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[dm-devel] [PATCH 2/2] dm-ioband v0.0.3: The I/O bandwidth controller: Document



Here is the document of dm-ioband.

Based on 2.6.24
Signed-off-by: Ryo Tsuruta <ryov valinux co jp>
Signed-off-by: Hirokazu Takahashi <taka valinux co jp>

diff -uprN linux-2.6.24.orig/Documentation/device-mapper/ioband.txt linux-2.6.24/Documentation/device-mapper/ioband.txt
--- linux-2.6.24.orig/Documentation/device-mapper/ioband.txt	1970-01-01 09:00:00.000000000 +0900
+++ linux-2.6.24/Documentation/device-mapper/ioband.txt	2008-02-05 19:09:41.000000000 +0900
@@ -0,0 +1,728 @@
+======================
+Document for dm-ioband
+======================
+
+Contents:
+  What's dm-ioband all about?
+  Differences from the CFQ I/O scheduler
+  How dm-ioband works
+  Setup and Installation
+  Getting started
+  Command Reference
+  Examples
+  TODO
+
+
+What's dm-ioband all about?
+===========================
+dm-ioband 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-ioband 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|  ioband groups
+  |       |       |  group |   |       |       |  group | 
+  +-------+-------+--------+   +-------+-------+--------+
+  |        ioband1         |   |       ioband2          |  ioband devices
+  +-----------|------------+   +-----------|------------+
+  +-----------V--------------+-------------V------------+
+  |                          |                          |
+  |          sdb1            |           sdb2           |  physical devices
+  +--------------------------+--------------------------+
+
+
+Differences from the CFQ I/O scheduler
+======================================
+
+Dm-ioband is flexible to configure the bandwidth settings. 
+
+Dm-ioband can work with any type of I/O scheduler such as the NOOP scheduler,
+which is often chosen for high-end storages, since it is implemented outside
+the I/O scheduling layer. It allows both of partition based bandwidth control
+and job --- a group of processes --- based control. In addition, it can
+set different configuration on each physical device to control its bandwidth.
+
+Meanwhile the current implementation of the CFQ scheduler has seven IO priority
+levels and all jobs whose processes have the same IO priority share the
+bandwidth assigned to this level between them. And IO priority is an attribute
+of a process so that it equally effects to all block devices.
+
+
+How dm-ioband works.
+====================
+Every ioband device has one ioband group, which by default is called the
+default group.
+
+Ioband devices can also have extra ioband groups in them. Each ioband group
+has a job to support and a weight. Proportional to the weight, dm-ioband 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-ioband 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 an ioband group with large weight is
+guaranteed to be able to issue a large number of I/O requests.
+
+
+Setup and Installation
+======================
+
+Build a kernel with these options enabled:
+
+  CONFIG_MD
+  CONFIG_BLK_DEV_DM
+  CONFIG_DM_IOBAND
+
+If compiled as module, use modprobe to load dm-ioband.
+
+  # make modules
+  # make modules_install
+  # depmod -a
+  # modprobe dm-ioband
+
+"dmsetup targets" command shows all available device-mapper targets.
+"ioband" is displayed if dm-ioband has been loaded.
+
+  # dmsetup targets
+  ioband           v0.0.3
+
+
+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.
+
+
+Create and map ioband devices
+-----------------------------
+Create two ioband devices "ioband1" and "ioband2" and map them to "/dev/sda1"
+and "/dev/sda2" respectively.
+
+ # echo "0 $(blockdev --getsize /dev/sda1) ioband /dev/sda1 1" | \
+     dmsetup create ioband1
+ # echo "0 $(blockdev --getsize /dev/sda2) ioband /dev/sda2 1" | \
+     dmsetup create ioband2
+
+If the commands are successful then the device files "/dev/mapper/ioband1"
+and "/dev/mapper/ioband2" will have been created.
+
+
+Bandwidth control
+-----------------
+In this example, weights of 40 and 10 will be assigned to "ioband1" and
+"ioband2" respectively. This is done using the following commands:
+
+ # dmsetup message ioband1 0 weight 40
+ # dmsetup message ioband2 0 weight 10
+
+After these commands, "ioband1" can use 80% --- 40/(40+10)*100 --- of the
+bandwidth of the physical disk "/dev/sda" while "ioband2" can use 20%.
