Documentation/cgroup-v1/blkio-controller.txt - linux/torvalds/linux - Git at Google

 				Block IO Controller
 				===================
 Overview
 ========
 cgroup subsys "blkio" implements the block io controller. There seems to be
 a need of various kinds of IO control policies (like proportional BW, max BW)
 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.

 One IO control policy 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
 =====
 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 (see cgroups.txt, Why are cgroups needed?)
         mount -t cgroup -o blkio none /sys/fs/cgroup/blkio

 - Specify a bandwidth rate on particular device for root group. The format
   for policy is "<major>:<minor>  <bytes_per_second>".

         echo "8:16  1048576" > /sys/fs/cgroup/blkio/blkio.throttle.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 iflag=direct if=/mnt/common/zerofile of=/dev/null bs=4K count=1024
         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.throttle.write_bps_device file.

 Hierarchical Cgroups
 ====================

 Throttling implements hierarchy support; however,
 throttling's hierarchy support is enabled iff "sane_behavior" is
 enabled from cgroup side, which currently is a development option and
 not publicly available.

 If somebody created a hierarchy like as follows.

 			root
 			/  \
 		     test1 test2
 			|
 		     test3

 Throttling with "sane_behavior" will handle the
 hierarchy correctly. For throttling, all limits apply
 to the whole subtree while all statistics are local to the IOs
 directly generated by tasks in that cgroup.

 Throttling without "sane_behavior" enabled from cgroup side will
 practically treat all groups at same level as if it looks like the
 following.

 				pivot
 			     /  /   \  \
 			root  test1 test2  test3

 Various user visible config options
 ===================================
 CONFIG_BLK_CGROUP
 	- Block IO controller.

 CONFIG_DEBUG_BLK_CGROUP
 	- Debug help. Right now some additional stats file show up in cgroup
 	  if this option is enabled.

 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.
 	  (See blkio.weight_device).
 	  Currently allowed range of weights is from 10 to 1000.

 - blkio.weight_device
 	- One can specify per cgroup per device rules using this interface.
 	  These rules override the default value of group weight as specified
 	  by blkio.weight.

 	  Following is the format.

 	  # echo dev_maj:dev_minor weight > blkio.weight_device
 	  Configure weight=300 on /dev/sdb (8:16) in this cgroup
 	  # echo 8:16 300 > blkio.weight_device
 	  # cat blkio.weight_device
 	  dev     weight
 	  8:16    300

 	  Configure weight=500 on /dev/sda (8:0) in this cgroup
 	  # echo 8:0 500 > blkio.weight_device
 	  # cat blkio.weight_device
 	  dev     weight
 	  8:0     500
 	  8:16    300

 	  Remove specific weight for /dev/sda in this cgroup
 	  # echo 8:0 0 > blkio.weight_device
 	  # cat blkio.weight_device
 	  dev     weight
 	  8:16    300

 - blkio.leaf_weight[_device]
 	- Equivalents of blkio.weight[_device] for the purpose of
           deciding how much weight tasks in the given cgroup has while
           competing with the cgroup's child cgroups. For details,
           please refer to Documentation/block/cfq-iosched.txt.

 - blkio.time
 	- disk time allocated to cgroup per device in milliseconds. First
 	  two fields specify the major and minor number of the device and
 	  third field specifies the disk time allocated to group in
 	  milliseconds.

 - blkio.sectors
 	- number of sectors transferred to/from disk by the group. First
 	  two fields specify the major and minor number of the device and
 	  third field specifies the number of sectors transferred by the
 	  group to/from the device.

 - blkio.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.

 - blkio.io_serviced
 	- Number of IOs (bio) issued to 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 IOs.

 - blkio.io_service_time
 	- Total amount of time between request dispatch and request completion
 	  for the IOs done by this cgroup. This is in nanoseconds to make it
 	  meaningful for flash devices too. For devices with queue depth of 1,
 	  this time represents the actual service time. When queue_depth > 1,
 	  that is no longer true as requests may be served out of order. This
 	  may cause the service time for a given IO to include the service time
 	  of multiple IOs when served out of order which may result in total
 	  io_service_time > actual time elapsed. This time is 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
 	  io_service_time in ns.

