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Subject[PATCH 4/4] blkio: Documentation update
Date
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
Documentation/cgroups/blkio-controller.txt | 89 +++++++++++++++++++++++++++-
1 files changed, 86 insertions(+), 3 deletions(-)

diff --git a/Documentation/cgroups/blkio-controller.txt b/Documentation/cgroups/blkio-controller.txt
index 48e0b21..ba895f0 100644
--- a/Documentation/cgroups/blkio-controller.txt
+++ b/Documentation/cgroups/blkio-controller.txt
@@ -8,12 +8,17 @@ both at leaf nodes as well as at intermediate nodes in a storage hierarchy.
Plan is to use the same cgroup based management interface for blkio controller
and based on user options switch IO policies in the background.

-In the first phase, this patchset implements proportional weight time based
-division of disk policy. It is implemented in CFQ. Hence this policy takes
-effect only on leaf nodes when CFQ is being used.
+Currently two IO control policies are implemented. First one is proportional
+weight time based division of disk policy. It is implemented in CFQ. Hence
+this policy takes effect only on leaf nodes when CFQ is being used. The second
+one is throttling policy which can be used to specify upper IO rate limits
+on devices. This policy is implemented in generic block layer and can be
+used on leaf nodes as well as higher level logical devices like device mapper.

HOWTO
=====
+Proportional Weight division of bandwidth
+-----------------------------------------
You can do a very simple testing of running two dd threads in two different
cgroups. Here is what you can do.

@@ -55,6 +60,35 @@ cgroups. Here is what you can do.
group dispatched to the disk. We provide fairness in terms of disk time, so
ideally io.disk_time of cgroups should be in proportion to the weight.

+Throttling/Upper Limit policy
+-----------------------------
+- Enable Block IO controller
+ CONFIG_BLK_CGROUP=y
+
+- Enable throttling in block layer
+ CONFIG_BLK_DEV_THROTTLING=y
+
+- Mount blkio controller
+ mount -t cgroup -o blkio none /cgroup/blkio
+
+- Specify a bandwidth rate on particular device for root group. The format
+ for policy is "<major>:<minor> <byes_per_second>".
+
+ echo "8:16 1048576" > /cgroup/blkio/blkio.read_bps_device
+
+ Above will put a limit of 1MB/second on reads happening for root group
+ on device having major/minor number 8:16.
+
+- Run dd to read a file and see if rate is throttled to 1MB/s or not.
+
+ # dd if=/mnt/common/zerofile of=/dev/null bs=4K count=1024
+ # iflag=direct
+ 1024+0 records in
+ 1024+0 records out
+ 4194304 bytes (4.2 MB) copied, 4.0001 s, 1.0 MB/s
+
+ Limits for writes can be put using blkio.write_bps_device file.
+
Various user visible config options
===================================
CONFIG_BLK_CGROUP
@@ -68,8 +102,13 @@ CONFIG_CFQ_GROUP_IOSCHED
- Enables group scheduling in CFQ. Currently only 1 level of group
creation is allowed.

+CONFIG_BLK_DEV_THROTTLING
+ - Enable block device throttling support in block layer.
+
Details of cgroup files
=======================
+Proportional weight policy files
+--------------------------------
- blkio.weight
- Specifies per cgroup weight. This is default weight of the group
on all the devices until and unless overridden by per device rule.
@@ -210,6 +249,50 @@ Details of cgroup files
and minor number of the device and third field specifies the number
of times a group was dequeued from a particular device.

+Throttling/Upper limit policy files
+-----------------------------------
+- blkio.throttle.read_bps_device
+ - Specifies upper limit on READ rate from the device. IO rate is
+ specified in bytes per second. Rules are per deivce. Following is
+ the format.
+
+ echo "<major>:<minor> <rate_bytes_per_second>" > /cgrp/blkio.read_bps_device
+
+- blkio.throttle.write_bps_device
+ - Specifies upper limit on WRITE rate to the device. IO rate is
+ specified in bytes per second. Rules are per deivce. Following is
+ the format.
+
+ echo "<major>:<minor> <rate_bytes_per_second>" > /cgrp/blkio.write_bps_device
+
+- blkio.throttle.io_serviced
+ - Number of IOs (bio) completed to/from the disk by the group (as
+ seen by throttling policy). These are further divided by the type
+ of operation - read or write, sync or async. First two fields specify
+ the major and minor number of the device, third field specifies the
+ operation type and the fourth field specifies the number of IOs.
+
+ blkio.io_serviced does accounting as seen by CFQ and counts are in
+ number of requests (struct request). On the other hand,
+ blkio.throttle.io_serviced counts number of IO in terms of number
+ of bios as seen by throttling policy. These bios can later be
+ merged by elevator and total number of requests completed can be
+ lesser.
+
+- blkio.throttle.io_service_bytes
+ - Number of bytes transferred to/from the disk by the group. These
+ are further divided by the type of operation - read or write, sync
+ or async. First two fields specify the major and minor number of the
+ device, third field specifies the operation type and the fourth field
+ specifies the number of bytes.
+
+ These numbers should roughly be same as blkio.io_service_bytes as
+ updated by CFQ. The difference between two is that
+ blkio.io_service_bytes will not be updated if CFQ is not operating
+ on request queue.
+
+Common files among various policies
+-----------------------------------
- blkio.reset_stats
- Writing an int to this file will result in resetting all the stats
for that cgroup.
--
1.7.2.2


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