Tag: qemu-img

Expand disk and the root partition of the QEMU virtual server

This article is to show how easy is to grow the size of QEMU virtual disk and its partitions (along with ext4 file system). Of course, you can use this article as an example of expanding the partitions of a physical disk.

Our setup is a QEMU virtual server using a raw image of 20G and the steps are as follow:

  1. Stop the virtual server
  2. Resize with qemu-img the raw image of the virtual server
  3. Start the virtual server
  4. Get a root ssh shell (probably by using openssh)
  5. Use parted to resize the partition (and fix the GPT of the disk – not the disk is larger, so the GPT table need fixing).
  6. Use resize2fs to resize the

STEP 1) Power off your virtual server.

The best way is to power it off within the server with the “poweroff” command. Be careful to check whether the host server killed the QEMU process. It is almost certain if the VNC port is released, the QEMU process has been exited.
If you use virsh (i.e. libvirt), you may execute:

virsh shutdown my-private-vm-01
virsh destroy my-private-vm-01

The destroy command ensures there is no QEMU process, which still operates over the image disk file. But it is dangerous for your data if you issue it on a running virtual server, because it may lose the unsaved data.
If you use QEMU manually wait for the process to exit or if you have enabled the management console connect to it using telnet and just quit – this will destory the QEMU virtual server process – again be careful with unsaved data.

[root@lsrv1 ~]# ps axuf|grep qemu
root     15575  2.3 50.1 13061032 8112212 ?    Sl   May08 1522:27 qemu-system-x86_64 -enable-kvm -smp 4,maxcpus=8 -daemonize -vnc :30 -cdrom /mnt/vm/isos/CentOS-7-x86_64-Minimal-1810.iso -drive file=/mnt/vm/images/templatesrv-wordpress.bin,cache=none,aio=threads,if=virtio -boot c -net nic,model=virtio,macaddr=00:00:00:00:00:30 -net tap,ifname=tap30,script=no,downscript=no -balloon virtio -m 8144 -monitor telnet:127.0.0.1:5830,server,nowait
[root@srv-host ~]# telnet 127.0.0.1 5830
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
QEMU 2.0.0 monitor - type 'help' for more information
(qemu) q
Connection closed by foreign host.

If you use a web interface (for example WebVirtMgr) check whether the virtual server is in power-off state.

STEP 2) Resize the image file of the virtual server.

Find where are located the virtual servers’ image files in your installation and use qemu-img. We want to increse the size with 174GB to 200GB.

qemu-img resize my-private-vm-01.img +174GB

STEP 3) Start your server.

Start your server by issuing a command with virsh or QEMU (qemu-system-x86_64) or from a web interface if use one (like WebVirtMgr).
*virsh and libvirt:

virsh start my-private-vm-01

*Manual start of QEMU emulator – qemu-system-x86_64:

qemu-system-x86_64 -enable-kvm -smp 4,maxcpus=8 -daemonize -vnc :30 -cdrom /mnt/vm/isos/CentOS-7-x86_64-Minimal-1810.iso -drive file=/mnt/vm/images/templatesrv-wordpress.bin,cache=none,aio=threads,if=virtio -boot c -net nic,model=virtio,macaddr=00:00:00:00:00:30 -net tap,ifname=tap30,script=no,downscript=no -balloon virtio -m 8144 -monitor telnet:127.0.0.1:5830,server,nowait

Or just use the web browser and start the virtual server from WebVirtMgr if it is what you use.

STEP 4) Open a shell to your server.

We use openssh client to connect to our server.

STEP 5) Use parted to resize the partition.