+
+
+Additional bandwidth control
+----------------------------
+In this example two extra ioband groups are created on "ioband1".
+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.
+
+ # dmsetup message ioband1 0 type user
+ # dmsetup message ioband1 0 attach 1000
+ # dmsetup message ioband1 0 attach 2000
+ # dmsetup message ioband1 0 weight 1000:30
+ # dmsetup message ioband1 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.
+
+ ioband device   ioband group                   weight
+ ioband1         user id 1000                     30
+ ioband1         user id 2000                     20
+ ioband1         default group(the other users)   40
+ ioband2         default group                    10
+
+
+Remove the ioband devices
+-------------------------
+Remove the ioband devices when no longer used.
+
+  # dmsetup remove ioband1
+  # dmsetup remove ioband2
+
+
+Command Reference
+=================
+
+
+Create an ioband device
+-----------------------
+SYNOPSIS
+  dmsetup create IOBAND_DEVICE
+
+DESCRIPTION
+  Create an ioband device with the given name IOBAND_DEVICE. The following
+  arguments, which dmsetup command reads from standard input, are also
+  required.
+
+      Logical starting sector. This must be "0."
+      The number of sectors to use.
+      "ioband" as a target type.
+      The path name of the physical device.
+      Device group ID.
+      I/O throttling value (optional)
+      I/O limiting value (optional)
+
+  The same device group ID must be set among the ioband devices that share
+  the same bandwidth, which means they work on the same physical disk.
+  "The number of sectors to use" should be "the number of sectors the physical
+  device." I/O throttling value and I/O limiting value, which are described
+  later in this document, are optional.
+
+  If the command is successful, the device file
+  "/dev/device-mapper/IOBAND_DEVICE" will have been created.
+  An ioband group is also created and attached to IOBAND_DEVICE as the default
+  ioband group.
+
+EXAMPLE
+  Create an ioband device with the following parameters:
+    physical device = "/dev/sda1"
+    ioband device name = "ioband1"
+    device group ID = "1"
+    I/O throttling value = "10"
+    I/O limiting value = "200"
+
+    # echo "0 $(blockdev --getsize /dev/sda1) ioband /dev/sda1 1 10 200" | \
+        dmsetup create ioband1
+
+  Create two device groups (ID=1,2). The bandwidths of these device groups
+  will be individually controlled.
+
+    # echo "0 $(blockdev --getsize /dev/sda1) ioband /dev/sda1 1" | \
+        dmsetup create ioband1
+    # echo "0 $(blockdev --getsize /dev/sda2) ioband /dev/sda2 1" | \
+        dmsetup create ioband2
+    # echo "0 $(blockdev --getsize /dev/sdb3) ioband /dev/sdb3 2" | \
+        dmsetup create ioband3
+    # echo "0 $(blockdev --getsize /dev/sdb4) ioband /dev/sdb4 2" | \
+        dmsetup create ioband4
+
+
+Remove the ioband device
+------------------------
+SYNOPSIS
+  dmsetup remove IOBAND_DEVICE
+
+DESCRIPTION
+  Remove the specified ioband device IOBAND_DEVICE. All the band groups
+  attached to the ioband device are also removed automatically.
+
+EXAMPLE
+  Remove ioband device "ioband1."
+
+  # dmsetup remove ioband1
+
+
+Set an ioband group type
+--------------------------
+SYNOPSIS
+  dmsetup message IOBAND_DEVICE 0 type TYPE
+
+DESCRIPTION
+  Set the ioband group type of the specified ioband device IOBAND_DEVICE. TYPE
+  must be one of "user", "gid", "pid" or "pgrp." Once the type is set, new
+  ioband groups can be created on IOBAND_DEVICE.
+
+EXAMPLE
+  Set the ioband group type of ioband device "ioband1" to "user."
+
+  # dmsetup message ioband1 0 type user
+
+
+Create an ioband group
+----------------------
+SYNOPSIS
+  dmsetup message IOBAND_DEVICE 0 attach ID
+
+DESCRIPTION
+  Create an ioband group and attach it to IOBAND_DEVICE.
+  ID specifies user-id, group-id, process-id or process-group-id depending
+  the ioband group type of IOBAND_DEVICE.
+
+EXAMPLE
+  Create an ioband group which consists of all processes with user-id 1000 and
+  attach it to ioband device "ioband1."
