# 九：如何在线添加/移除虚拟机的vCPU
KVM支持vCPU热插拔么？Linux KVM虚拟机能够感知到新添加的vCPU么？答案当然是"YES"。
类似[KVM 内存管理](https://github.com/huataihuang/cloud-atlas-draft/tree/6f3204fffc11cf006abd394631e2598d98b415c3/virtual/kvm/startup/how_to_add_remove_memory_to_guest_on_fly/README.md)，你可以使用`virsh`命令给正在运行的VM添加或删除vCPU。不过，这个工作是需要预先配置KVM虚拟机的最大vCPU参数才能使用。所以当部署一个新的虚拟机，你应该首先考虑这个参数。给KVM虚拟机设置最大vCPU数量并不会对资源有所小号，因为虚拟机只会使用分配的vCPU。
## 理解VM的vCPU配置
* 列出KVM主机配置的VM
```
[root@UA-HA ~]# virsh list --all
Id Name State
----------------------------------------------------
- UAKVM2 shut off
[root@UA-HA ~]#
```
* 检查当前VM配置
```
[root@UA-HA ~]# virsh dumpxml UAKVM2 | grep vcpu
2
```
以上显示这个VM当前使用1个vCPU，并且可以通过`virsh vsetcpus`命令增加vCPU。但死你不能在VM运行时增加超过2个vCPU。
* 现在我们启动VM
```
[root@UA-HA ~]# virsh start UAKVM2
Domain UAKVM2 started
[root@UA-HA ~]#
```
* 登陆到KVM虚拟机并检查分配的vCPU数量：
```
[root@UA-KVM1 ~]# lscpu
Architecture: x86_64
CPU op-mode(s): 32-bit, 64-bit
Byte Order: Little Endian
CPU(s): 1
On-line CPU(s) list: 0
Thread(s) per core: 1
Core(s) per socket: 1
Socket(s): 1
NUMA node(s): 1
Vendor ID: GenuineIntel
CPU family: 6
Model: 44
Model name: Westmere E56xx/L56xx/X56xx (Nehalem-C)
Stepping: 1
CPU MHz: 2594.058
BogoMIPS: 5188.11
Hypervisor vendor: KVM
Virtualization type: full
L1d cache: 32K
L1i cache: 32K
L2 cache: 4096K
NUMA node0 CPU(s): 0
[root@UA-KVM1 ~]#
```
## 向运行的KVM虚拟机增加vCPU
* 切换到KVM主机并使用`virsh`命令增加vCPU，从1增加到2
```
[root@UA-HA ~]# virsh setvcpus UAKVM2 2
[root@UA-HA ~]# virsh dominfo UAKVM2
Id: 38
Name: UAKVM2
UUID: 6013be3b-08f9-4827-83fb-390bd5a86de6
OS Type: hvm
State: running
CPU(s): 2
CPU time: 51.5s
Max memory: 1048576 KiB
Used memory: 1048576 KiB
Persistent: yes
Autostart: disable
Managed save: no
Security model: selinux
Security DOI: 0
Security label: system_u:system_r:svirt_t:s0:c709,c868 (permissive)
[root@UA-HA ~]#
```
* 返回到KVM guest虚拟机，检查新添加的vCPU。此时VM从`UP code(uniprocessor)`切换到`SMP code (symmetric multiprocessor`)
```
[root@UA-KVM1 ~]# tail -f /var/log/messages
Dec 16 12:48:28 UA-KVM1 kernel: CPU1 has been hot-added
Dec 16 12:48:28 UA-KVM1 kernel: SMP alternatives: switching to SMP code
Dec 16 12:48:57 UA-KVM1 kernel: smpboot: Booting Node 0 Processor 1 APIC 0x1
Dec 16 12:48:57 UA-KVM1 kernel: kvm-clock: cpu 1, msr 0:3ff87041, secondary cpu clock
Dec 16 12:48:57 UA-KVM1 kernel: TSC synchronization [CPU#0 -> CPU#1]:
Dec 16 12:48:57 UA-KVM1 kernel: Measured 906183720569 cycles TSC warp between CPUs, turning off TSC clock.
