attach 1000
 # dmsetup message band1 0 attach 2000
 # dmsetup message band1 0 weight 1000:30
 # dmsetup message band1 0 weight 2000:20

Now the processes in the user-id 1000 group can use 30% ---
30/(30+20+40+10)*100 --- of the bandwidth of the physical disk.

 Band Device    Band Group                     Weight
  band1         user id 1000                     30
  band1         user id 2000                     20
  band1         default group(the other users)   40
  band2         default group                    10


Remove band devices
-------------------
Remove the band devices when no longer used.

  # dmsetup remove band1
  # dmsetup remove band2


TODO
========================
  - Cgroup support. 
  - Control read and write requests separately.
  - Support WRITE_BARRIER.
  - Optimization.
  - More configuration tools. Or is the dmsetup command sufficient?
  - Other policies to schedule BIOs. Or is the weight policy sufficient?

Thanks,
Ryo Tsuruta

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