The program “parted” will report that the partition table does not use the whole available disk, which is perfectly normal because we’ve just increased the disk size. Just confirm to fix the GPT partition table:

parted /dev/vda
GNU Parted 3.2
Using /dev/vda
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) p                                                                
Warning: Not all of the space available to /dev/vda appears to be used, you can fix the GPT to use all of the space (an extra 367001600 blocks) or continue with the current
setting? 
Fix/Ignore? Fix                                                           
Model: Virtio Block Device (virtblk)
Disk /dev/vda: 215GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags: 

Number  Start   End     Size    File system     Name  Flags
 1      1049kB  2097kB  1049kB                        bios_grub
 2      2097kB  4096MB  4094MB  linux-swap(v1)
 3      4096MB  24.0GB  19.9GB  ext4

(parted) resizepart 3 -1                                                  
Warning: Partition /dev/vda3 is being used. Are you sure you want to continue?
parted: invalid token: -1
Yes/No? Yes
End?  [24.0GB]? -1
(parted) p
Model: Virtio Block Device (virtblk)
Disk /dev/vda: 215GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags:

Number  Start   End     Size    File system     Name  Flags
 1      1049kB  2097kB  1049kB                        bios_grub
 2      2097kB  4096MB  4094MB  linux-swap(v1)
 3      4096MB  215GB   211GB   ext4

(parted) q                                                                
Information: You may need to update /etc/fstab.

There is a warning the partition is in use but it is perfectly OK to continue.
*parted: reports “invalid token: -1”, but it is accepted for the “End” parameter.

STEP 6) Resize the ext4 file system online

Use the tool resize2fs to resize EXT4.

resize2fs /dev/vda3
resize2fs 1.42.13 (17-May-2015)
Filesystem at /dev/vda3 is mounted on /; on-line resizing required
old_desc_blocks = 2, new_desc_blocks = 13
The filesystem on /dev/vda3 is now 51428620 (4k) blocks long.

To check the resize operation:

srv1-vm ~ # dmesg|grep EXT4
[  449.330140] EXT4-fs (vda3): resizing filesystem from 4859392 to 51428620 blocks
[  449.936044] EXT4-fs (vda3): resized filesystem to 51428620
srv1-vm ~ # df -h
Filesystem      Size  Used Avail Use% Mounted on
udev            3.9G     0  3.9G   0% /dev
tmpfs           798M  3.6M  795M   1% /run
/dev/vda3       193G  5.7G  180G   4% /
tmpfs           3.9G  196K  3.9G   1% /dev/shm

Output log of the whole resize operation

srv-vm1 ~ # parted /dev/vda
GNU Parted 3.2
Using /dev/vda
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) p                                                                
Model: Virtio Block Device (virtblk)
Disk /dev/vda: 26.8GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags: 

Number  Start   End     Size    File system     Name  Flags
 1      1049kB  2097kB  1049kB                        bios_grub
 2      2097kB  4096MB  4094MB  linux-swap(v1)
 3      4096MB  24.0GB  19.9GB  ext4

(parted) q                                                                
srv-vm1 ~ # poweroff
Connection to srv-vm1 closed by remote host.
Connection to srv-vm1 closed.
myuser@gw1:~$ sshh srv1-host
srv1-host ~ # cd /mnt/vm/images
srv1-host images # qemu-img resize srv-vm1.img +174GB
Image resized.
srv1-host images # logout
Connection to srv1-host closed.
myuser@gw1:~$ sshh srv-vm1
srv-vm1 ~ # df -h
Filesystem      Size  Used Avail Use% Mounted on
udev            3.9G     0  3.9G   0% /dev
tmpfs           798M  3.5M  795M   1% /run
/dev/vda3        19G  5.7G   12G  33% /
tmpfs           3.9G     0  3.9G   0% /dev/shm
tmpfs           5.0M     0  5.0M   0% /run/lock
tmpfs           3.9G     0  3.9G   0% /sys/fs/cgroup
tmpfs           798M     0  798M   0% /run/user/0
srv-vm1 ~ # parted /dev/vda
GNU Parted 3.2
Using /dev/vda
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) p                                                                
Warning: Not all of the space available to /dev/vda appears to be used, you can fix the GPT to use all of the space (an extra 367001600 blocks) or continue with the current
setting? 
Fix/Ignore? Fix                                                           
Model: Virtio Block Device (virtblk)
Disk /dev/vda: 215GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags: 

