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.
yum remove kernel
That fixed my problem. Hopefully it fixes yours.
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 energy. It is also possible to suspend-to-disk (S4, S5), with STD current state of computer saved to disk. STD is called hibernate. In hibernate cold reboot will be performed so, it will take longer to restore the state.
This post is about the errors while getting physical machine metrics with ceilometer specifically Openstack Liberty. Simply errors can be:
– No metrics shown in:
specifically metrics listed below:
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 the key: I, wait few seconds
- While holding Alt and SysRq keys down, press the key: S, wait few seconds
- While holding Alt and SysRq keys down, press the key: U, wait few seconds
- While holding Alt and SysRq keys down, press the key: B, wait few seconds
Computer will reboot now!
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 create a file that takes all space available in backing store (here qcow2 file). This file will only contain zeros. That way qemu-img will skip copying zeros and it will only mark the meta-data of zero sequence (for example length of it).
To do this, turn on virtual machine and in virtual machine’s terminal (you can use ssh to connect):
dd if=/dev/zero of=/tempzerofile
dd will read from /dev/zero and write to /tempzerofile until all disk space is full. Now remove the file:
2. Reclaim unused disk space from qcow2 file
Now turn virtual machine off, and run below command in your host machine to reclaim unused space back. Remember this space is marked with zeros and these type of files are called “sparse file”.
qemu-img convert -O qcow2 image.qcow2 shrinked_image.qcow2
where image is the original image whose size is big and we want to shrink it. shrinked_image is the destination image we want to create.
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 variable holds singleton object's reference
s = None
print "this will print only once!!!"
self._name = None
#set _name with setter
self.name = "yuppii"
self._name = n
#static method to get instance
#if no object is created before, create new one
if Mysingleton.s == None:
Mysingleton.s = Mysingleton()
You should see an output like:
this will print only once!!!
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: agilemanifesto.org for details.