Dec 16 12:48:57 UA-KVM1 kernel: tsc: Marking TSC unstable due to check_tsc_sync_source failed
Dec 16 12:48:57 UA-KVM1 kernel: KVM setup async PF for cpu 1
Dec 16 12:48:57 UA-KVM1 kernel: kvm-stealtime: cpu 1, msr 3fd0d240
Dec 16 12:48:57 UA-KVM1 kernel: microcode: CPU1 sig=0x206c1, pf=0x1, revision=0x1
Dec 16 12:48:57 UA-KVM1 kernel: Will online and init hotplugged CPU: 1
[root@UA-KVM1 ~]# lscpu
Architecture: x86_64
CPU op-mode(s): 32-bit, 64-bit
Byte Order: Little Endian
CPU(s): 2
On-line CPU(s) list: 0,1
Thread(s) per core: 1
Core(s) per socket: 1
Socket(s): 2
NUMA node(s): 1
Vendor ID: GenuineIntel
CPU family: 6
Model: 44
Model name: Westmere E56xx/L56xx/X56xx (Nehalem-C)
Stepping: 1
CPU MHz: 2594.058
BogoMIPS: 5188.11
Hypervisor vendor: KVM
Virtualization type: full
L1d cache: 32K
L1i cache: 32K
L2 cache: 4096K
NUMA node0 CPU(s): 0,1
[root@UA-KVM1 ~]#
```
* 如果想配置永久生效，保存VM配置
```
[root@UA-HA ~]# virsh setvcpus UAKVM2 2 --config
[root@UA-HA ~]#
```
现在我们已经成功给KVM虚拟机添加了vCPU并使配置永久生效。
## 从运行的KVM虚拟机去除vCPU
没有直接从KVM guest虚拟机直接移除vCPU的方法。但是可以通过禁止CPU核心方式将CPU核心的性能返还给KVM主机。
* 登陆到KVM host主机
* 假设当前KVM guest虚拟机UAKVM2有2个活动的vCPU
* 注意不要使用以下命令来移除vCPU，你会收到错误消息`error: internal error: cannot change vcpu count of this domain`
```
[root@UA-HA ~]# virsh setvcpus UAKVM2 1
error: internal error: cannot change vcpu count of this domain
[root@UA-HA ~]#
```
* 使用以下命令来缩减UAKVM2的vCPU数量（vCPU从2缩减到1）
```
[root@UA-HA ~]# virsh setvcpus --live --guest UAKVM2 1
[root@UA-HA ~]#
```
* 保存VM配置
```
[root@UA-HA ~]# virsh setvcpus --config UAKVM2 1
[root@UA-HA ~]#
```
* 登陆到KVM guest虚拟机`UAKVM2`验证
```
[root@UA-KVM1 ~]# tail -f /var/log/messages
Dec 16 13:01:01 UA-KVM1 systemd: Starting Session 2 of user root.
Dec 16 13:19:08 UA-KVM1 kernel: Unregister pv shared memory for cpu 1
Dec 16 13:19:08 UA-KVM1 kernel: smpboot: CPU 1 is now offline
[root@UA-KVM1 ~]# lscpu
Architecture: x86_64
CPU op-mode(s): 32-bit, 64-bit
Byte Order: Little Endian
CPU(s): 2
On-line CPU(s) list: 0
Off-line CPU(s) list: 1
Thread(s) per core: 1
Core(s) per socket: 1
Socket(s): 1
NUMA node(s): 1
Vendor ID: GenuineIntel
CPU family: 6
Model: 44
Model name: Westmere E56xx/L56xx/X56xx (Nehalem-C)
Stepping: 1
CPU MHz: 2594.058
BogoMIPS: 5188.11
Hypervisor vendor: KVM
Virtualization type: full
L1d cache: 32K
L1i cache: 32K
L2 cache: 4096K
NUMA node0 CPU(s): 0
[root@UA-KVM1 ~]#
```
注意：
在KVM物理主机中过，你依然会看到两个vCPU呗分配给虚拟机`UAKVM2`，只有重启VM才会生效。
```
[root@UA-HA ~]# virsh dominfo UAKVM2
Id: 38
Name: UAKVM2
UUID: 6013be3b-08f9-4827-83fb-390bd5a86de6
OS Type: hvm
State: running
CPU(s): 2
CPU time: 90.4s
Max memory: 1048576 KiB