+
+  # dmsetup message ioband1 0 type user
+  # dmsetup message ioband1 0 attach 1000
+
+
+Detach the ioband group
+-----------------------
+SYNOPSIS
+  dmsetup message IOBAND_DEVICE 0 detach ID
+
+DESCRIPTION
+  Detach the ioband group specified by ID from ioband device IOBAND_DEVICE.
+
+EXAMPLE
+  Detach the ioband group with ID "2000" from ioband device "ioband2."
+
+  # dmsetup message ioband2 0 detach 1000
+
+
+Set the weight of an ioband group
+---------------------------------
+SYNOPSIS
+  dmsetup message IOBAND_DEVICE 0 weight VAL
+  dmsetup message IOBAND_DEVICE 0 weight ID:VAL
+
+DESCRIPTION
+  Set the weight of the ioband group specified by ID. Set the weight of the
+  default ioband group of IOBAND_DEVICE if ID isn't specified.
+  The following example means that "ioband1" can use 80% --- 40/(40+10)*100
+   --- of the bandwidth of the physical disk while "ioband2" can use 20%.
+
+    # dmsetup message ioband1 0 weight 40
+    # dmsetup message ioband2 0 weight 10
+
+  The following lines have the same effect as the above:
+
+    # dmsetup message ioband1 0 weight 4
+    # dmsetup message ioband2 0 weight 1  
+
+  VAL must be an integer larger than 0. The default value, which is assigned
+  to newly created ioband groups, is 100.
+
+EXAMPLE
+  Set the weight of the default ioband group of "ioband1" to 40.
+
+  # dmsetup message ioband1 0 weight 40
+
+  Set the weight of the ioband group of "ioband1" with ID "1000" to 10.
+
+  # dmsetup message ioband1 0 weight 1000:10
+
+
+Set the number of tokens
+------------------------
+SYNOPSIS
+  dmsetup message IOBAND_DEVICE 0 token VAL
+
+DESCRIPTION
+  Set the number of tokens to VAL. According to their weight, this number of
+  tokens will be distributed to all the ioband groups on the physical device
+  to which ioband device IOBAND_DEVICE belongs when they use up their tokens.
+
+  VAL must be an integer greater than 0. The default is 2048.
+
+EXAMPLE
+  Set the number of tokens of the physical device to which "ioband1" belongs
+  to 256.
+
+  # dmsetup message ioband1 0 token 256
+
+
+Set I/O throttling
+------------------
+SYNOPSIS
+  dmsetup message IOBAND_DEVICE 0 io_throttle VAL
+
+DESCRIPTION
+  Set the I/O throttling value of the physical disk to which ioband device
+  IOBAND_DEVICE belongs to VAL. Dm-ioband start to control the bandwidth
+  when the number of BIOs in progress on the physical disk exceeds this value.
+
+EXAMPLE  
+  Set the I/O throttling value of "ioband1" to 16.
+
+  # dmsetup message ioband1 0 io_throttle 16
+
+
+Set I/O limiting
+----------------
+SYNOPSIS
+  dmsetup message IOBAND_DEVICE 0 io_limit VAL
+
+DESCRIPTION
+  Set the I/O limiting value of the physical disk to which ioband device
+  IOBAND_DEVICE belongs to VAL. Dm-ioband will block all I/O requests for
+  the physical device if the number of BIOs in progress on the physical disk
+  exceeds this value.
+
+EXAMPLE  
+  Set the I/O limiting value of "ioband1" to 128.
+
+  # dmsetup message ioband1 0 io_limit 128
+
+
+Display settings
+----------------
+SYNOPSIS
+  dmsetup table --target ioband
+
+DESCRIPTION
+  Display the settings of all the ioband devices whose target type is "ioband."
+
+  The output format is as below:
+    ioband device name
+    starting sector of partition
+    partition size in sectors
+    target type
+    device number (major:minor)
+    device group ID
+    I/O throttle
+    I/O limit
+
+EXAMPLE
+  Display the setting of an ioband device configured such as:
+    device name = "ioband1"
+    starting sector of partition = "0"
+    partition size in sectors = "44371467"
+    target type = "ioband"
+    device number (major:minor) = "202:33"
+    device group ID = "128"
+    I/O throttle = "10"
+    I/O limit = "400"
+
+  # dmsetup table --target ioband
+  ioband1: 0 44371467 ioband 202:33 128 10 400
+
+Display Statistics
+------------------
+SYNOPSIS
+  dmsetup status --target ioband
+
+DESCRIPTION
+  Display the statistics of all the ioband devices whose target type is
+  "ioband."
