Virtual Machines on a CentOS Host

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A powerful rack server can be used as a host for installing lots of virtual machines, and it can be used as a data storage as well. This article will describe how to use such a server, installed with CentOS, as a host for virtual machines.

The server is a SuperMicro X9DRL-3F/IF with these parameters:

HDD 2x320GB + 2x3TB
Network 2 Gb interfaces + 1 KVM/IMPI interface

The first two disks (2x320GB) are used in RAID1 configuration to keep the host and the virtual servers, and the 2x3TB diska are used for the data.

1 Installation of CentOS

During boot-up, we press Ctrl+I and configure a RAID1 device with the first two disks (320Gb each).

Installation was done with CentOS-6.2-x86_64-minimal.iso (standard installation, where the installer automatically partitions the first disk drive (the raid one). The standard installation of CentOS is very easy (with a GUI interface), it use RAID automatically, partitions the disk automatically, and uses LVM for the partitions.

2 Disk partitioning and formating

We have two disks of 3TB each that we need to partition and manage, however the partition tables of type msdos (most commonly used) cannot manage more than 2TB of disk space. The solution is to use partion tables of type GPT. Fortunately, the partition editor (parted) of Linux, supports them quite well.

The GPT partitions can also have more than 4 primary partitions, so there is no need for extended partitions and tricks like this. I split both of the disks in 6 primary partitions of 500GB each, formating them with ext4. Later, I am going to manage these partitions with LVM.

The steps below show roughly how I did the partitioning:

yum install parted
parted /dev/sdc
parted /dev/sdd
(parted) print
(parted) mklabel gpt
(parted) mkpart primary 0.0GB 500.0GB
(parted) mkpart primary 0.0GB 500.0GB                                             
(parted) mkpart primary 500.0GB 1.0TB                               
(parted) mkpart primary 1.0TB 1.5TB                               
(parted) mkpart primary 1.5TB 2.0TB
(parted) mkpart primary 2.0TB 2.5TB                                 
(parted) mkpart primary 2.5TB 3.0TB                                 
(parted) print                                                            
Model: ATA ST3000DM001-9YN1 (scsi)
Disk /dev/sdd: 3001GB
Sector size (logical/physical): 512B/4096B
Partition Table: gpt

Number  Start   End     Size   File system  Name     Flags
 1      1049kB  500GB   500GB               primary
 2      500GB   1000GB  500GB               primary
 3      1000GB  1500GB  500GB               primary
 4      1500GB  2000GB  500GB               primary
 5      2000GB  2500GB  500GB               primary
 6      2500GB  3001GB  501GB               primary


And this is how I formated them with ext4:

mkfs.ext4 /dev/sdc1
mkfs.ext4 /dev/sdc2
mkfs.ext4 /dev/sdc3
mkfs.ext4 /dev/sdc4
mkfs.ext4 /dev/sdc5
mkfs.ext4 /dev/sdc6
for i in 1 2 3 4 5 6; do  mkfs.ext4 /dev/sdd$i ; done


3 Managing partitions with LVM

One of the best advantages of Logical Volume Management (LVM) is the flexibility. LVM disks and partitions can be resized easily, when needed. Actually, in the terminology of LVM, logical disks are called Volume Groups (VG), and logical partitions are called Logical Volumes (LV). We can create several VGs, and inside each of them we can create LVs. The sizes of VGs and LVs are flexible, we can extend them later, if needed.

Let's create a volume group named vg_data by including some Physical Volumes (PV, physical disk partitions) to it:

vgcreate vg_data /dev/sdc1 /dev/sdc2 /dev/sdd1 /dev/sdd2

Then we can extend it by adding some more PVs (partitions) to it:

vgextend vg_data /dev/sdc3 /dev/sdc4 /dev/sdd3 /dev/sdd4

Now, inside the VG named vg_data, let's create an LV (logical partition) named /dev/vg_data/lv_mirror, of size 1TB:

lvcreate vg_data -L 1T -n /dev/vg_data/lv_mirror

We can create an ext4 filesystem on it like this:

mkfs.ext4 -L mirror /dev/vg_data/lv_mirror

Another LV can be created like this:

lvcreate vg_data -L 500G -n /dev/vg_data/lv_cache

4 Creating bridged interfaces on CentOS

We want the virtual machines to be connected directly to the network, and for this reason we should create bridged interfaces on the host system. We create a bridged interface for each of the network interfaces of the server. The steps below show how it can be done on CentOS.

  • Edit /etc/sysconfig/network-scripts/ifcfg-eth0:
  • Edit /etc/sysconfig/network-scripts/ifcfg-eth1:
  • Edit /etc/sysconfig/network-scripts/ifcfg-br0:
  • Edit /etc/sysconfig/network-scripts/ifcfg-br1:
  • Restart the network: service network restart


5 Installing KVM and libvirt

  • First check if the CPU supports hardware virtualization:
    egrep '(vmx|svm)' --color=always /proc/cpuinfo
  • Install kvm and libvirt:
    rpm --import /etc/pki/rpm-gpg/RPM-GPG-KEY*
    yum install kvm libvirt python-virtinst qemu-kvm
  • Modify /etc/libvirt/libvirtd.conf and uncomment mdns_adv = 0. Then restart libvirtd and check it with virsh:
    service libvirtd restart
    virsh -c qemu:///system list
  • Add a user that can manage the virtual machines:
    useradd virtadmin
    passwd virtadmin
    usermod -a -G kvm virtadmin

    We would like to be able to manage the virtual machines remotely (for example with virt-manager), and it is not a good idea to use the root account for doing it. So we create another account, virtadmin, that has permissions to manage the virtual machines. These permissions are assigned to it simply by adding it to the group kvm.

  • Set SELINUX=disabled on /etc/selinux/config and then reboot:
    # This file controls the state of SELinux on the system.
    # SELINUX= can take one of these three values:
    #     enforcing - SELinux security policy is enforced.
    #     permissive - SELinux prints warnings instead of enforcing.
    #     disabled - No SELinux policy is loaded.
    # SELINUXTYPE= can take one of these two values:
    #     targeted - Targeted processes are protected,
    #     mls - Multi Level Security protection.
  • For easy backup, we keep all the configurations and images on a separate directory, called /systems (which can also be on a separate partition). Move all configurations and images to /systems:
    mkdir /systems
    mv /etc/libvirt /systems/etc
    ln -s /systems/etc /etc/libvirt
    mv /var/lib/libvirt/ /systems/var
    ln -s /systems/var/ /var/lib/libvirt
    mkdir /systems/images/

    Modify /systems/etc/storage/default.xml like this:

    <!- - <path>/var/lib/libvirt/images</path> -->


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