Fun with ones and zeros - kvm

Still playing with KVM (Kernel-based Virtual Machine), this time checking out some networking features. I've been running Ubuntu 8.04 LTS Server (Hardy Heron), both as the host and as a VM on that host. Networking is setup to use a bridge.

KVM offers different emulated NICs, I took a quick look at running iperf between the VM and the host, and got these speeds for a few select NIC models:

• RTL-8139C+ (the default): ~210 Mb/sec
• Intel e1000 (somewhat recommended here): ~ 330Mb/sec
• virtio: ~ 700Mb/sec

The thing about virtio though is that it doesn't work when the VMs RAM is set to 4GB. So I guess you can have fast networking, or lots of memory, but not both.

posted at: 09:16 | tags: kvm linux ubuntu | 0 comments | permanent link to this entry

I'm still experimenting with Ubuntu 8.04 Server (Hardy Heron), and have switched from Xen to KVM (Kernel-based Virtual Machine). Xen worked well on a little test machine I had, but when I tried it on a brand-new Supermicro server, it turned out to have a problem with the Intel NIC. Since it seems Ubuntu is recommending KVM over Xen, and the server supports hardware virtualization, I figured I'd give it a try.

One big difference is that KVM does full emulation, which means any disk space you give it from LVM (Logical Volume Manager), will be a full virtual disk, with a partition table. It's a little more complicated to access filesystems within the virtual disk that it was with Xen, I wanted to jot some notes down here mostly for myself on how to do that.

If I've created a logical volume named /dev/myvg/test_vm and installed another linux on it with a single ext3 filesystem (/dev/sda1 from the point of view of the VM) and some swap space (/dev/sda5), it can be accessed when the VM isn't running with the help of the kpartx utility...

kpartx -av /dev/myvg/test_vm

would read the partition table on the virtual disk and create:

/dev/mapper/myvg-test_vm1 
/dev/mapper/myvg-test_vm2 
/dev/mapper/myvg-test_vm5

Then you can

mount /dev/mapper/myvg-test_vm1 /mnt

to mess with the VMs /dev/sda1. To clean things up when finished, run:

umount /mnt
kpartx -d /dev/myvg/test_vm

Snapshots

If you want to look at the contents of a running VM's disks (perhaps for backing it up) you can use LVM snapshots. For example:

lvcreate --snapshot --size 1G --name test_snap /dev/myvg/test_vm
kpartx -av /dev/myvg/test_snap
mount /dev/mapper/myvg-test_snap1 /mnt
   .
   (play with VM's /dev/sda1 in /mnt)
   .
umount /mnt
kpartx -dv /dev/myvg/test_snap
lvremove /dev/myvg/test_snap

posted at: 11:25 | tags: kvm linux ubuntu xen | 1 comment | permanent link to this entry