Recently I had some spare time and compared Docker with KVM. Docker is impressive and has a lot of room to grow, but it still falls short in a few areas — for example, isolation is not great. I will revisit it in a couple of years once it has matured. For now KVM feels more practical. Here is a write-up I put together from playing around with it, shared for anyone interested:
Base environment
[root@localhost ~]# cat /etc/redhat-release
CentOS Linux release 7.1.1503 (Core)
[root@localhost ~]# uname -r
3.10.0-229.el7.x86_64
Check whether virtualization is supported
grep -E '(vmx|svm)' /proc/cpuinfo
Install
yum install qemu-kvm libvirt libvirt-python libguestfs-tools virt-install qemu-kvm-toools virt-manager
Start
systemctl start libvirtd
systemctl status libvirtd
Create a virtual machine
qemu-img create -f raw Centos-7.raw 10G # create an empty 10G raw disk in the current directory
virt-install --name Centos-7 --ram 1024 --vcpus=1 --disk path=Centos-7.raw --cdrom=CentOS-7-x86_64-DVD-1503-01_2.iso --graphics vnc,listen=0.0.0.0
Then connect to the host with a VNC client (installing the OS is out of scope here).
List the running virtual machines:
virsh list --all
Id Name State
----------------------------------------------------
7 Centos-7 running
Show detailed information about a virtual machine:
[root@localhost media]# virsh dominfo Centos-7
Id: 7
Name: Centos-7
UUID: 544cdb00-6c2c-4c93-9e11-086920b89320
OS Type: hvm
State: running
CPU(s): 1
CPU time: 46.9s
Max memory: 1048576 KiB
Used memory: 1048576 KiB
Persistent: yes
Autostart: disable
Managed save: no
Security model: none
Security DOI: 0
Scaling KVM up and down
Memory
virsh setmem Centos-7 512M
Disk
Disks can also be resized dynamically, but in my experience this is not advisable — you can lose data. If a disk runs out of space, just add another disk instead.
Format conversion
[root@linux-node2 opt]# qemu-img convert -f raw -O qcow2 Centos-7.raw Centos-7.qcow2
[root@linux-node2 opt]# ll -h Centos-7.*
-rw-r--r-- 1 root root 1017M May 4 16:18 Centos-7.qcow2
-rw-r--r-- 1 qemu qemu 10G May 4 16:11 Centos-7.raw
Pros and cons of RAW vs QCOW2
raw is the most primitive format and has the best I/O performance, bar none.
Its downside is a very limited feature set: snapshots, encryption, and
compression are all unsupported, and it is huge — allocating 10G really
does consume 10G from the start.
qcow2 is the opposite: feature-rich, supporting snapshots, encryption, and
compression, and it uses space efficiently by storing only the changes on
top of the base image — but performance is worse than raw.
Bridging the virtual machine to a physical NIC
At this point the virtual machine still has no network access — it is just running a system. To get it online:
brctl addbr br0
Create a virtual bridge interface named
br0
:
brctl addif br0 eth0
Bridge it to
eth0
. Be careful: this will drop your connection to the server, because the server’s
eth0
has no IP address.
ip addr del dev eth0 192.168.56.11/24
Remove the original IP address from
eth0
:
ifconfig br0 192.168.56.11/24 up
You can now reach the server again, but networking still will not work because the route is wrong. Add:
route add default gw 1.1.1.1（gateway address）
This will disconnect you from the KVM virtual machine — that is fine. Edit the virtual machine’s XML file:
virsh edit Centos-7 # change it to the following two lines
<interface type='bridge'>
<source bridge='br0'/>
Reboot:
virsh reboot Centos-7
Then configure the IP address, gateway, DNS, and so on, and it will be online. I will not demonstrate that here.
A few commonly used commands:
Power on:
virsh start test1
Power off:
virsh shutdown test1
Force power off:
virsh destroy test1
Reboot:
virsh reboot test1
Start a domain from a config file:
virsh create /etc/libvirt/qemu/test1.xml
Check domain status:
virsh list --all
Suspend a virtual machine:
virsh suspend test1
Save a virtual machine:
virsh save test1
or
virsh dumpxml Centos-7 > dump_kvm.xml
Restore a virtual machine:
virsh resume test1
Snapshots
Snapshot files are stored in
/var/lib/libvirt/qemu/snapshot/
by default.
Take a snapshot:
virsh snapshot-create test1
List snapshots:
virsh snapshot-list test1
Name Creation Time State
------------------------------------------------------------
1486394873 2017-02-06 23:27:53 +0800 shutoff #previous snapshot
1486394993 2017-02-06 23:29:53 +0800 shutoff #this snapshot
Restore a snapshot:
virsh snapshot-revert test1 1486394873
Example 1: changing the CPU count
virsh edit Centos-7
<vcpu placement='auto' current='1'>4</vcpu> #max 4 CPUs, currently 1
virsh setvcpus Centos-7 2 --live #you can also use --config
virsh reboot Centos-7 #restart optional
Example 2: adding a disk
qemu-img create -f qcow2 add.img 20G
virsh attach-disk Centos-7 /opt/add.img sdb #hot-add a disk (lost on reboot)
virsh edit Centos-7 #write it into the config file
…………
</disk>
<disk type='file' device='disk'>
<driver name='qemu' type='qcow2' cache='none'/>
<source file='/opt/add.img'/>
<target dev='sdb' bus='virtio'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
</disk>
virsh reboot Centos-7
Later I found a simpler approach:
virsh attach-disk Centos-7 /opt/add.img sdb --config