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diff --git a/docs/throttle.txt b/docs/throttle.txt new file mode 100644 index 00000000..0a0453a5 --- /dev/null +++ b/docs/throttle.txt @@ -0,0 +1,359 @@ +The QEMU throttling infrastructure +================================== +Copyright (C) 2016,2020 Igalia, S.L. +Author: Alberto Garcia <berto@igalia.com> + +This work is licensed under the terms of the GNU GPL, version 2 or +later. See the COPYING file in the top-level directory. + +Introduction +------------ +QEMU includes a throttling module that can be used to set limits to +I/O operations. The code itself is generic and independent of the I/O +units, but it is currently used to limit the number of bytes per second +and operations per second (IOPS) when performing disk I/O. + +This document explains how to use the throttling code in QEMU, and how +it works internally. The implementation is in throttle.c. + + +Using throttling to limit disk I/O +---------------------------------- +Two aspects of the disk I/O can be limited: the number of bytes per +second and the number of operations per second (IOPS). For each one of +them the user can set a global limit or separate limits for read and +write operations. This gives us a total of six different parameters. + +I/O limits can be set using the throttling.* parameters of -drive, or +using the QMP 'block_set_io_throttle' command. These are the names of +the parameters for both cases: + +|-----------------------+-----------------------| +| -drive | block_set_io_throttle | +|-----------------------+-----------------------| +| throttling.iops-total | iops | +| throttling.iops-read | iops_rd | +| throttling.iops-write | iops_wr | +| throttling.bps-total | bps | +| throttling.bps-read | bps_rd | +| throttling.bps-write | bps_wr | +|-----------------------+-----------------------| + +It is possible to set limits for both IOPS and bps at the same time, +and for each case we can decide whether to have separate read and +write limits or not, but note that if iops-total is set then neither +iops-read nor iops-write can be set. The same applies to bps-total and +bps-read/write. + +The default value of these parameters is 0, and it means 'unlimited'. + +In its most basic usage, the user can add a drive to QEMU with a limit +of 100 IOPS with the following -drive line: + + -drive file=hd0.qcow2,throttling.iops-total=100 + +We can do the same using QMP. In this case all these parameters are +mandatory, so we must set to 0 the ones that we don't want to limit: + + { "execute": "block_set_io_throttle", + "arguments": { + "device": "virtio0", + "iops": 100, + "iops_rd": 0, + "iops_wr": 0, + "bps": 0, + "bps_rd": 0, + "bps_wr": 0 + } + } + + +I/O bursts +---------- +In addition to the basic limits we have just seen, QEMU allows the +user to do bursts of I/O for a configurable amount of time. A burst is +an amount of I/O that can exceed the basic limit. Bursts are useful to +allow better performance when there are peaks of activity (the OS +boots, a service needs to be restarted) while keeping the average +limits lower the rest of the time. + +Two parameters control bursts: their length and the maximum amount of +I/O they allow. These two can be configured separately for each one of +the six basic parameters described in the previous section, but in +this section we'll use 'iops-total' as an example. + +The I/O limit during bursts is set using 'iops-total-max', and the +maximum length (in seconds) is set with 'iops-total-max-length'. So if +we want to configure a drive with a basic limit of 100 IOPS and allow +bursts of 2000 IOPS for 60 seconds, we would do it like this (the line +is split for clarity): + + -drive file=hd0.qcow2, + throttling.iops-total=100, + throttling.iops-total-max=2000, + throttling.iops-total-max-length=60 + +Or, with QMP: + + { "execute": "block_set_io_throttle", + "arguments": { + "device": "virtio0", + "iops": 100, + "iops_rd": 0, + "iops_wr": 0, + "bps": 0, + "bps_rd": 0, + "bps_wr": 0, + "iops_max": 2000, + "iops_max_length": 60, + } + } + +With this, the user can perform I/O on hd0.qcow2 at a rate of 2000 +IOPS for 1 minute before it's throttled down to 100 IOPS. + +The user will be able to do bursts again if there's a sufficiently +long period of time with unused I/O (see below for details). + +The default value for 'iops-total-max' is 0 and it means that bursts +are not allowed. 