Used memory: 1048576 KiB
Persistent: yes
Autostart: disable
Managed save: no
Security model: selinux
Security DOI: 0
Security label: system_u:system_r:svirt_t:s0:c709,c868 (permissive)
[root@UA-HA ~]#
```
* 关闭`UAKVM2` KVM虚拟机并验证
```
[root@UA-HA ~]# virsh destroy UAKVM2
Domain UAKVM2 destroyed
[root@UA-HA ~]# virsh dominfo UAKVM2
Id: -
Name: UAKVM2
UUID: 6013be3b-08f9-4827-83fb-390bd5a86de6
OS Type: hvm
State: shut off
CPU(s): 1
Max memory: 1048576 KiB
Used memory: 0 KiB
Persistent: yes
Autostart: disable
Managed save: no
Security model: selinux
Security DOI: 0
[root@UA-HA ~]#
```
此时可以看到`vCPU`被修改成1个。
如果尝试添加比预分配更多的vCPU会看到如下错误信息`error: invalid argument: requested vcpus is greater than max allowable vcpus for the domain`
```
[root@UA-HA ~]# virsh setvcpus UAKVM2 3
error: invalid argument: requested vcpus is greater than max allowable vcpus for the domain: 3 > 2
[root@UA-HA ~]#
```
请重新参考前述的"理解VM vCPU配置"段落。使用`virsh`命令不能超过vCPU最大配置数量。这个最大vCPU数量不能在虚拟机运行时修改。
## 如何修改每个虚拟机的最大vCPU数量？（只能使用offline模式）
* 作为root用户登陆到KVM host主机
* 优雅地停止虚拟机
```
[root@UA-HA ~]# virsh list
Id Name State
----------------------------------------------------
39 UAKVM2 running
[root@UA-HA ~]# virsh shutdown UAKVM2
Domain UAKVM2 is being shutdown
[root@UA-HA ~]#
```
* 编辑VM配置类似如下，这里将最大vCPU数量从2修改成4:
```
2
```
修改成
```
4
```
vCPU XML 格式：
```
M
```
这里N时当前激活的CPU数量，而M是CPU的最大数量
* 启动KVM虚拟机`UAKVM2`
```
[root@UA-HA ~]# virsh start UAKVM2
Domain UAKVM2 started
[root@UA-HA ~]#
```
* 验证UAKVM2虚拟机的vCPU信息
```
[root@UA-HA ~]# virsh vcpuinfo UAKVM2
VCPU: 0
CPU: 1
State: running
CPU time: 24.0s
CPU Affinity: yy
[root@UA-HA ~]#
```
如上所述，此时UAKVM2只分配了1个vCPU。由于我们配置了vCPU最大数量是4，所以我们可以动态将虚拟机的vCPU增加到4个。
* 增加vCPU数量到4
```
[root@UA-HA ~]# virsh setvcpus UAKVM2 4
[root@UA-HA ~]# virsh vcpuinfo UAKVM2
VCPU: 0
CPU: 0
State: running
CPU time: 27.6s
CPU Affinity: yy
VCPU: 1
CPU: 0
State: running
CPU time: 0.4s
CPU Affinity: yy
VCPU: 2
CPU: 0
State: running
CPU time: 0.1s
CPU Affinity: yy
VCPU: 3
CPU: 0
State: running
CPU time: 0.1s
CPU Affinity: yy
[root@UA-HA ~]#
[root@UA-HA ~]# virsh setvcpus UAKVM2 4 --config
[root@UA-HA ~]#
```
* 登陆到KVM虚拟机`UAKVM2`中并列出vCPU
```
[root@UA-KVM1 ~]# lscpu
Architecture: x86_64
CPU op-mode(s): 32-bit, 64-bit
Byte Order: Little Endian
CPU(s): 4
On-line CPU(s) list: 0-3
Thread(s) per core: 1
Core(s) per socket: 1
Socket(s): 4
NUMA node(s): 1
Vendor ID: GenuineIntel
CPU family: 6
Model: 44
Model name: Westmere E56xx/L56xx/X56xx (Nehalem-C)
Stepping: 1
CPU MHz: 2594.058
BogoMIPS: 5188.11
Hypervisor vendor: KVM
Virtualization type: full
L1d cache: 32K
L1i cache: 32K
