Create a Linux VM with KVM in 6 easy steps

Posted on September 10, 2022November 5, 2022 by gary

A Step-by-step guide to creating a Linux virtual machine on a Linux host with KVM,qemu,libvirt and ubuntu cloud images.

Duplicate IP issues with Linux and virtual machine cloning.

Posted on July 10, 2019January 3, 2023 by gary

TL;DR – Some modern Linux distributions use a newer method of identification which, when combined with DHCP can result in duplicate IP addresses when cloning VMs, even when the VMs have unique MAC addresses.

To resolve, do the following ( remove file, run the systemd-machine-id-setup command, reboot):

# rm /etc/machine-id

# systemd-machine-id-setup

# reboot

When hypervisor management tools make clones of virtual machines, the tools usually make sure to create a unique MAC address for every clone. Combined with DHCP, this is normally enough to boot the clones and have them receive a unique IP. Recently, when I cloned several Bitnami guest VMs which are based on Debian, I started to get duplicate IP addresses on the clones. The issue can be resolved manually by following the above procedure.

To create a VM template to clone from which will generate a new machine-id for very clone, simply create an empty /etc/machine-id file (do not rm the file, otherwise the machine-id will not be generated)

# echo "" |  tee /etc/machine-id

The machine-id man page is a well written explanation of the implementation and motivation.

Impact of Paravirtual SCSI driver VS LSI Emulation with Data.

Posted on March 30, 2015September 7, 2022 by gary

TL;DR Comparison of Paravirtual SCSI Vs Emulated SCSI in with measurements. PVSCSI gives measurably better response times at high load.

During a performance debugging session, I noticed that the response time on two of the SCSI devices was much higher than the others (Linux host under vmware ESX). The difference was unexpected since all the devices were part of the same stripe doing a uniform synthetic workload.

iostat output from the system under investigation.

The immediate observation is that queue length is higher, as is wait time. All these devices reside on the same back-end storage so I am looking for something else. When I traced back the devices it turned out that the “slow devices” were attached to LSI emulated controllers in ESX. Whereas the “fast devices” are attached to para-virtual controllers.

I was surprised to see how much difference using para virtual (PV) SCSI drivers made to the guest response time once IOPS started to ramp up. In these plots the y-axis is iostat “await” time. The x-axis is time (each point is a 3 second average).

PVSCSI = Gey Dots
LSI Emulated SCSI = Red Dots
Lower is better.

Each plot is from a workload which uses a different offered IO rate. The offered rates are 8000,9000 and 10,000 the storage is able to meet the rates even though latency increases because there is a lot of outstanding IO. The workload is mixed read/write with bursts.