'iops-total-max-length' can only be set if +'iops-total-max' is set as well, and its default value is 1 second. + +Here's the complete list of parameters for configuring bursts: + +|----------------------------------+-----------------------| +| -drive | block_set_io_throttle | +|----------------------------------+-----------------------| +| throttling.iops-total-max | iops_max | +| throttling.iops-total-max-length | iops_max_length | +| throttling.iops-read-max | iops_rd_max | +| throttling.iops-read-max-length | iops_rd_max_length | +| throttling.iops-write-max | iops_wr_max | +| throttling.iops-write-max-length | iops_wr_max_length | +| throttling.bps-total-max | bps_max | +| throttling.bps-total-max-length | bps_max_length | +| throttling.bps-read-max | bps_rd_max | +| throttling.bps-read-max-length | bps_rd_max_length | +| throttling.bps-write-max | bps_wr_max | +| throttling.bps-write-max-length | bps_wr_max_length | +|----------------------------------+-----------------------| + + +Controlling the size of I/O operations +-------------------------------------- +When applying IOPS limits all I/O operations are treated equally +regardless of their size. This means that the user can take advantage +of this in order to circumvent the limits and submit one huge I/O +request instead of several smaller ones. + +QEMU provides a setting called throttling.iops-size to prevent this +from happening. This setting specifies the size (in bytes) of an I/O +request for accounting purposes. Larger requests will be counted +proportionally to this size. + +For example, if iops-size is set to 4096 then an 8KB request will be +counted as two, and a 6KB request will be counted as one and a +half. This only applies to requests larger than iops-size: smaller +requests will be always counted as one, no matter their size. + +The default value of iops-size is 0 and it means that the size of the +requests is never taken into account when applying IOPS limits. + + +Applying I/O limits to groups of disks +-------------------------------------- +In all the examples so far we have seen how to apply limits to the I/O +performed on individual drives, but QEMU allows grouping drives so +they all share the same limits. + +The way it works is that each drive with I/O limits is assigned to a +group named using the throttling.group parameter. If this parameter is +not specified, then the device name (i.e. 'virtio0', 'ide0-hd0') will +be used as the group name. + +Limits set using the throttling.* parameters discussed earlier in this +document apply to the combined I/O of all members of a group. + +Consider this example: + + -drive file=hd1.qcow2,throttling.iops-total=6000,throttling.group=foo + -drive file=hd2.qcow2,throttling.iops-total=6000,throttling.group=foo + -drive file=hd3.qcow2,throttling.iops-total=3000,throttling.group=bar + -drive file=hd4.qcow2,throttling.iops-total=6000,throttling.group=foo + -drive file=hd5.qcow2,throttling.iops-total=3000,throttling.group=bar + -drive file=hd6.qcow2,throttling.iops-total=5000 + +Here hd1, hd2 and hd4 are all members of a group named 'foo' with a +combined IOPS limit of 6000, and hd3 and hd5 are members of 'bar'. hd6 +is left alone (technically it is part of a 1-member group). + +Limits are applied in a round-robin fashion so if there are concurrent +I/O requests on several drives of the same group they will be +distributed evenly. + +When I/O limits are applied to an existing drive using the QMP command +'block_set_io_throttle', the following things need to be taken into +account: + + - I/O limits are shared within the same group, so new values will + affect all members and overwrite the previous settings. In other + words: if different limits are applied to members of the same + group, the last one wins. + + - If 'group' is unset it is assumed to be the current group of that + drive. If the drive is not in a group yet, it will be added to a + group named after the device name. + + - If 'group' is set then the drive will be moved to that group if + it was member of a different one. In this case the limits + specified in the parameters will be applied to the new group + only. + + - I/O limits can be disabled by setting all of them to 0. In this + case the device will be removed from its group and the rest of + its members will not be affected. The 'group' parameter is + ignored. + + +The Leaky Bucket algorithm +-------------------------- +I/O limits in QEMU are implemented using the leaky bucket algorithm +(specifically the "Leaky bucket as a meter" variant). + +This algorithm uses the analogy of a bucket that leaks water +constantly. The water that gets into the bucket represents the I/O +that has been performed, and no more I/O is allowed once the bucket is +full. + +To see the way this corresponds to the throttling parameters in QEMU, +consider the following values: + + iops-total=100 + iops-total-max=2000 + iops-total-max-length=60 + + - Water leaks from the bucket at a rate