+
+  The output format is as below. the first five columns shows:
+      ioband device name
+      logical start sector of the device (must be 0)
+      device size in sectors
+      target type (must be "ioband")
+      device group ID
+
+   The remaining columns show the statistics of each ioband group on the
+   band device. Each group uses seven columns for its statistics.
+      ioband group ID (-1 means default)
+      total read requests
+      delayed read requests
+      total read sectors
+      total write requests
+      delayed write requests
+      total write sectors
+
+EXAMPLE
+  The following output shows the statistics of two ioband devices. Ioband2 only
+  has the default ioband group and ioband1 has three (default, 1001, 1002)
+  ioband groups. 
+
+  # dmsetup status
+  ioband2: 0 44371467 ioband 128 -1 143 90 424 122 78 352
+  ioband1: 0 44371467 ioband 128 -1 223 172 408 211 136 600 1001 166 107 \
+  472 139 95 352 1002 211 146 520 210 147 504
+
+Reset status counter
+--------------------
+SYNOPSIS
+  dmsetup message IOBAND_DEVICE 0 reset
+
+DESCRIPTION
+  Reset the statistics of ioband device IOBAND_DEVICE.
+
+EXAMPLE
+  Reset the statistics of "ioband1."
+
+  # dmsetup message ioband1 0 reset
+
+
+Examples
+========
+
+
+Example #1: Bandwidth control on Partitions
+-------------------------------------------
+This example describes how to control the bandwidth with disk partitions.
+The following diagram illustrates the configuration of this example.
+You may want to run a database on /dev/mapper/ioband1 and web applications
+on /dev/mapper/ioband2.
+
+              /mnt1                          /mnt2            mount points
+                |                              |
+  +-------------V------------+   +-------------V------------+ 
+  |   /dev/mapper/ioband1    |   |   /dev/mapper/ioband2    | ioband devices
+  +--------------------------+   +--------------------------+
+  |       default group      |   |       default group      | ioband groups
+  |           (80)           |   |           (40)           |    (weight)
+  +-------------|------------+   +-------------|------------+
+                |                              |
+  +-------------V--------------+---------------V------------+
+  |         /dev/sda1          |           /dev/sda2        | physical devices
+  +----------------------------+----------------------------+
+
+To setup the above configuration, follow these steps:
+
+  1) Create ioband devices with the same device group ID.
+
+    # echo "0 $(blockdev --getsize /dev/sda1) ioband /dev/sda1 1" | \
+        dmsetup create ioband1
+    # echo "0 $(blockdev --getsize /dev/sda2) ioband /dev/sda2 1" | \
+        dmsetup create ioband2
+
+  2) Assign weights of 80 and 40 to the default ioband groups respectively.
+
+    # dmsetup message ioband1 0 weight 80
+    # dmsetup message ioband2 0 weight 40
+
+  3) Create filesystems on the ioband devices and mount them.
+
+    # mkfs.ext3 /dev/mapper/ioband1
+    # mount /dev/mapper/ioband1 /mnt1
+
+    # mkfs.ext3 /dev/mapper/ioband2
+    # mount /dev/mapper/ioband2 /mnt2
+
+
+
+Example #2: Bandwidth control on Logical Volumes
+------------------------------------------------
+This example is similar to the example #1 but it uses LVM logical volumes
+instead of disk partitions. This example shows how to configure ioband devices
+on two striped logical volumes.
+
+              /mnt1                          /mnt2            mount points
+                |                              |
+  +-------------V------------+   +-------------V------------+ 
+  |   /dev/mapper/ioband1    |   |   /dev/mapper/ioband2    | ioband devices
+  +--------------------------+   +--------------------------+
+  |       default group      |   |       default group      | ioband groups
+  |           (80)           |   |           (40)           |    (weight)
+  +-------------|------------+   +-------------|------------+
+                |                              |
+  +-------------V------------+   +-------------V------------+
+  |      /dev/mapper/lv0     |   |     /dev/mapper/lv1      | striped logical
+  |                          |   |                          | volumes
+  +---------------------------------------------------------+
+  |                           vg0                           | volume group
+  +-------------|------------------------------|------------+
+                |                              |
+  +-------------V------------+   +-------------V------------+
+  |         /dev/sdb         |   |         /dev/sdc         | physical devices
+  +--------------------------+   +--------------------------+
+
+To setup the above configuration, follow these steps:
+
+  1) Initialize the partitions for use by LVM.