 - blkio.io_wait_time
 	- Total amount of time the IOs for this cgroup spent waiting in the
 	  scheduler queues for service. This can be greater than the total time
 	  elapsed since it is cumulative io_wait_time for all IOs. It is not a
 	  measure of total time the cgroup spent waiting but rather a measure of
 	  the wait_time for its individual IOs. For devices with queue_depth > 1
 	  this metric does not include the time spent waiting for service once
 	  the IO is dispatched to the device but till it actually gets serviced
 	  (there might be a time lag here due to re-ordering of requests by the
 	  device). This is in nanoseconds to make it meaningful for flash
 	  devices too. This time is 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 io_wait_time in ns.

 - blkio.io_merged
 	- Total number of bios/requests merged into requests belonging to this
 	  cgroup. This is further divided by the type of operation - read or
 	  write, sync or async.

 - blkio.io_queued
 	- Total number of requests queued up at any given instant for this
 	  cgroup. This is further divided by the type of operation - read or
 	  write, sync or async.

 - blkio.avg_queue_size
 	- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
 	  The average queue size for this cgroup over the entire time of this
 	  cgroup's existence. Queue size samples are taken each time one of the
 	  queues of this cgroup gets a timeslice.

 - blkio.group_wait_time
 	- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
 	  This is the amount of time the cgroup had to wait since it became busy
 	  (i.e., went from 0 to 1 request queued) to get a timeslice for one of
 	  its queues. This is different from the io_wait_time which is the
 	  cumulative total of the amount of time spent by each IO in that cgroup
 	  waiting in the scheduler queue. This is in nanoseconds. If this is
 	  read when the cgroup is in a waiting (for timeslice) state, the stat
 	  will only report the group_wait_time accumulated till the last time it
 	  got a timeslice and will not include the current delta.

 - blkio.empty_time
 	- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
 	  This is the amount of time a cgroup spends without any pending
 	  requests when not being served, i.e., it does not include any time
 	  spent idling for one of the queues of the cgroup. This is in
 	  nanoseconds. If this is read when the cgroup is in an empty state,
 	  the stat will only report the empty_time accumulated till the last
 	  time it had a pending request and will not include the current delta.

 - blkio.idle_time
 	- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
 	  This is the amount of time spent by the IO scheduler idling for a
 	  given cgroup in anticipation of a better request than the existing ones
 	  from other queues/cgroups. This is in nanoseconds. If this is read
 	  when the cgroup is in an idling state, the stat will only report the
 	  idle_time accumulated till the last idle period and will not include
 	  the current delta.

 - blkio.dequeue
 	- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y. This
 	  gives the statistics about how many a times a group was dequeued
 	  from service tree of the device. First two fields specify the major
 	  and minor number of the device and third field specifies the number
 	  of times a group was dequeued from a particular device.

 - blkio.*_recursive
 	- Recursive version of various stats. These files show the
           same information as their non-recursive counterparts but
           include stats from all the descendant cgroups.

 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 device. Following is
 	  the format.

   echo "<major>:<minor>  <rate_bytes_per_second>" > /cgrp/blkio.throttle.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 device. Following is
 	  the format.

   echo "<major>:<minor>  <rate_bytes_per_second>" > /cgrp/blkio.throttle.write_bps_device

 - blkio.throttle.read_iops_device
 	- Specifies upper limit on READ rate from the device. IO rate is
 	  specified in IO per second. Rules are per device. Following is
 	  the format.

   echo "<major>:<minor>  <rate_io_per_second>" > /cgrp/blkio.throttle.read_iops_device

 - blkio.throttle.write_iops_device
 	- Specifies upper limit on WRITE rate to the device. IO rate is
 	  specified in io per second. Rules are per device. Following is
 	  the format.

   echo "<major>:<minor>  <rate_io_per_second>" > /cgrp/blkio.throttle.write_iops_device

 Note: If both BW and IOPS rules are specified for a device, then IO is
       subjected to both the constraints.

