Online resizing block devices and file systems

Here is a short reference sheet for resizing block devices and file systems.

Resizing block devices

Logical volumes

Add 100 GiB to the logical volume with name logicalvolume in volume group volumegroup.

# lvextend -L +100G volumegroup/logicalvolume 

QEMU block device

If you resized a block device which is used to store a virtual disk for a QEMU VM, you will need to expand the virtual disk itself. First we need to know the name of the virtual disk. If you are managing your QEMU VMs via libvirt, you can use this command to see all virtual disks:

# virsh qemu-monitor-command VMname --hmp "info block"
drive-virtio-disk0 (#block108): /dev/vm/web-www (raw)
    Attached to:      /machine/peripheral/virtio-disk0/virtio-backend
    Cache mode:       writeback, direct

drive-virtio-disk1 (#block302): /dev/vm/web-logs (raw)
    Attached to:      /machine/peripheral/virtio-disk1/virtio-backend
    Cache mode:       writeback, direct

Then if you resized the logical volume /dev/vm/web-www from 100 to 200 GiB using the command mentioned before, you can resize the corresponding QEMU virtual disk drive-virtio-disk0 using this command:

# virsh qemu-monitor-command VMname --hmp "block_resize drive-virtio-disk0 200G"

Resizing file systems

When you have resized a block device, you will need to resize the file system on the block device too in order to use the disk space.


# resize2fs /dev/vda


# xfs_growfs /var/www/


# btrfs filesystem resize max /var/www

Importing a VMWare virtual machine in qemu/kvm/libvirtd

So you have a VMWare virtual machine and you want to migrate it to Qemu/KVM setup managed by libvirt? This is very easy, using libguestfs.

You will need libguestfs 1.37.10 or higher, which unfortunately is not available for Debian Stretch. The libguestfs-tools package in Debian Buster is fine though.

The command you need is this:

$ virt-v2v -i vmx /mnt/storage/vmware/vm/vm.vmx -o libvirt -of qcow2 -os storage-pool -n network

Replace storage-pool with the name of the libvirt storage pool where you want to store the new VM it, and network by the network name. In this example the disk images will be converted to qemu’s qcow2 format.

To get a list of all available storage pools, use this:

$ virsh pool-list

This command will show all available networks:

$ virsh net-list

Linux performance improvements

Two years ago I wrote an article presenting some Linux performance improvements. These performance improvements are still valid, but it is time to talk about some new improvements available. As I am using Debian now, I will focus on that distribution, but you should be able to easily implement these things on other distributions too. Some of these improvements are best suited for desktop systems, other for server systems and some are useful for both. Continue reading “Linux performance improvements”

Server migration to Debian

Since this afternoon, this server is now running Debian GNU/Linux Squeeze. Just like the previous system, this is a KVM virtual machine running on a HP Proliant DL185G5 host. The host server has always been running Debian. This was my last production system still running Mandriva. I might have forgotten to move over a few things or there might be some breakage somewhere, so let me know if you encounter a problem.

Migration to virtual machine

I just finished migrating this website to a new KVM virtual machine. The virtual machine is running Mandriva Cooker (2010.0) now with kernel 2.6.31-rc9 and the virtio drivers. The host machine is a HP DL185 G5 running Debian Lenny (kernel 2.6.26) and kvm 85.

During the next days, there might be some short downtime now and then while I continue configuring things

Server migration

Since two days, I have merged the main servers used by two research laboratories at work. One server was an old Linux server which really needed a hardware upgrade, and the other one was a Mac Pro machine running a flaky OS X Leopard. The new server is of course running Linux: Debian Lenny.

It was a very interesting experience: working out procedures to migrate the mailboxes (from Dovecot on the Linux server and Cyrus on the Mac server to Cyrus on the new server), finding out how to set up one NIC in two different subnets (especially the routing is a little bit tricky), getting all services hooked up to LDAP and managed by GOSA, getting dhcpd to do exactly what we want in a shared-network set up, and much more.