Number  Start   End     Size    File system     Name  Flags
 1      1049kB  2097kB  1049kB                        bios_grub
 2      2097kB  4096MB  4094MB  linux-swap(v1)
 3      4096MB  24.0GB  19.9GB  ext4

(parted) q
Information: You may need to update /etc/fstab.

srv-vm1 ~ # parted /dev/vda                                           
GNU Parted 3.2
Using /dev/vda
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) resizepart 3 -1                                                  
Warning: Partition /dev/vda3 is being used. Are you sure you want to continue?
parted: invalid token: -1                                                 
Yes/No? Yes                                                               
End?  [24.0GB]? -1                                                        
(parted) p                                                                
Model: Virtio Block Device (virtblk)
Disk /dev/vda: 215GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags: 

Number  Start   End     Size    File system     Name  Flags
 1      1049kB  2097kB  1049kB                        bios_grub
 2      2097kB  4096MB  4094MB  linux-swap(v1)
 3      4096MB  215GB   211GB   ext4

(parted) q                                                                
Information: You may need to update /etc/fstab.

srv-vm1 ~ # resize2fs /dev/vda3
resize2fs 1.42.13 (17-May-2015)
Filesystem at /dev/vda3 is mounted on /; on-line resizing required
old_desc_blocks = 2, new_desc_blocks = 13
The filesystem on /dev/vda3 is now 51428620 (4k) blocks long.

srv-vm1 ~ # dmesg|grep EXT4
[  449.330140] EXT4-fs (vda3): resizing filesystem from 4859392 to 51428620 blocks
[  449.936044] EXT4-fs (vda3): resized filesystem to 51428620

srv-vm1 ~ # touch /forcefsck
srv-vm1 ~ # reboot

We rebooted the virtual machine with force check for precaution, but it is not reqiured.

Bonus – physical disks setup

Probably you would need an additional first step of copying your old disk to the new disk – basically, there are two ways to do it:

  • blind copy everything with a hardware or the Linux “dd” command.
  • use gparted to copy the GPT table and the partitions to the new disk

Howto do QEMU full virtualization with bridged networking

This howto rather continues the previous one “Howto do QEMU full virtualization with MacVTap networking” with the exception it will be showed how to use a classic setup of the networking – the use of bridge device. Because this setup requires a specific configuration for every Linux distro if we do not just add the bridge manually it is separated in this howto. For the clear and full howto, we would repeat the two first steps just to enable this howto to be independent of the original one mentioned above.
So use full virtualization under Linux you can use QEMU and no other library or manager like virt-manager. QEMU is simple enough and with a couple of parameters to it, you can start KVM virtual machines with near-native performance. To use KVM you must enable it in the BIOS of your server (or desktop machine).

Here are the main steps:

STEP 1) Enable KVM in the BIOS

  • For Intel machine you must find option Intel Virtualization Technology (or Intel VT-x) probably in BIOS menu of Chipset, Advanced CPU Configuration or other.
  • For AMD machine the virtualization cannot be disabled so it is enabled by default, but you can check for additional virtualization features to enable like Virtualization Extensions, Vanderpool and other.
  • Enable also additional features – Intel VT-d or AMD IOMMU, if they are available.

Reboot your machine and check if the KVM is supported:

srv@local ~$ cat /proc/cpuinfo |grep -E "vmx|svm"
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid sse4_1 sse4_2 x2apic movbe popcnt aes xsave avx f16c rdrand lahf_lm abm 3dnowprefetch cpuid_fault epb invpcid_single pti intel_pt tpr_shadow vnmi flexpriority ept vpid fsgsbase tsc_adjust bmi1 avx2 smep bmi2 erms invpcid rdseed adx smap xsaveopt dtherm ida arat pln pts
...