+
+    # pvcreate /dev/sdb
+    # pvcreate /dev/sdc
+
+  2) Create a new volume group named "vg0" with /dev/sdb and /dev/sdc.
+
+    # vgcreate vg0 /dev/sdb /dev/sdc
+
+  3) Create two logical volumes in "vg0." The volumes have to be striped.
+
+    # lvcreate -n lv0 -i 2 -l 64 vg0 -L 1024M
+    # lvcreate -n lv1 -i 2 -l 64 vg0 -L 1024M
+
+The rest is the same as the example #1.
+
+  4) Create ioband devices corresponding to each logical volume.
+
+    # echo "0 $(blockdev --getsize /dev/mapper/lv0) ioband /dev/mapper/lv0 1"\
+       | dmsetup create ioband1
+    # echo "0 $(blockdev --getsize /dev/mapper/lv1) ioband /dev/mapper/lv1 1"\
+       | dmsetup create ioband2
+
+  5) Assign weights of 80 and 40 to the default ioband groups respectively.
+
+    # dmsetup message ioband1 0 weight 80
+    # dmsetup message ioband2 0 weight 40
+
+  6) Create filesystems on the ioband devices and mount them.
+
+    # mkfs.ext3 /dev/mapper/ioband1
+    # mount /dev/mapper/ioband1 /mnt1
+
+    # mkfs.ext3 /dev/mapper/ioband2
+    # mount /dev/mapper/ioband2 /mnt2
+
+
+
+Example #3: Bandwidth control on processes
+------------------------------------------
+This example describes how to control the bandwidth with groups of processes.
+You may also want to run an additional application on the same machine
+described in the example #1. This example shows how to add a new ioband group
+for this application.
+
+
+              /mnt1                          /mnt2            mount points
+                |                              |
+  +-------------V------------+   +-------------V------------+ 
+  |   /dev/mapper/ioband1    |   |   /dev/mapper/ioband2    | ioband devices
+  +-------------+------------+   +-------------+------------+ 
+  |          default         |   |  user=1000  |   default  | ioband groups
+  |           (80)           |   |     (20)    |    (40)    |   (weight)
+  +-------------+------------+   +-------------+------------+ 
+                |                              |
+  +-------------V--------------+---------------V------------+
+  |         /dev/sda1          |           /dev/sda2        | physical device
+  +----------------------------+----------------------------+
+
+The following shows to set up a new ioband group on the machine that is already
+configured as the example #1. The application will have a weight of 20 and run
+with user-id 1000 on /dev/mapper/ioband2.
+
+
+  1) Set the type of ioband2 to "user."
+    # dmsetup message ioband2 0 type user.
+
+  2) Create a new ioband group on ioband2.
+    # dmsetup message ioband2 0 attach 1000
+
+  3) Assign weight of 10 to this newly created ioband group.
+    # dmsetup message ioband2 0 weight 1000:20
+
+
+
+Example #4: Bandwidth control for Xen virtual block devices
+-----------------------------------------------------------
+
+This example describes how to control the bandwidth for Xen virtual
+block devices. The following diagram illustrates the configuration of
+this example.
+
+        Virtual Machine 1              Virtual Machine 2      virtual machines
+                |                              |
+  +-------------V------------+   +-------------V------------+
+  |         /dev/xvda1       |   |         /dev/xvda1       | virtual block
+  +-------------|------------+   +-------------|------------+    devices
+                |                              |
+  +-------------V------------+   +-------------V------------+ 
+  |   /dev/mapper/ioband1    |   |   /dev/mapper/ioband2    | ioband devices
+  +--------------------------+   +--------------------------+
+  |       default group      |   |       default group      | ioband groups
+  |           (80)           |   |           (40)           |    (weight)
+  +-------------|------------+   +-------------|------------+
+                |                              |
+  +-------------V--------------+---------------V------------+
+  |         /dev/sda1          |           /dev/sda2        | physical device
+  +----------------------------+----------------------------+
+
+The followings shows how to map ioband device "ioband1" and "ioband2" to
+virtual block device "/dev/xvda1 on Virtual Machine 1" and  "/dev/xvda1 on
+Virtual Machine 2" respectively on the machine configured as the example #1.