L2 cache: 4096K
NUMA node0 CPU(s): 0-3
[root@UA-KVM1 ~]#
```
以上我们已经在离线情况下成功增加了最大vCPU数量，并在VM运行时增加了分配到vCPU。
## 系列教程编译
本系列教程根据 [UnixArena Linux KVM](http://www.unixarena.com/category/redhat-linux/linux-kvm) 系列教程编译，共分以下文章：
* [一：基于内核的虚拟机（KVM）概览](https://github.com/huataihuang/cloud-atlas-draft/tree/6f3204fffc11cf006abd394631e2598d98b415c3/virtual/kvm/startup/kernel_based_virtual_machine_kvm_overview/README.md)
* [二：Redhat企业Linux安装KVM](https://github.com/huataihuang/cloud-atlas-draft/tree/6f3204fffc11cf006abd394631e2598d98b415c3/virtual/kvm/startup/redhat_enterprise_linux_kvm_installation/README.md)
* [三：RHEL 7.2 配置KVM主机](https://github.com/huataihuang/cloud-atlas-draft/tree/6f3204fffc11cf006abd394631e2598d98b415c3/virtual/kvm/startup/rhel_7_2_configuring_kvm_hosts/README.md)
* [四：使用命令行启动第一个KVM实例](https://github.com/huataihuang/cloud-atlas-draft/tree/6f3204fffc11cf006abd394631e2598d98b415c3/virtual/kvm/startup/launch_the_first_kvm_instance_using_cli/README.md)
* [五：使用Virt-Manger(VMM)部署KVM实例](https://github.com/huataihuang/cloud-atlas-draft/tree/6f3204fffc11cf006abd394631e2598d98b415c3/virtual/kvm/startup/deploy_kvm_instance_using_virt_manger_vmm_gui/README.md)
* [六：如何克隆一个KVM虚拟机并重置该虚拟机](https://github.com/huataihuang/cloud-atlas-draft/tree/6f3204fffc11cf006abd394631e2598d98b415c3/virtual/kvm/startup/how_to_clone_a_kvm_virtual_machines_and_reset_the_vm/README.md)
* [七：如何在线添加/更改虚拟磁盘](https://github.com/huataihuang/cloud-atlas-draft/tree/6f3204fffc11cf006abd394631e2598d98b415c3/virtual/kvm/startup/how_to_add_resize_virtual_disk_on_fly/README.md)
* [八：如何在线添加/移除虚拟机的内存](https://github.com/huataihuang/cloud-atlas-draft/tree/6f3204fffc11cf006abd394631e2598d98b415c3/virtual/kvm/startup/how_to_add_remove_memory_to_guest_on_fly/README.md)
* [九：如何在线添加/移除虚拟机的vCPU](https://github.com/huataihuang/cloud-atlas-draft/tree/6f3204fffc11cf006abd394631e2598d98b415c3/virtual/kvm/startup/how_to_add_remove_vcpu_to_guest_on_fly/README.md)
* [十：更改libvirt虚拟机镜像存储路径](https://github.com/huataihuang/cloud-atlas-draft/tree/6f3204fffc11cf006abd394631e2598d98b415c3/virtual/kvm/startup/change_libvirt_vm_image_store_path/README.md)
* [十一：实现Linux KVM在线迁移](https://github.com/huataihuang/cloud-atlas-draft/tree/6f3204fffc11cf006abd394631e2598d98b415c3/virtual/kvm/startup/perform_live_migration_on_linux_kvm/README.md)
* [十二：RHEL7 Pacemaker - 配置高可用KVM虚拟机](https://github.com/huataihuang/cloud-atlas-draft/tree/6f3204fffc11cf006abd394631e2598d98b415c3/virtual/kvm/startup/rhel_7_pacemaker_configuring_ha_kvm_guest/README.md)
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