STEP 2) Install QEMU

Under CentOS 7 you can just install couple of packets – that’s all you need:

yum install -y qemu qemu-common qemu-img qemu-kvm-common qemu-system-x86 qemu-user bridge-utils

Or under Ubuntu

apt-get install qemu-kvm bridge-utils

STEP 3) Prepare the network 1 – the bridge device

Under CentOS 7 add the following configuration file

/etc/sysconfig/network-scripts/ifcfg-br0

with the content of

DEVICE=br0
TYPE=Bridge
BOOTPROTO=none
ONBOOT=yes
IPADDR0=192.168.0.1
PREFIX0=24
#GATEWAY0=192.168.0.1
NETMASK=255.255.255.0
IPV6_FAILURE_FATAL=no
NM_CONTROLLED=no
ZONE=public

If you want to use a real IP set to your virtual machine, you should set a real IP here and uncomment the GATEWAY0 with the real gateway IP. If real IP is used then you should include the main Internet network interface to the bridge by adding at the end of the configuration file /etc/sysconfig/network-scripts/ifcfg-eth0 (if eth0 is your network interface):

...
BRIDGE=br0

And restart the network

srv@local ~$ systemctl restart network

Under Ubuntu add to the file

/etc/network/interfaces

the following:

# Bridge
auto br0
iface br0 inet static
  address 192.168.0.10
  netmask 255.255.255.0
#  gateway 192.168.0.1
  bridge_ports none
  bridge_stp off
  bridge_fd 0
  bridge_maxwait 0

If you want to use real IP set to your virtual machine, you should set a real IP here and uncomment the GATEWAY0 with the real gateway IP and replace the “none” in the option “bridge_ports” with the name of your main Internet network interface. For example:

  ...
  bridge_ports eth0
  ...

And restart the network

srv@local ~$ /etc/init.d/networking restart

Or we can add the bridge device manually:

srv@local ~$ brctl addbr br0
srv@local ~$ ip link set dev br0 up
srv@local ~$ ip addr add 192.168.0.1/24 dev br0

If we use real IP we have to add the main Internet network interface to the bridge, so when you set up the network in our virtual machine with a real IP it will work with no more additional configurations, but if we use a local IPs like our setup here and we want to have Internet in our virtual machine we must enable masquerade and linux routing. You can do it with:

srv@local ~$ echo 1 > /proc/sys/net/ipv4/ip_forward
#NAT with firewalld
srv@local ~$ firewall-cmd --add-masquerade --permanent
#NAT with iptables
srv@local ~$ iptables -t nat -A POSTROUTING -o br0 -j MASQUERADE

Use either firewalld or iptables setup, depends on your system configuration, just check if firewalld is running with:

srv@local ~$ firewall-cmd --list-all

If you receive an error, saying command not found or firewalld is not running, you should use the “NAT with iptables”
So the network is ready!

STEP 4) Prepare the network 2 – the tun/tap for the virtual machine

After we have added a bridge device tun/tap device, which will be used for the QEMU virtual machine must be added:

srv@local ~$ ip tuntap add tap0 mode tap
srv@local ~$ brctl addif br0 tap0

STEP 5) Create a QEMU hard drive

Create a 100G file

srv@local ~$ cd /mnt/storage1/disks/
srv@local ~$ qemu-img create -f qcow2 vm_harddisk.qcow2 100G
Formatting 'vm_harddisk.qcow2', fmt=qcow2 size=107374182400 encryption=off cluster_size=65536 lazy_refcounts=off

or you can enable encryption (but on every start of your virtual machine you must set the key through the qemu console to start the virtual machine):

srv@local ~$ qemu-img create -f qcow2 vm_harddisk_e.bin -o encryption 100G
Formatting 'vm_harddisk_e.bin', fmt=qcow2 size=107374182400 encryption=on cluster_size=65536 lazy_refcounts=off

