297 lines
12 KiB
ReStructuredText
297 lines
12 KiB
ReStructuredText
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===================
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Block IO Controller
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===================
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Overview
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========
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cgroup subsys "blkio" implements the block io controller. There seems to be
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a need of various kinds of IO control policies (like proportional BW, max BW)
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both at leaf nodes as well as at intermediate nodes in a storage hierarchy.
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Plan is to use the same cgroup based management interface for blkio controller
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and based on user options switch IO policies in the background.
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One IO control policy is throttling policy which can be used to
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specify upper IO rate limits on devices. This policy is implemented in
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generic block layer and can be used on leaf nodes as well as higher
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level logical devices like device mapper.
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HOWTO
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=====
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Throttling/Upper Limit policy
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-----------------------------
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- Enable Block IO controller::
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CONFIG_BLK_CGROUP=y
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- Enable throttling in block layer::
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CONFIG_BLK_DEV_THROTTLING=y
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- Mount blkio controller (see cgroups.txt, Why are cgroups needed?)::
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mount -t cgroup -o blkio none /sys/fs/cgroup/blkio
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- Specify a bandwidth rate on particular device for root group. The format
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for policy is "<major>:<minor> <bytes_per_second>"::
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echo "8:16 1048576" > /sys/fs/cgroup/blkio/blkio.throttle.read_bps_device
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Above will put a limit of 1MB/second on reads happening for root group
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on device having major/minor number 8:16.
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- Run dd to read a file and see if rate is throttled to 1MB/s or not::
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# dd iflag=direct if=/mnt/common/zerofile of=/dev/null bs=4K count=1024
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1024+0 records in
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1024+0 records out
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4194304 bytes (4.2 MB) copied, 4.0001 s, 1.0 MB/s
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Limits for writes can be put using blkio.throttle.write_bps_device file.
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Hierarchical Cgroups
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====================
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Throttling implements hierarchy support; however,
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throttling's hierarchy support is enabled iff "sane_behavior" is
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enabled from cgroup side, which currently is a development option and
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not publicly available.
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If somebody created a hierarchy like as follows::
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root
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/ \
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test1 test2
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test3
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Throttling with "sane_behavior" will handle the
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hierarchy correctly. For throttling, all limits apply
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to the whole subtree while all statistics are local to the IOs
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directly generated by tasks in that cgroup.
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Throttling without "sane_behavior" enabled from cgroup side will
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practically treat all groups at same level as if it looks like the
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following::
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pivot
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/ / \ \
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root test1 test2 test3
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Various user visible config options
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===================================
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CONFIG_BLK_CGROUP
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- Block IO controller.
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CONFIG_BFQ_CGROUP_DEBUG
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- Debug help. Right now some additional stats file show up in cgroup
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if this option is enabled.
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CONFIG_BLK_DEV_THROTTLING
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- Enable block device throttling support in block layer.
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Details of cgroup files
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=======================
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Proportional weight policy files
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--------------------------------
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- blkio.weight
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- Specifies per cgroup weight. This is default weight of the group
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on all the devices until and unless overridden by per device rule.
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(See blkio.weight_device).
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Currently allowed range of weights is from 10 to 1000.
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- blkio.weight_device
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- One can specify per cgroup per device rules using this interface.
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These rules override the default value of group weight as specified
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by blkio.weight.
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Following is the format::
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# echo dev_maj:dev_minor weight > blkio.weight_device
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Configure weight=300 on /dev/sdb (8:16) in this cgroup::
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# echo 8:16 300 > blkio.weight_device
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# cat blkio.weight_device
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dev weight
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8:16 300
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Configure weight=500 on /dev/sda (8:0) in this cgroup::
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# echo 8:0 500 > blkio.weight_device
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# cat blkio.weight_device
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dev weight
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8:0 500
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8:16 300
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Remove specific weight for /dev/sda in this cgroup::
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# echo 8:0 0 > blkio.weight_device
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# cat blkio.weight_device
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dev weight
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8:16 300
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- blkio.time
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- disk time allocated to cgroup per device in milliseconds. First
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two fields specify the major and minor number of the device and
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third field specifies the disk time allocated to group in
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milliseconds.
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- blkio.sectors
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- number of sectors transferred to/from disk by the group. First
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two fields specify the major and minor number of the device and
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third field specifies the number of sectors transferred by the
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group to/from the device.
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- blkio.io_service_bytes
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- Number of bytes transferred to/from the disk by the group. These
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are further divided by the type of operation - read or write, sync
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or async. First two fields specify the major and minor number of the
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device, third field specifies the operation type and the fourth field
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specifies the number of bytes.
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- blkio.io_serviced
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- Number of IOs (bio) issued to the disk by the group. These
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are further divided by the type of operation - read or write, sync
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or async. First two fields specify the major and minor number of the
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device, third field specifies the operation type and the fourth field
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specifies the number of IOs.
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- blkio.io_service_time
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- Total amount of time between request dispatch and request completion
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for the IOs done by this cgroup. This is in nanoseconds to make it
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meaningful for flash devices too. For devices with queue depth of 1,
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this time represents the actual service time. When queue_depth > 1,
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that is no longer true as requests may be served out of order. This
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may cause the service time for a given IO to include the service time
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of multiple IOs when served out of order which may result in total
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io_service_time > actual time elapsed. This time is further divided by
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the type of operation - read or write, sync or async. First two fields
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specify the major and minor number of the device, third field
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specifies the operation type and the fourth field specifies the
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io_service_time in ns.
