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authorRajithaY <rajithax.yerrumsetty@intel.com>2017-04-25 03:31:15 -0700
committerRajitha Yerrumchetty <rajithax.yerrumsetty@intel.com>2017-05-22 06:48:08 +0000
commitbb756eebdac6fd24e8919e2c43f7d2c8c4091f59 (patch)
treeca11e03542edf2d8f631efeca5e1626d211107e3 /qemu/docs/throttle.txt
parenta14b48d18a9ed03ec191cf16b162206998a895ce (diff)
Adding qemu as a submodule of KVMFORNFV
This Patch includes the changes to add qemu as a submodule to kvmfornfv repo and make use of the updated latest qemu for the execution of all testcase Change-Id: I1280af507a857675c7f81d30c95255635667bdd7 Signed-off-by:RajithaY<rajithax.yerrumsetty@intel.com>
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-The QEMU throttling infrastructure
-==================================
-Copyright (C) 2016 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 currenly 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 and 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 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.