docs/throttle.txt

   1 The QEMU throttling infrastructure
   2 ==================================
   3 Copyright (C) 2016 Igalia, S.L.
   4 Author: Alberto Garcia <berto@igalia.com>
   5
   6 This work is licensed under the terms of the GNU GPL, version 2 or
   7 later. See the COPYING file in the top-level directory.
   8
   9 Introduction
  10 ------------
  11 QEMU includes a throttling module that can be used to set limits to
  12 I/O operations. The code itself is generic and independent of the I/O
  13 units, but it is currently used to limit the number of bytes per second
  14 and operations per second (IOPS) when performing disk I/O.
  15
  16 This document explains how to use the throttling code in QEMU, and how
  17 it works internally. The implementation is in throttle.c.
  18
  19
  20 Using throttling to limit disk I/O
  21 ----------------------------------
  22 Two aspects of the disk I/O can be limited: the number of bytes per
  23 second and the number of operations per second (IOPS). For each one of
  24 them the user can set a global limit or separate limits for read and
  25 write operations. This gives us a total of six different parameters.
  26
  27 I/O limits can be set using the throttling.* parameters of -drive, or
  28 using the QMP 'block_set_io_throttle' command. These are the names of
  29 the parameters for both cases:
  30
  31 |-----------------------+-----------------------|
  32 | -drive                | block_set_io_throttle |
  33 |-----------------------+-----------------------|
  34 | throttling.iops-total | iops                  |
  35 | throttling.iops-read  | iops_rd               |
  36 | throttling.iops-write | iops_wr               |
  37 | throttling.bps-total  | bps                   |
  38 | throttling.bps-read   | bps_rd                |
  39 | throttling.bps-write  | bps_wr                |
  40 |-----------------------+-----------------------|
  41
  42 It is possible to set limits for both IOPS and bps at the same time,
  43 and for each case we can decide whether to have separate read and
  44 write limits or not, but note that if iops-total is set then neither
  45 iops-read nor iops-write can be set. The same applies to bps-total and
  46 bps-read/write.
  47
  48 The default value of these parameters is 0, and it means 'unlimited'.
  49
  50 In its most basic usage, the user can add a drive to QEMU with a limit
  51 of 100 IOPS with the following -drive line:
  52
  53    -drive file=hd0.qcow2,throttling.iops-total=100
  54
  55 We can do the same using QMP. In this case all these parameters are
  56 mandatory, so we must set to 0 the ones that we don't want to limit:
  57
  58    { "execute": "block_set_io_throttle",
  59      "arguments": {
  60         "device": "virtio0",
  61         "iops": 100,
  62         "iops_rd": 0,
  63         "iops_wr": 0,
  64         "bps": 0,
  65         "bps_rd": 0,
  66         "bps_wr": 0
  67      }
  68    }
  69
  70
  71 I/O bursts
  72 ----------
  73 In addition to the basic limits we have just seen, QEMU allows the
  74 user to do bursts of I/O for a configurable amount of time. A burst is
  75 an amount of I/O that can exceed the basic limit. Bursts are useful to
  76 allow better performance when there are peaks of activity (the OS
  77 boots, a service needs to be restarted) while keeping the average
  78 limits lower the rest of the time.
  79
  80 Two parameters control bursts: their length and the maximum amount of
  81 I/O they allow. These two can be configured separately for each one of
  82 the six basic parameters described in the previous section, but in
  83 this section we'll use 'iops-total' as an example.
  84
  85 The I/O limit during bursts is set using 'iops-total-max', and the
  86 maximum length (in seconds) is set with 'iops-total-max-length'. So if
  87 we want to configure a drive with a basic limit of 100 IOPS and allow
  88 bursts of 2000 IOPS for 60 seconds, we would do it like this (the line
  89 is split for clarity):
  90
  91    -drive file=hd0.qcow2,
  92           throttling.iops-total=100,
  93           throttling.iops-total-max=2000,
  94           throttling.iops-total-max-length=60
  95
  96 Or, with QMP:
  97
  98    { "execute": "block_set_io_throttle",
  99      "arguments": {
 100         "device": "virtio0",
 101         "iops": 100,
 102         "iops_rd": 0,
 103         "iops_wr": 0,
 104         "bps": 0,
 105         "bps_rd": 0,
 106         "bps_wr": 0,
 107         "iops_max": 2000,
 108         "iops_max_length": 60,
 109      }
 110    }
 111
 112 With this, the user can perform I/O on hd0.qcow2 at a rate of 2000
 113 IOPS for 1 minute before it's throttled down to 100 IOPS.
 114
 115 The user will be able to do bursts again if there's a sufficiently
 116 long period of time with unused I/O (see below for details).
 117
 118 The default value for 'iops-total-max' is 0 and it means that bursts
 119 are not allowed. 'iops-total-max-length' can only be set if
 120 'iops-total-max' is set as well, and its default value is 1 second.
 121
 122 Here's the complete list of parameters for configuring bursts:
 123
 124 |----------------------------------+-----------------------|
 125 | -drive                           | block_set_io_throttle |
 126 |----------------------------------+-----------------------|
 127 | throttling.iops-total-max        | iops_max              |
 128 | throttling.iops-total-max-length | iops_max_length       |
 129 | throttling.iops-read-max         | iops_rd_max           |
 130 | throttling.iops-read-max-length  | iops_rd_max_length    |
 131 | throttling.iops-write-max        | iops_wr_max           |
 132 | throttling.iops-write-max-length | iops_wr_max_length    |
 133 | throttling.bps-total-max         | bps_max               |
 134 | throttling.bps-total-max-length  | bps_max_length        |
 135 | throttling.bps-read-max          | bps_rd_max            |
 136 | throttling.bps-read-max-length   | bps_rd_max_length     |
 137 | throttling.bps-write-max         | bps_wr_max            |
 138 | throttling.bps-write-max-length  | bps_wr_max_length     |
 139 |----------------------------------+-----------------------|
 140
 141
 142 Controlling the size of I/O operations
 143 --------------------------------------
 144 When applying IOPS limits all I/O operations are treated equally
 145 regardless of their size. This means that the user can take advantage
 146 of this in order to circumvent the limits and submit one huge I/O
 147 request instead of several smaller ones.
