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