STEP 6) Boot up the QEMU KVM virtual server

srv@local ~$ qemu-system-x86_64 -enable-kvm -cpu host -smp 4 -runas qemu -daemonize -vnc 127.0.0.1:1 \
-drive file=/mnt/storage1/disks/vm_harddisk.qcow2,index=0,cache=none,aio=threads,if=virtio \
-boot d -net nic,model=virtio,macaddr=00:00:00:00:00:01 -net tap,ifname=tap0 \
-balloon virtio -m 2048 -monitor telnet:127.0.0.1:5801,server,nowait

The command above will :

  • “-enable-kvm” – enable the KVM – full virtualization with near native performance
  • “-cpu host” – will expose all supported host CPU features (only supported in KVM mode)
  • “-smp 4” – sets 4 processors to the virtual machine
  • “-daemonize” – start the command in daemon mode
  • “-runas qemu” – run under user, you can run thwo whole virtual machine from a user created especially for it, no need to run it with root, even it is recommended to run it under unprivileged user
  • “-vnc 192.168.1.10:1” – start a VNC server on this IP:PORT = 192.168.1.10:5901, the IP must present on the server or you can use 0.0.0.0:1 for 0.0.0.0:5901, but in every situation limit the access by a firewall
  • “-drive file=/mnt/storage1/disks/vm_harddisk.qcow2,index=0,cache=none,aio=threads,if=virtio” – set the main hard drive of the system
  • “-boot d” – boot from the first hard drive
  • “-net nic,model=virtio,macaddr=00:00:00:00:00:01 -net tap,ifname=tap0” – set the network interface using the tap device created by STEP 3) and STEP 4)
  • “-balloon virtio” – use balloon driver to be able to hot add or hot remove RAM (newer version this option is depricated and it can be skipped)
  • “-m 2048” – set virtual RAM size to megs
  • “-monitor telnet:127.0.0.1:5801,server,nowait” – set the management console for the this virtual server, you can connect with: 
    srv@local ~$ telnet 127.0.0.1 5801
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
QEMU 2.0.0 monitor - type 'help' for more information
(qemu) 
<Press "CTRL+]">
telnet> Connection closed.

    When quitting the management console you must NOT exit the console with quite/exit or CTRL+d, becuause it will terminate the virtual server, you must disconnect from the console with “CTRL+]” and then quit the telnet shell. With the console you can hot add/remove CPU, RAM, network cards, pci devices, harddrives, start/stop/shutdown/reset the virtual machine and a lot more.

Boot the virtual machine from the hard drive given by “-drive” with network “-net” (couple of options), the RAM uses baloon memory and could be adjusted on-the-fly and sets the vncserver to listen for connection on port IP:port = 192.168.1.1:5901 (probably you’ll want to change this with a the real IP of your server, but be careful to set up a firewall rule for 5901 – the vnc port) and a management console listening on IP:port 127.0.0.1:5801.

* Boot the virtual server from a virtual CD/DVD

Probably the first time booting you might need to boot from an installation disk, this could be done by the following command:

srv@local ~$ qemu-system-x86_64 -enable-kvm -cpu host -smp 4 -runas qemu -daemonize -vnc 127.0.0.1:1 -cdrom /mnt/storage1/disks/isos/CentOS-7-x86_64-NetInstall-1708.iso -boot c -drive file=/mnt/storage1/disks/vm_harddisk.qcow2,index=0,cache=none,aio=threads,if=virtio -net nic,model=virtio,macaddr=00:00:00:00:00:01 -net tap,ifname=tap0 -balloon virtio -m 2048 -monitor telnet:127.0.0.1:5805,server,nowait

The changes:

  1. “-boot c” – First boot device is now CD/DVD. “c” is for CD, “d” is for disk
  2. “-cdrom /mnt/storage1/disks/isos/CentOS-7-x86_64-NetInstall-1708.iso” – added the installation disk to the virtual machine

A newer QEMU version may need adding “script=no,downscript=no” to the tap0 interface!