 - blkio.throttle.io_serviced
 	- Number of IOs (bio) issued to 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 IOs.

 - 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.

 Common files among various policies
 -----------------------------------
 - blkio.reset_stats
 	- Writing an int to this file will result in resetting all the stats
 	  for that cgroup.
	Block IO Controller
	===================
	Overview
	========
	cgroup subsys "blkio" implements the block io controller. There seems to be
	a need of various kinds of IO control policies (like proportional BW, max BW)
	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.

	One IO control policy 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
	=====
	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 (see cgroups.txt, Why are cgroups needed?)
	mount -t cgroup -o blkio none /sys/fs/cgroup/blkio

	- Specify a bandwidth rate on particular device for root group. The format
	for policy is "<major>:<minor> <bytes_per_second>".

	echo "8:16 1048576" > /sys/fs/cgroup/blkio/blkio.throttle.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 iflag=direct if=/mnt/common/zerofile of=/dev/null bs=4K count=1024
	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.throttle.write_bps_device file.

	Hierarchical Cgroups
	====================

	Throttling implements hierarchy support; however,
	throttling's hierarchy support is enabled iff "sane_behavior" is
	enabled from cgroup side, which currently is a development option and
	not publicly available.

	If somebody created a hierarchy like as follows.

	root
	/ \
	test1 test2
	\|
	test3

	Throttling with "sane_behavior" will handle the
	hierarchy correctly. For throttling, all limits apply
	to the whole subtree while all statistics are local to the IOs
	directly generated by tasks in that cgroup.

	Throttling without "sane_behavior" enabled from cgroup side will
	practically treat all groups at same level as if it looks like the
	following.

	pivot
	/ / \ \
	root test1 test2 test3

	Various user visible config options
	===================================
	CONFIG_BLK_CGROUP
	- Block IO controller.

	CONFIG_DEBUG_BLK_CGROUP
	- Debug help. Right now some additional stats file show up in cgroup
	if this option is enabled.

	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.
	(See blkio.weight_device).
	Currently allowed range of weights is from 10 to 1000.

	- blkio.weight_device
	- One can specify per cgroup per device rules using this interface.
	These rules override the default value of group weight as specified
	by blkio.weight.

	Following is the format.

	# echo dev_maj:dev_minor weight > blkio.weight_device
	Configure weight=300 on /dev/sdb (8:16) in this cgroup
	# echo 8:16 300 > blkio.weight_device
	# cat blkio.weight_device
	dev weight
	8:16 300

	Configure weight=500 on /dev/sda (8:0) in this cgroup
	# echo 8:0 500 > blkio.weight_device
	# cat blkio.weight_device
	dev weight
	8:0 500
	8:16 300

	Remove specific weight for /dev/sda in this cgroup
	# echo 8:0 0 > blkio.weight_device
	# cat blkio.weight_device
	dev weight
	8:16 300

	- blkio.leaf_weight[_device]
	- Equivalents of blkio.weight[_device] for the purpose of
	deciding how much weight tasks in the given cgroup has while
	competing with the cgroup's child cgroups. For details,
	please refer to Documentation/block/cfq-iosched.txt.

	- blkio.time
	- disk time allocated to cgroup per device in milliseconds. First
	two fields specify the major and minor number of the device and
	third field specifies the disk time allocated to group in
	milliseconds.

	- blkio.sectors
	- number of sectors transferred to/from disk by the group. First
	two fields specify the major and minor number of the device and
	third field specifies the number of sectors transferred by the
	group to/from the device.

	- blkio.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.

	- blkio.io_serviced
	- Number of IOs (bio) issued to 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 IOs.

	- blkio.io_service_time
	- Total amount of time between request dispatch and request completion
	for the IOs done by this cgroup. This is in nanoseconds to make it
	meaningful for flash devices too. For devices with queue depth of 1,
	this time represents the actual service time. When queue_depth > 1,
	that is no longer true as requests may be served out of order. This
	may cause the service time for a given IO to include the service time
	of multiple IOs when served out of order which may result in total
	io_service_time > actual time elapsed. This time is 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
	io_service_time in ns.

