Zugschlusbeobachtungen

In the last few days, I found the time to spend some with KVM and libvirt. Unfortunately, there is a subject that I haven’t yet found a satisfying solution: Naming of block devices in guest instances.

This is surely a common issue, but solutions are rare. Neither an article on Usenet (in German) nor the German version of this blog article has found solutions for the main question. I should have written this in English in the first place and am thus translating from German to english, hoping that there will be some answers and suggestions.

KVM is quite inflexible when it coms to configure block devices. It is possible to define on the host, which files or whole devices from the host should be visible in the guest. The documentation suggests that devices should be brought into the guest with the virtio model, which needs suppport in the guest kernel. Importing a device as emulated ATA or SCSI device brings a performance penalty.

The devices brought into the guest via virtio appear in the guest’s dev as /dev/vd<x> and do also have their corresponding entries in /dev/disk/by-uuid and /dev/disk/by-path. The vd<x> node is simply numbered in consecutive order as hd<x> and sd<x>. /dev/disk/by-uuid is the correct UUID of the file system found on the device, at least if it’s a block device partitioned inside the guest and formatted with ext3 (I didn’t try anything else yet). The terminology of the /dev/disk/by-path node is not yet understood, and I am somewhat reluctant to assume the PCI paths of emulated hardware as stable.