of 100 IOPS. + - Water can be added to the bucket at a rate of 2000 IOPS. + - The size of the bucket is 2000 x 60 = 120000 + - If 'iops-total-max-length' is unset then it defaults to 1 and the + size of the bucket is 2000. + - If 'iops-total-max' is unset then 'iops-total-max-length' must be + unset as well. In this case the bucket size is 100. + +The bucket is initially empty, therefore water can be added until it's +full at a rate of 2000 IOPS (the burst rate). Once the bucket is full +we can only add as much water as it leaks, therefore the I/O rate is +reduced to 100 IOPS. If we add less water than it leaks then the +bucket will start to empty, allowing for bursts again. + +Note that since water is leaking from the bucket even during bursts, +it will take a bit more than 60 seconds at 2000 IOPS to fill it +up. After those 60 seconds the bucket will have leaked 60 x 100 = +6000, allowing for 3 more seconds of I/O at 2000 IOPS. + +Also, due to the way the algorithm works, longer burst can be done at +a lower I/O rate, e.g. 1000 IOPS during 120 seconds. + + +The 'throttle' block filter +--------------------------- +Since QEMU 2.11 it is possible to configure the I/O limits using a +'throttle' block filter. This filter uses the exact same throttling +infrastructure described above but can be used anywhere in the node +graph, allowing for more flexibility. + +The user can create an arbitrary number of filters and each one of +them must be assigned to a group that contains the actual I/O limits. +Different filters can use the same group so the limits are shared as +described earlier in "Applying I/O limits to groups of disks". + +A group can be created using the object-add QMP function: + + { "execute": "object-add", + "arguments": { + "qom-type": "throttle-group", + "id": "group0", + "limits" : { + "iops-total": 1000, + "bps-write": 2097152 + } + } + } + +throttle-group has a 'limits' property (of type ThrottleLimits as +defined in qapi/block-core.json) which can be set on creation or later +with 'qom-set'. + +A throttle-group can also be created with the -object command line +option but at the moment there is no way to pass a 'limits' parameter +that contains a ThrottleLimits structure. The solution is to set the +individual values directly, like in this example: + + -object throttle-group,id=group0,x-iops-total=1000,x-bps-write=2097152 + +Note however that this is not a stable API (hence the 'x-' prefixes) and +will disappear when -object gains support for structured options and +enables use of 'limits'. + +Once we have a throttle-group we can use the throttle block filter, +where the 'file' property must be set to the block device that we want +to filter: + + { "execute": "blockdev-add", + "arguments": { + "options": { + "driver": "qcow2", + "node-name": "disk0", + "file": { + "driver": "file", + "filename": "/path/to/disk.qcow2" + } + } + } + } + + { "execute": "blockdev-add", + "arguments": { + "driver": "throttle", + "node-name": "throttle0", + "throttle-group": "group0", + "file": "disk0" + } + } + +A similar setup can also be done with the command line, for example: + + -drive driver=throttle,throttle-group=group0, + file.driver=qcow2,file.file.filename=/path/to/disk.qcow2 + +The scenario described so far is very simple but the throttle block +filter allows for more complex configurations. For example, let's say +that we have three different drives and we want to set I/O limits for +each one of them and an additional set of limits for the combined I/O +of all three drives. + +First we would define all throttle groups, one for each one of the +drives and one that would apply to all of them: + + -object throttle-group,id=limits0,x-iops-total=2000 + -object throttle-group,id=limits1,x-iops-total=2500 + -object throttle-group,id=limits2,x-iops-total=3000 + -object throttle-group,id=limits012,x-iops-total=4000 + +Now we can define the drives, and for each one of them we use two +chained throttle filters: the drive's own filter and the combined +filter. + + -drive driver=throttle,throttle-group=limits012, + file.driver=throttle,file.throttle-group=limits0 + file.file.driver=qcow2,file.file.file.filename=/path/to/disk0.qcow2 + -drive driver=throttle,throttle-group=limits012, + file.driver=throttle,file.throttle-group=limits1 + file.file.driver=qcow2,file.file.file.filename=/path/to/disk1.qcow2 + -drive driver=throttle,throttle-group=limits012, + file.driver=throttle,file.throttle-group=limits2 + file.file.driver=qcow2,file.file.file.filename=/path/to/disk2.qcow2 + +In this example the individual drives have IOPS limits of 2000, 2500 +and 3000 respectively but the total combined I/O can never exceed 4000 +IOPS. |