Memory Overcommit Uncovered
Posted by James B. in Technology on January 29, 2010
There was a good discussion on twitter on memory overcommit and the value of memory overcommit and whether you should or should not use it in production. What struck me in this was that on a subject like this, there is so much misunderstanding although there is a lot of documentation available that can explain the subtle difference between good and bad overcommit of memory.
Memory overcommit, the basics.
In short: When you assign more RAM to your VMs than available in your host.
Good memory overcommit: When you assign more RAM to your VMs than available in your host BUT never cross the line where the amount of RAM that is USED by your VMs is more than available in your host.
Bad memory overcommit: When you assign more RAM to your VMs than available in your host AND cross the line where the amount of RAM that is USED by your VMs is more than available in your host.
A simple example:
Host has 48GB of RAM and just for the sake of argument we’ll pretend the hypervisor doesn’t use any RAM and we don’t have memory overhead per VM. I now start loading it with VMs that have 4GB RAM assigned. Without any memory overcommit I can load this host with 12 VMs of each 4GB.
Now let’s say, these VMs normally use only 2.5GB of RAM but sometimes they peak to 4GB. With memory overcommit I could now load the host with 19 VMs of 4GB RAM assigning a total of 76GB RAM and demanding 19 x 2.5GB = 47.5GB of physical memory. Even to me this is a bit on the edge, so I’d reserve some RAM for spikes and would go back to 17 VMs, which would leave me with 17 x 2.5GB = 42.5GB of actively used physical RAM, 17 x 4GB = 68GB of RAM assigned and therefore 68GB-48GB = 20GB of overcommitted RAM. So, 20GB of RAM I didn’t have to pay for. This is a good use of memory overcommit.
Bad use of memory overcommit is when in the previous example I would place more VMs on this host, to the point where the use of physical RAM is higher than the amount of physical RAM present in the host. ESX will start some memory optimization and reclaim techniques, but in the end it will swap host memory to disk, which is bad. It is essential to carefully monitor your hosts to see if you’re moving from good memory overcommit to bad memory overcommit. Read the rest of this entry »
Cloning Hyper-V Virtual Machines – Part 3
Posted by James B. in Technology on January 25, 2010
Although Microsoft’s System Center Virtual Machine Manager automates the process of cloning a virtual machine, you can perform the cloning process manually without the aid of additional software. This article starts with the explanation of the manual cloning process.
In my previous article in this series, I showed you how you could use SYSPREP to prepare a virtual machine for cloning. Once we prepared the system image, we used System Center Virtual Machine Manager to perform the actual cloning process. Although this technique works really well, I realize that not everyone owns a copy of System Center Virtual Machine Manager. That being the case, I want to show you a cloning method that you can perform using only native Windows tools.
Cloning Hyper-V Virtual Machines – Part 2
Posted by James B. in Technology on January 23, 2010
In my previous article in this series, I explained that cloning a virtual machine is not quite as intuitive as you might have been lead to believe. Part of the reason for this is that in order for the cloning process to be successful, you need to use Sysprep to strip certain attributes from the host that is being cloned. In this article, I want to continue the discussion by showing you one method that you can use for cloning virtual machines.
Cloning Hyper-V Virtual Machines – Part 1
Posted by James B. in Technology on January 21, 2010
Although it is relatively easy to make a clone of a virtual machine, the cloning process is less than intuitive. This article series shows you how to avoid disaster by creating a virtual machine clone in the proper way.
You have probably heard that server virtualization products such as Hyper-V make it easy to clone virtual server images or to move virtual servers to new hardware. However, if you ever actually tried to build a server image and then clone it, you might have been surprised by how non intuitive the process actually was.
The first time that I ever had to create a virtual machine clone, I was a little taken back. All I kept hearing about was how easy the process was. Because of that, I really expected the Hyper-V Manager to have a Clone Virtual Machine button. As you have probably already guessed though, such a button simply does not exist.
Since that time, I have tried a lot of different methods to cloning virtual machines. Some of them have worked, and some of them have not. Since there are so many techniques that you could potentially use, I wanted to take the opportunity and explain which cloning techniques actually work, and why.
Switch Android Screens from Home Screen
Posted by James B. in Technology on January 19, 2010
Click on thumbnail for full image...
I am just now starting to get into Android and in v2.1 there are some “easter eggs” that you can run across. One of the recent finds is that you can switch to your different running screens from the home screen without swiping.
If you take a look at the screenshot you will see that you can view your current screens in a thumbnail view and then select the one you would like to go to without swiping.
Here is how you do it:
- Tap and hold the dots on the bottom left and/or right.
- Tap on the thumbnail of the screen you would like to switch to.
It is really that easy…Now, I understand that some people would rather use the cool transition of swiping but this is for those of us who might have a few more screens than normal. The thumbnail backgrounds do not show the wallpaper but instead show an all white background for better visibility. It does show the icon and widget layouts for easier identification. Virtual desktop users will feel at home as the motion is somewhat familiar.
If you don’t want to spend the time to hunt down all of the Easter Eggs then please feel free to check out this blog post over at Smarterware.
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