Nested Virtualization

Nested virtualization is very important feature if you’re using virtual machines for your daily tasks. If you’ve heard but not know much about it this blog post is for you. You’ve probably watched the movie “Inception”. In this movie you dream in an other dream. This is a nested dream. Virtualization can also be nested like this. You have virtual machine but within the virtual machine you may have another virtual machine. This is not an easy thing (just like dream) since your CPU architecture…

Solution to: Getting Kernel Panic After Update of Virtual Machine

If you recently updated your Centos VM you may started getting the error below: Kernel panic — not syncing: VFS: Unable to mount root fs on unknown-block(0,0) unknown-block may change depending on your configuration, but symptoms stay same: Kernel panic after the update, and this error message. Solution is: yum remove kernel yum update That fixed my problem. Hopefully it fixes yours.    

System Sleep States

When you put your computer to suspend, it will turn some of its devices to power saving mode to preserve energy consumption. But computers have many sleep states from deeper to shallow just like humans. The most shallow one is the freeze (or S0-idle) which only puts CPU to idle and if possible turn other devices to less power consuming mode. The most deeper one is the “Sleep-to-Memory” (or S3) in which CPU is turned off, and nearly only memory and some critical devices uses…

Physical Host Metrics in Ceilometer – An Openstack Liberty Tale

This post is about the errors while getting physical machine metrics with ceilometer specifically  Openstack Liberty. Simply errors can be: – No metrics shown in: ceilometer meter-list specifically metrics listed below: compute.node.cpu.frequency compute.node.cpu.idle.percent compute.node.cpu.idle.time compute.node.cpu.iowait.percent compute.node.cpu.iowait.time

3D Rotations about Arbitrary Axis – Euler–Rodrigues and Matrix Methods

Code is written in python, uses pyglet to draw lines and vertices. All vertex calculations is done without using 3D methods of any library. Euler-Roudrigues rotation and rotation about x, y, and z axis are given separately. Rotation method can be selected from code. Left, right, up, and down arrow keys moves the cube. x, y, and z keys rotates cube with the specified axes. This code is for demonstration purposes only, so it is neither optimal nor complete. Use it as is.

SysReq, Raising Elephants Is So Utterly Boring

SysRq keyboard key is very magical. You can perform a safe reboot a locked up computer with this key. To achieve this remember mnemonics: “Raising Elephants Is So Utterly Boring” To use SysRq: Press Alt and SysRq (Print Screen) keys at the same time (but do NOT release them) While holding Alt and SysRq keys down, press the key: R, wait few seconds While holding Alt and SysRq keys down, press the key: E, wait few seconds While holding Alt and SysRq keys down, press…

Reclaim empty space from, Shrink disk of qcow2 disk file

To reclaim disk space (in other words to shrink disk space) of qcow2 disk file you need to run following steps. 1. Fill guest disk with empty file This is required since disk does not really hold its configured size, instead it has a sparse file format in creation time. After a while disk gets writes, temporary file creations/deletions that result non zero blocks in qcow2 file which are useless in reality (i.e. temporarily created file and deleted later). So to gain this space we’ll…

Singleton Design Pattern

Singleton design pattern is well known design pattern which uses a single object to access its methods at all time. Assume that you have a configuration file to read settings from. You are expecting to access various settings from different source files. With singleton you’ll hold a single object in memory and use this object to access its methods. Sample python code demonstrating singleton is given below: class Mysingleton(): #class variable holds singleton object’s reference s = None #constructor def __init__(self): print “this will print…

Agile Manifesto

Scrum has been used by Google since 2005. Scrum is widely used agile framework. In short Agile can be described with the following manifesto: Individuals and interactions,over,processes and tools Working software,over,comprehensive documentation Customer collaboration,over,contract negotiation Responding to change,over,following a plan Hats off for this manifesto. look at: for details.