srv@local ~$ qemu-system-x86_64 -enable-kvm -cpu host -smp 4 -runas qemu -daemonize -vnc 127.0.0.1:1 -cdrom /mnt/storage1/disks/isos/CentOS-7-x86_64-NetInstall-1708.iso -boot c -drive file=/mnt/storage1/disks/vm_harddisk.qcow2,index=0,cache=none,aio=threads,if=virtio -net nic,model=virtio,macaddr=00:00:00:00:00:01 -net tap,ifname=tap0,script=no,downscript=no -m 2048 -monitor telnet:127.0.0.1:5805,server,nowait

Howto do QEMU full virtualization with MacVTap networking

To use full virtualization under linux you can use QEMU and no other library or manager like virt-manager. QEMU is simple enough and with couple of parameters to it you can start KVM virtual machines with near native performance. To use KVM you must enable it in the BIOS of your server (or desktop machine).

Here a several simple step to start a KVM virtual server:

STEP 1) Enable KVM in the BIOS

  • For Intel machine you must find option Intel Virtualization Technology (or Intel VT-x) probably in BIOS menu of Chipset, Advanced CPU Configuration or other.
  • For AMD machine the virtualization cannot be disabled so it is enabled by default, but you can check for additional virtualization features to enable like Virtualization Extensions, Vanderpool and other.
  • Enable also additional features – Intel VT-d or AMD IOMMU, if they are available.

Reboot your machine and check if the KVM is supported:

srv@local ~$ cat /proc/cpuinfo |grep -E "vmx|svm"
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid sse4_1 sse4_2 x2apic movbe popcnt aes xsave avx f16c rdrand lahf_lm abm 3dnowprefetch cpuid_fault epb invpcid_single pti intel_pt tpr_shadow vnmi flexpriority ept vpid fsgsbase tsc_adjust bmi1 avx2 smep bmi2 erms invpcid rdseed adx smap xsaveopt dtherm ida arat pln pts
...

STEP 2) Install QEMU

Under CentOS 7 you can just install couple of packets – that’s all you need:

yum install -y qemu qemu-common qemu-img qemu-kvm-common qemu-system-x86 qemu-user bridge-utils

Or under Ubuntu

apt-get install qemu-kvm bridge-utils

STEP 3) Prepare the network

srv@local ~$ ip link add link enp8s0f1 name macvtap0 type macvtap mode bridge
srv@local ~$ ip link set macvtap0 up

Here we create a macvtap0 device in bridge mode, these commands will create a tap device bridged to the network interface “enp8s0f1” (in our case, you must replace this device name with the device name you want to bridge your virtual machine network, probably the main interface of your server/desktop machine?). Only these two commands are needed, no other devices or network reload is needed.
The device will show in “ip addr”

7: macvtap0@enp8s0f1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UNKNOWN qlen 500
    link/ether 2e:51:7e:bb:44:ee brd ff:ff:ff:ff:ff:ff
    inet6 fe80::2c51:7eff:febb:44ee/64 scope link 
       valid_lft forever preferred_lft forever

This setup could expose the MAC address of the macvtap device to the router port connected

STEP 4) Create a QEMU hard drive

Create a 100G file

srv@local ~$ cd /mnt/storage1/disks/
srv@local ~$ qemu-img create -f qcow2 vm_harddisk.qcow2 100G
Formatting 'vm_harddisk.qcow2', fmt=qcow2 size=107374182400 encryption=off cluster_size=65536 lazy_refcounts=off

or you can enable encryption (but on every start of your virtual machine you must set the key through the qemu console to start the virtual machine):

srv@local ~$ qemu-img create -f qcow2 vm_harddisk_e.bin -o encryption 100G
Formatting 'vm_harddisk_e.bin', fmt=qcow2 size=107374182400 encryption=on cluster_size=65536 lazy_refcounts=off