	- blkio.io_wait_time
	- Total amount of time the IOs for this cgroup spent waiting in the
	scheduler queues for service. This can be greater than the total time
	elapsed since it is cumulative io_wait_time for all IOs. It is not a
	measure of total time the cgroup spent waiting but rather a measure of
	the wait_time for its individual IOs. For devices with queue_depth > 1
	this metric does not include the time spent waiting for service once
	the IO is dispatched to the device but till it actually gets serviced
	(there might be a time lag here due to re-ordering of requests by the
	device). This is in nanoseconds to make it meaningful for flash
	devices too. This time is 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 io_wait_time in ns.

	- blkio.io_merged
	- Total number of bios/requests merged into requests belonging to this
	cgroup. This is further divided by the type of operation - read or
	write, sync or async.

	- blkio.io_queued
	- Total number of requests queued up at any given instant for this
	cgroup. This is further divided by the type of operation - read or
	write, sync or async.

	- blkio.avg_queue_size
	- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
	The average queue size for this cgroup over the entire time of this
	cgroup's existence. Queue size samples are taken each time one of the
	queues of this cgroup gets a timeslice.

	- blkio.group_wait_time
	- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
	This is the amount of time the cgroup had to wait since it became busy
	(i.e., went from 0 to 1 request queued) to get a timeslice for one of
	its queues. This is different from the io_wait_time which is the
	cumulative total of the amount of time spent by each IO in that cgroup
	waiting in the scheduler queue. This is in nanoseconds. If this is
	read when the cgroup is in a waiting (for timeslice) state, the stat
	will only report the group_wait_time accumulated till the last time it
	got a timeslice and will not include the current delta.

	- blkio.empty_time
	- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
	This is the amount of time a cgroup spends without any pending
	requests when not being served, i.e., it does not include any time
	spent idling for one of the queues of the cgroup. This is in
	nanoseconds. If this is read when the cgroup is in an empty state,
	the stat will only report the empty_time accumulated till the last
	time it had a pending request and will not include the current delta.

	- blkio.idle_time
	- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
	This is the amount of time spent by the IO scheduler idling for a
	given cgroup in anticipation of a better request than the existing ones
	from other queues/cgroups. This is in nanoseconds. If this is read
	when the cgroup is in an idling state, the stat will only report the
	idle_time accumulated till the last idle period and will not include
	the current delta.

	- blkio.dequeue
	- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y. This
	gives the statistics about how many a times a group was dequeued
	from service tree of the device. First two fields specify the major
	and minor number of the device and third field specifies the number
	of times a group was dequeued from a particular device.

	- blkio.*_recursive
	- Recursive version of various stats. These files show the
	same information as their non-recursive counterparts but
	include stats from all the descendant cgroups.

	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 device. Following is
	the format.

	echo "<major>:<minor> <rate_bytes_per_second>" > /cgrp/blkio.throttle.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 device. Following is
	the format.

	echo "<major>:<minor> <rate_bytes_per_second>" > /cgrp/blkio.throttle.write_bps_device

	- blkio.throttle.read_iops_device
	- Specifies upper limit on READ rate from the device. IO rate is
	specified in IO per second. Rules are per device. Following is
	the format.

	echo "<major>:<minor> <rate_io_per_second>" > /cgrp/blkio.throttle.read_iops_device

	- blkio.throttle.write_iops_device
	- Specifies upper limit on WRITE rate to the device. IO rate is
	specified in io per second. Rules are per device. Following is
	the format.

	echo "<major>:<minor> <rate_io_per_second>" > /cgrp/blkio.throttle.write_iops_device

	Note: If both BW and IOPS rules are specified for a device, then IO is
	subjected to both the constraints.

	- blkio.throttle.io_serviced
	- Number of IOs (bio) issued to 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 IOs.

	- 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.

	Common files among various policies
	-----------------------------------
	- blkio.reset_stats
	- Writing an int to this file will result in resetting all the stats
	for that cgroup.