qemu/docs/throttle.txt

6 This work is licensed under the terms of the GNU GPL, version 2 or
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
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
25 write operations. This gives us a total of six different parameters.
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
48 The default value of these parameters is 0, and it means 'unlimited'.
51 of 100 IOPS with the following -drive line:
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
78 limits lower the rest of the time.
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
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
112 With this, the user can perform I/O on hd0.qcow2 at a rate of 2000
116 long period of time with unused I/O (see below for details).
122 Here's the complete list of parameters for configuring bursts:
142 Controlling the size of I/O operations
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.
150 from happening. This setting specifies the size (in bytes) of an I/O
159 The default value of iops-size is 0 and it means that the size of the
163 Applying I/O limits to groups of disks
175 document apply to the combined I/O of all members of a group.
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).
191 I/O requests on several drives of the same group they will be
200      words: if different limits are applied to members of the same
203    - If 'group' is unset it is assumed to be the current group of that
208      it was member of a different one. In this case the limits
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
223 This algorithm uses the analogy of a bucket that leaks water
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
239     size of the bucket is 2000.
244 full at a rate of 2000 IOPS (the burst rate). Once the bucket is full
252 6000, allowing for 3 more seconds of I/O at 2000 IOPS.
265 The user can create an arbitrary number of filters and each one of
268 described earlier in "Applying I/O limits to groups of disks".
283 throttle-group has a 'limits' property (of type ThrottleLimits as
296 enables use of 'limits'.
332 each one of them and an additional set of limits for the combined I/O
333 of all three drives.
335 First we would define all throttle groups, one for each one of the
336 drives and one that would apply to all of them:
343 Now we can define the drives, and for each one of them we use two
357 In this example the individual drives have IOPS limits of 2000, 2500