The new server is a HP DL185 G5 with an AMD Opteron quad core CPU and 8 GB of RAM and hosts two KVM virtual machines, one for public services and another one running internal services. You can visit the two websites, which are also hosted on this machine of course, of the concerned research labs:

Maybe in the not too far away future, I should try to move the services hosted on the underpowered desktop machine running this website, also to a virtual machine…

Using KVM on Mandriva 2009.0


My new Dell Latitude E6400 laptop, is my first system which support hardware visualization. It also has 4 GB RAM, making this an ideal machine to start experimenting with KVM, the kernel based virtual machine system which is now integrated in Linux itself.

In the past I had already used different virtualization systems. VMWare Server is annoying because you have to rebuild the kernel modules by hand each time you install a new kernel and it’s not Free (as in Free Speech). Virtualbox handles the modules rebuild nicely thanks to the use of DKMS in Mandriva, but always randomly crashes on an assert, making it totally unusable for serious work. Also it does not really include a server mode, where virtual machines just run in the background. Xen is especially nice if you have a system which does not support hardware virtualization and you just want to run a paravirtualized Linux in it. However, the host system (Dom0) needs to run a very specific kernel version (normally version 2.6.18, although Debian has a 2.6.26 based one now). OpenVZ is nice for lightweight Linux server containers (some kind of chroot on steroids), but is not a complete virtualization solution.

Because KVM is built into the standard Linux kernel, this means there won’t be any problems with kernel modules which have to be installed separately. It’s a complete virtualization system which means that you can run all kind of operating systems on it, like for example also Windows. KVM is evolving very quickly and its performance and features are constantly improving. Thanks to libvirt and virt-manager, it also has a nice GUI front-end which make configuring and managing virtual machines very easy (libvirt and virt-manager also include support for Xen and OpenVZ, but I don’t know how well that works).

Installing and using KVM on Mandriva 2009.0

Before installing KVM, it’s important to verify that the CPU on your system has hardware virtualization support. Start up a terminal, and run

$ cat /proc/cpuinfo

If you have an Intel CPU, you should have vmx in the flags. If you are using an AMD CPU, you should have svm.

It’s best to use KVM, libvirt and virt-manager from Mandriva’s backports repositories. The versions included there, are the most recent ones, and fix a few problems which made networking not work out of the box with the ones included in the standard repositories. So start up the Mandriva Control Center (Tools – System Tools – Configure Your Computer), and in the Software category, launch the media sources configuration tool and enable the Main Backports and Contrib Backports sources by checking the checkboxes.

Before we install the necessary packages, it’s necessary to make a change to udev’s configuration file to prevent the virbr* and vnet* virtual ethernet devices and bridges being configured automatically by Mandriva’s configuration tools. To do so, edit the file /etc/sysconfig/udev_net and add virbr* and vnet* to the BLACKLIST_ALL line if they are not there yet. The line should look like this:


Then install the kvm and virt-manager packages with the graphical software installation utility or with urpmi. Some extra dependencies will be installed, such as dnsmasq-base, libvirt-utils and python-virtinst.

After the installation, make sure that the libvirtd service is running. You can use the graphical tool in the Mandriva Control Center in the category System – Manager system services, or you can run “service libvirtd status” to verify whether it’s running and “service libvirtd start” to start it.

Now start the Virtual Machine Manager from Tools – Emulators. Choose File – Add Connection and set up a Local QEMU connection. Double click on the localhost/qemu connection in the virtual machine manager, and enter your root password. You can now create new virtual machines by clicking on the New button. To use KVM, choose a fully virtualized system, and choose KVM as hypervisor. If you want to run Mandriva 2009.0 or later in a KVM based virtual machine, you can probably choose Linux as OS type and Ubuntu Hardy as OS variant, because that should give better performance than the generic options.

I currently have Windows Vista and Debian Lenny running in KVM. It works very fast and very reliably. The only problem I encounter, is that sometimes I cannot switch back from my Windows virtual machine to my host system: the mouse and keyboard cannot leave the VNC window anymore and also the Ctrl-Alt keystroke does not release the mouse pointer. The only solution is to shut down the Windows system.