 148
 149 QEMU provides a setting called throttling.iops-size to prevent this
 150 from happening. This setting specifies the size (in bytes) of an I/O
 151 request for accounting purposes. Larger requests will be counted
 152 proportionally to this size.
 153
 154 For example, if iops-size is set to 4096 then an 8KB request will be
 155 counted as two, and a 6KB request will be counted as one and a
 156 half. This only applies to requests larger than iops-size: smaller
 157 requests will be always counted as one, no matter their size.
 158
 159 The default value of iops-size is 0 and it means that the size of the
 160 requests is never taken into account when applying IOPS limits.
 161
 162
 163 Applying I/O limits to groups of disks
 164 --------------------------------------
 165 In all the examples so far we have seen how to apply limits to the I/O
 166 performed on individual drives, but QEMU allows grouping drives so
 167 they all share the same limits.
 168
 169 The way it works is that each drive with I/O limits is assigned to a
 170 group named using the throttling.group parameter. If this parameter is
 171 not specified, then the device name (i.e. 'virtio0', 'ide0-hd0') will
 172 be used as the group name.
 173
 174 Limits set using the throttling.* parameters discussed earlier in this
 175 document apply to the combined I/O of all members of a group.
 176
 177 Consider this example:
 178
 179    -drive file=hd1.qcow2,throttling.iops-total=6000,throttling.group=foo
 180    -drive file=hd2.qcow2,throttling.iops-total=6000,throttling.group=foo
 181    -drive file=hd3.qcow2,throttling.iops-total=3000,throttling.group=bar
 182    -drive file=hd4.qcow2,throttling.iops-total=6000,throttling.group=foo
 183    -drive file=hd5.qcow2,throttling.iops-total=3000,throttling.group=bar
 184    -drive file=hd6.qcow2,throttling.iops-total=5000
 185
 186 Here hd1, hd2 and hd4 are all members of a group named 'foo' with a
 187 combined IOPS limit of 6000, and hd3 and hd5 are members of 'bar'. hd6
 188 is left alone (technically it is part of a 1-member group).
 189
 190 Limits are applied in a round-robin fashion so if there are concurrent
 191 I/O requests on several drives of the same group they will be
 192 distributed evenly.
 193
 194 When I/O limits are applied to an existing drive using the QMP command
 195 'block_set_io_throttle', the following things need to be taken into
 196 account:
 197
 198    - I/O limits are shared within the same group, so new values will
 199      affect all members and overwrite the previous settings. In other
 200      words: if different limits are applied to members of the same
 201      group, the last one wins.
 202
 203    - If 'group' is unset it is assumed to be the current group of that
 204      drive. If the drive is not in a group yet, it will be added to a
 205      group named after the device name.
 206
 207    - If 'group' is set then the drive will be moved to that group if
 208      it was member of a different one. In this case the limits
 209      specified in the parameters will be applied to the new group
 210      only.
 211
 212    - I/O limits can be disabled by setting all of them to 0. In this
 213      case the device will be removed from its group and the rest of
 214      its members will not be affected. The 'group' parameter is
 215      ignored.
 216
 217
 218 The Leaky Bucket algorithm
 219 --------------------------
 220 I/O limits in QEMU are implemented using the leaky bucket algorithm
 221 (specifically the "Leaky bucket as a meter" variant).
 222
 223 This algorithm uses the analogy of a bucket that leaks water
 224 constantly. The water that gets into the bucket represents the I/O
 225 that has been performed, and no more I/O is allowed once the bucket is
 226 full.
 227
 228 To see the way this corresponds to the throttling parameters in QEMU,
 229 consider the following values:
 230
 231   iops-total=100
 232   iops-total-max=2000
 233   iops-total-max-length=60
 234
 235   - Water leaks from the bucket at a rate of 100 IOPS.
 236   - Water can be added to the bucket at a rate of 2000 IOPS.
 237   - The size of the bucket is 2000 x 60 = 120000
 238   - If 'iops-total-max-length' is unset then it defaults to 1 and the
 239     size of the bucket is 2000.
 240   - If 'iops-total-max' is unset then 'iops-total-max-length' must be
 241     unset as well. In this case the bucket size is 100.
 242
 243 The bucket is initially empty, therefore water can be added until it's
 244 full at a rate of 2000 IOPS (the burst rate). Once the bucket is full
 245 we can only add as much water as it leaks, therefore the I/O rate is
 246 reduced to 100 IOPS. If we add less water than it leaks then the
 247 bucket will start to empty, allowing for bursts again.
 248
 249 Note that since water is leaking from the bucket even during bursts,
 250 it will take a bit more than 60 seconds at 2000 IOPS to fill it
 251 up. After those 60 seconds the bucket will have leaked 60 x 100 =
 252 6000, allowing for 3 more seconds of I/O at 2000 IOPS.
 253
 254 Also, due to the way the algorithm works, longer burst can be done at
 255 a lower I/O rate, e.g. 1000 IOPS during 120 seconds.