+Add the following lines to the configuration files that are referenced when
+creating "Virtual Machine 1" and "Virtual Machine 2."
+
+    For "Virtual Machine 1"
+    disk = [ 'phy:/dev/mapper/ioband1,xvda,w' ]
+
+    For "Virtual Machine 2"
+    disk = [ 'phy:/dev/mapper/ioband2,xvda,w' ]
+
+
+
+Example #5: Bandwidth control for Xen blktap devices
+----------------------------------------------------
+This example describes how to control the bandwidth for Xen virtual
+block devices when Xen blktap devices are used. The following diagram
+illustrates the configuration of this example.
+
+        Virtual Machine 1              Virtual Machine 2      virtual machines
+                |                              |
+  +-------------V------------+   +-------------V------------+
+  |         /dev/xvda1       |   |         /dev/xvda1       | virtual block
+  +-------------|------------+   +-------------|------------+    devices
+                |                              |
+  +-------------V------------------------------V------------+
+  |                   /dev/mapper/ioband1                   | ioband device
+  +----------------------------+----------------------------+
+  |       default group        |         default group      | ioband groups
+  |           (80)             |             (40)           |    (weight)
+  +-------------|--------------+---------------|------------+
+                |                              |
+  +-------------|------------------------------|------------+
+  |  +----------V----------+        +----------V---------+  |
+  |  |       vm1.img       |        |       vm2.img      |  | disk image files
+  |  +---------------------+        +--------------------+  |
+  |                         /vmdisk                         | mount point
+  +----------------------------|----------------------------+
+                               |
+  +----------------------------V----------------------------+
+  |                        /dev/sda1                        | physical device
+  +---------------------------------------------------------+
+
+To setup the above configuration, follow these steps:
+
+  1) Create an ioband device.
+
+    # echo "0 $(blockdev --getsize /dev/sda1) ioband /dev/sda1 1" | \
+        dmsetup create ioband1
+
+  2) Add the following lines to the configuration files that are referenced
+     when creating "Virtual Machine 1" and "Virtual Machine 2."
+     Disk image files "/vmdisk/vm1.img" and "/vmdisk/vm2.img" will be used.
+
+    For "Virtual Machine 1"
+    disk = [ 'tap:aio:/vmdisk/vm1.img,xvda,w', ]
+
+    For "Virtual Machine 1"
+    disk = [ 'tap:aio:/vmdisk/vm2.img,xvda,w', ]
+
+  3) Run the virtual machines.
+
+    # xm create vm1
+    # xm create vm2
+
+  4) Find out the process IDs of the daemons which control the blktap devices.
+
+    # lsof /vmdisk/disk[12].img
+    COMMAND   PID USER   FD   TYPE DEVICE       SIZE  NODE NAME
+    tapdisk 15011 root   11u   REG  253,0 2147483648 48961 /vmdisk/vm1.img
+    tapdisk 15276 root   13u   REG  253,0 2147483648 48962 /vmdisk/vm2.img
+
+  5) Create new ioband groups of pid 15011 and pid 15276, which are process
+     IDs of the tapdisks, and assign weight of 80 and 40 to the groups
+     respectively.
+
+    # dmsetup message ioband1 0 type pid
+    # dmsetup message ioband1 0 attach 15011
+    # dmsetup message ioband1 0 weight 15011:80
+    # dmsetup message ioband1 0 attach 15276
+    # dmsetup message ioband1 0 weight 15276:40
+
+
+
+
+TODO
+====
+  - Cgroup support. 
+  - Create a mechanism to track down which I/O is originally issued by which
+    process or cgroup.
+  - Control read and write requests separately.
+  - Support WRITE_BARRIER.
+  - Hierarchical ioband groups support.
+  - Optimization.
+  - More configuration tools. Or is the dmsetup command sufficient?
+  - Other policies to schedule BIOs. Or is the weight policy sufficient?
+    Is a new policy for LUNs, which may partially shared the bandwidth, needed?
+  - Other accounting policies to determine the bandwidths. Or is the number
+    of BIOs sufficient?


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