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

[Anthony Liguori <anthony@xxxxxxxxxxxxx>]

Hi,

I believe this work is very important especially in the context of 
virtual machines.  I think it would be more useful though implemented in 
the context of the IO scheduler.  Since we already support a notion of 
IO priority, it seems reasonable to add a notion of an IO cap.
Regards,

Anthony Liguori

Ryo Tsuruta wrote:
> 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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