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diff --git a/Documentation/admin-guide/cgroup-v1/blkio-controller.rst b/Documentation/admin-guide/cgroup-v1/blkio-controller.rst new file mode 100644 index 000000000..16253eda1 --- /dev/null +++ b/Documentation/admin-guide/cgroup-v1/blkio-controller.rst @@ -0,0 +1,301 @@ +=================== +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 + +This 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_BFQ_CGROUP_DEBUG + 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.bfq.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.bfq.weight_device` below). + + Currently allowed range of weights is from 1 to 1000. For more details, + see Documentation/block/bfq-iosched.rst. + + blkio.bfq.weight_device + Specifes per cgroup per device weights, overriding the default group + weight. For more details, see Documentation/block/bfq-iosched.rst. + + Following is the format:: + + # echo dev_maj:dev_minor weight > blkio.bfq.weight_device + + Configure weight=300 on /dev/sdb (8:16) in this cgroup:: + + # echo 8:16 300 > blkio.bfq.weight_device + # cat blkio.bfq.weight_device + dev weight + 8:16 300 + + Configure weight=500 on /dev/sda (8:0) in this cgroup:: + + # echo 8:0 500 > blkio.bfq.weight_device + # cat blkio.bfq.weight_device + dev weight + 8:0 500 + 8:16 300 + + Remove specific weight for /dev/sda in this cgroup:: + + # echo 8:0 0 > blkio.bfq.weight_device + # cat blkio.bfq.weight_device + dev weight + 8:16 300 + + 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_BFQ_CGROUP_DEBUG=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_BFQ_CGROUP_DEBUG=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_BFQ_CGROUP_DEBUG=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_BFQ_CGROUP_DEBUG=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_BFQ_CGROUP_DEBUG=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. |