STEP 5) Boot up the QEMU KVM virtual server

srv@local ~$ qemu-system-x86_64 -enable-kvm -cpu host -smp 4 -runas qemu -daemonize -vnc 127.0.0.1:1 \
-drive file=/mnt/storage1/disks/vm_harddisk.qcow2,index=0,cache=none,aio=threads,if=virtio \
-boot d -net nic,model=virtio,macaddr=$(cat /sys/class/net/macvtap0/address) \
-net tap,fd=3 3<>/dev/tap$(cat /sys/class/net/macvtap0/ifindex) \
-balloon virtio -m 2048 -monitor telnet:127.0.0.1:5801,server,nowait

The command above will :

  • “-enable-kvm” – enable the KVM – full virtualization with near native performance
  • “-cpu host” – will expose all supported host CPU features (only supported in KVM mode)
  • “-smp 4” – sets 4 processors to the virtual machine
  • “-daemonize” – start the command in daemon mode
  • “-runas qemu” – run under user, you can run thwo whole virtual machine from a user created especially for it, no need to run it with root, even it is recommended to run it under unprivileged user
  • “-vnc 192.168.1.10:1” – start a VNC server on this IP:PORT = 192.168.1.10:5901, the IP must present on the server or you can use 0.0.0.0:1 for 0.0.0.0:5901, but in every situation limit the access by a firewall
  • “-drive file=/mnt/storage1/disks/vm_harddisk.qcow2,index=0,cache=none,aio=threads,if=virtio” – set the main hard drive of the system
  • “-boot d” – boot from the first hard drive
  • “-net nic,model=virtio,macaddr=$(cat /sys/class/net/macvtap0/address) -net tap,fd=3 3<>/dev/tap$(cat /sys/class/net/macvtap0/ifindex)” – set the network interface using the tap device created by macvtap0 device (STEP 3)
  • “-balloon virtio” – use balloon driver to be able to hot add or hot remove RAM
  • “-m 2048” – set virtual RAM size to megs
  • “-monitor telnet:127.0.0.1:5801,server,nowait” – set the management console for the this virtual server, you can connect with: 
    srv@local ~$ telnet 127.0.0.1 5801
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
QEMU 2.0.0 monitor - type 'help' for more information
(qemu) 
<Press "CTRL+]">
telnet> Connection closed.

    When quitting the management console you must NOT exit the console with quite/exit or CTRL+d, becuause it will terminate the virtual server, you must disconnect from the console with “CTRL+]” and then quit the telnet shell. With the console you can hot add/remove CPU, RAM, network cards, pci devices, harddrives, start/stop/shutdown/reset the virtual machine and a lot more.

Boot the virtual machine from the hard drive given by “-drive” with network “-net” (couple of options), the RAM uses baloon memory and could be adjusted on-the-fly and sets the vncserver to listen for connection on port IP:port = 192.168.1.1:5901 (probably you’ll want to change this with a the real IP of your server, but be careful to set up a firewall rule for 5901 – the vnc port) and a management console listening on IP:port 127.0.0.1:5801.

* Boot the virtual server from a virtual CD/DVD

Probably the first time booting you might need to boot from an installation disk, this could be done by the following command:

srv@local ~$ qemu-system-x86_64 -enable-kvm -cpu host -smp 4 -runas qemu -daemonize -vnc 127.0.0.1:1 -cdrom /mnt/storage1/disks/isos/CentOS-7-x86_64-NetInstall-1708.iso -boot c -drive file=/mnt/storage1/disks/vm_harddisk.qcow2,index=0,cache=none,aio=threads,if=virtio -net nic,model=virtio,macaddr=$(cat /sys/class/net/macvtap0/address) -net tap,fd=3 3<>/dev/tap$(cat /sys/class/net/macvtap0/ifindex) -balloon virtio -m 2048 -monitor telnet:127.0.0.1:5801,server,nowait

The changes:

  1. “-boot c” – First boot device is now CD/DVD. “c” is for CD, “d” is for disk
  2. “-cdrom /mnt/storage1/disks/isos/CentOS-7-x86_64-NetInstall-1708.iso” – added the installation disk to the virtual machine