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- blkio.io_wait_time
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- Total amount of time the IOs for this cgroup spent waiting in the
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scheduler queues for service. This can be greater than the total time
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elapsed since it is cumulative io_wait_time for all IOs. It is not a
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measure of total time the cgroup spent waiting but rather a measure of
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the wait_time for its individual IOs. For devices with queue_depth > 1
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this metric does not include the time spent waiting for service once
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the IO is dispatched to the device but till it actually gets serviced
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(there might be a time lag here due to re-ordering of requests by the
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device). This is in nanoseconds to make it meaningful for flash
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devices too. This time is further divided by the type of operation -
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read or write, sync or async. First two fields specify the major and
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minor number of the device, third field specifies the operation type
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and the fourth field specifies the io_wait_time in ns.
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- blkio.io_merged
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- Total number of bios/requests merged into requests belonging to this
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cgroup. This is further divided by the type of operation - read or
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write, sync or async.
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- blkio.io_queued
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- Total number of requests queued up at any given instant for this
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cgroup. This is further divided by the type of operation - read or
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write, sync or async.
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- blkio.avg_queue_size
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- Debugging aid only enabled if CONFIG_BFQ_CGROUP_DEBUG=y.
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The average queue size for this cgroup over the entire time of this
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cgroup's existence. Queue size samples are taken each time one of the
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queues of this cgroup gets a timeslice.
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- blkio.group_wait_time
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- Debugging aid only enabled if CONFIG_BFQ_CGROUP_DEBUG=y.
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This is the amount of time the cgroup had to wait since it became busy
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(i.e., went from 0 to 1 request queued) to get a timeslice for one of
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its queues. This is different from the io_wait_time which is the
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cumulative total of the amount of time spent by each IO in that cgroup
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waiting in the scheduler queue. This is in nanoseconds. If this is
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read when the cgroup is in a waiting (for timeslice) state, the stat
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will only report the group_wait_time accumulated till the last time it
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got a timeslice and will not include the current delta.
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- blkio.empty_time
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- Debugging aid only enabled if CONFIG_BFQ_CGROUP_DEBUG=y.
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This is the amount of time a cgroup spends without any pending
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requests when not being served, i.e., it does not include any time
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spent idling for one of the queues of the cgroup. This is in
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nanoseconds. If this is read when the cgroup is in an empty state,
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the stat will only report the empty_time accumulated till the last
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time it had a pending request and will not include the current delta.
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- blkio.idle_time
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- Debugging aid only enabled if CONFIG_BFQ_CGROUP_DEBUG=y.
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This is the amount of time spent by the IO scheduler idling for a
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given cgroup in anticipation of a better request than the existing ones
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from other queues/cgroups. This is in nanoseconds. If this is read
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when the cgroup is in an idling state, the stat will only report the
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idle_time accumulated till the last idle period and will not include
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the current delta.
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- blkio.dequeue
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- Debugging aid only enabled if CONFIG_BFQ_CGROUP_DEBUG=y. This
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gives the statistics about how many a times a group was dequeued
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from service tree of the device. First two fields specify the major
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and minor number of the device and third field specifies the number
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of times a group was dequeued from a particular device.
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- blkio.*_recursive
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- Recursive version of various stats. These files show the
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same information as their non-recursive counterparts but
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include stats from all the descendant cgroups.
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Throttling/Upper limit policy files
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-----------------------------------
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- blkio.throttle.read_bps_device
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- Specifies upper limit on READ rate from the device. IO rate is
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specified in bytes per second. Rules are per device. Following is
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the format::
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echo "<major>:<minor> <rate_bytes_per_second>" > /cgrp/blkio.throttle.read_bps_device
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- blkio.throttle.write_bps_device
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- Specifies upper limit on WRITE rate to the device. IO rate is
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specified in bytes per second. Rules are per device. Following is
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the format::
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echo "<major>:<minor> <rate_bytes_per_second>" > /cgrp/blkio.throttle.write_bps_device
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- blkio.throttle.read_iops_device
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- Specifies upper limit on READ rate from the device. IO rate is
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specified in IO per second. Rules are per device. Following is
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the format::
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echo "<major>:<minor> <rate_io_per_second>" > /cgrp/blkio.throttle.read_iops_device
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- blkio.throttle.write_iops_device
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- Specifies upper limit on WRITE rate to the device. IO rate is
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specified in io per second. Rules are per device. Following is
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the format::
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echo "<major>:<minor> <rate_io_per_second>" > /cgrp/blkio.throttle.write_iops_device
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Note: If both BW and IOPS rules are specified for a device, then IO is
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subjected to both the constraints.
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- blkio.throttle.io_serviced
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- Number of IOs (bio) issued to the disk by the group. These
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are further divided by the type of operation - read or write, sync
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or async. First two fields specify the major and minor number of the
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device, third field specifies the operation type and the fourth field
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specifies the number of IOs.
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- blkio.throttle.io_service_bytes
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- Number of bytes transferred to/from the disk by the group. These
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are further divided by the type of operation - read or write, sync
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or async. First two fields specify the major and minor number of the
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device, third field specifies the operation type and the fourth field
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specifies the number of bytes.
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Common files among various policies
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-----------------------------------
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- blkio.reset_stats
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- Writing an int to this file will result in resetting all the stats
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for that cgroup.
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