CHECKPOINTS AND SNAPSHOTS in Software

Maker QR-Code in Software CHECKPOINTS AND SNAPSHOTS

Depending on the console that you are using to manage your virtual machines, a snapshot can be called by a different name If you are using the Hyper-V console, a snapshot is a called a snapshot If you are using SCVMM 2008 console, then the snapshot is called a checkpoint No matter what you call it, at the end of the day a snapshot or checkpoint is a point-in-time image of a virtual machine You can take a single point-in-time image, or you can take multiple point-in-time images of a virtual machine Once you ve taken a snapshot or checkpoint you can revert to that snapshot Alternately, you can continue to take snapshots This functionality comes in extremely handy when you are troubleshooting, patching, installing software, or just trying stuff out to see what happens When you take a snapshot of a virtual machine, Hyper-V goes through several steps to actually create the virtual machine The following steps happen whether the snapshot is taken from the Hyper-V Manager or from SCVMM 2008: 1 The virtual machine is put into a paused state 2 A differencing disk is created for each virtual hard disk and is linked to the virtual machine 3 A copy of the virtual machine s configuration file is created 4 The virtual machine is resumed 5 When the virtual machine has resumed, the contents of the virtual machine s memory are saved to disk While the contents of the virtual machine s memory are being saved to disk, the virtual machine is being monitored for memory activity If the guest operating system

8:

tries to modify something in memory that hasn t been copied yet, those write attempts are queued until the original contents are copied to the disk When this process is going on, the virtual machine is still available to your end users they may experience some slowed performance for a very small period of time One of the nice features of being able to take a point-in-time view is that you can take multiple snapshots as you go

Here s an example of how this works: First you create a virtual machine and you install your base Server 2008 image with IIS and ASPNET Now you want to install an application, but you re not sure how the installation is going to go and you don t want to have to start over if something doesn t work out To mitigate this issue, you take a snapshot before you start the installation This snapshot is created as an AVHD file An AVHD file is essentially a differencing disk and is linked to the parent VHD When changes happen on the virtual machine, the changes occur on the AVHD file and not the parent So you ve taken the snapshot, you ve installed the application, and everything worked out Now the application owner wants to make some changes, but isn t sure about the changes they want to make You can take another snapshot of the system that has Server 2008, IIS with ASPNET, and the application installed, so that now the application owner can make their changes If something doesn t work out, you can revert to either the snapshot where the application is installed, or the base server installation with IIS and ASPNET Once you ve created a snapshot of a system, you can take another snapshot; delete a snapshot; revert your system to a previous snapshot; or you can accept your snapshot Keep in mind that every time you create a snapshot, it uses physical disk space Also, if you ve created a highly available virtual machine, you will want to make sure that your snapshots are on a shared volume This makes it possible to move the virtual machine between nodes, and the snapshots will move along as well You don t want to move your virtual machine and have your snapshots back on the other node