• Preetam Zare

Using Snapshots To Manage Virtual Machines

Snapshots are useful as a short term solution for testing software with unknown or potentially harmful effects.For example, you can use a snapshot as a restoration point during a linear or iterative process, such as installing update packages, or during a branching process, such as installing different versions of a program.

Using snapshots ensures that each installation begins from an identical baseline.With snapshots, you can preserve a baseline before diverging a virtual machine in the snapshot tree.

The Snapshot Manager in the vSphere Web Client and the vSphere Client provide several operations for creating and managing virtual machine snapshots and snapshot trees. These operations let you create snapshots, restore any snapshot in the snapshot hierarchy, delete snapshots, and more. You can create extensive snapshot trees that you can use to save the virtual machine state at any specific time and restore the virtual machine state later. Each branch in a snapshot tree can have up to 32 snapshots.

A snapshot preserves the following information:

· Virtual machine settings. The virtual machine directory, which includes disks that were added or changed after you took the snapshot.

· Power state. The virtual machine can be powered on, powered off, or suspended.

· Disk state. State of all the virtual machine’s virtual disks.

· (Optional) Memory state. The contents of the virtual machine’s memory.

The Snapshot Hierarchy

The Snapshot Manager presents the snapshot hierarchy as a tree with one or more branches. The relationship between snapshots is like that of a parent to a child. In the linear process, each snapshot has one parent snapshot and one child snapshot, except for the last snapshot, which has no child snapshots. Each parent snapshot can have more than one child. You can revert to the current parent snapshot or restore any parent or child snapshot in the snapshot tree and create more snapshots from that snapshot. Each time you restore a snapshot and take another snapshot, a branch, or child snapshot, is created.

Parent Snapshots The first virtual machine snapshot that you create is the base parent snapshot.The parent snapshot is the most recently saved version of the current state of the virtual machine. Taking a snapshot creates a delta disk file for each disk attached to the virtual machine and optionally, a memory file. The delta disk files and memory file are stored with the virtual machine’s files. The parent snapshot is always the snapshot that appears immediately above the You are here icon in the Snapshot Manager. If you revert or restore a snapshot, that snapshot becomes the parent of the You are here current state.

No snapshot is taken in above screen.

Screen capture while taking snapshot

After snapshot is taken

Child Snapshots A snapshot that is taken of the same virtual machine after the parent snapshot.Each child constitutes delta files for each attached virtual disk, and optionally a memory file that points from the present state of the virtual disk (You are here). Each child snapshot’s delta files merge with each previous child snapshot until reaching the parent disks. A child disk can later be a parent disk for future child disks.The relationship of parent and child snapshots can change if you have multiple branches in the snapshot tree.

IMPORTANT Do not manually manipulate individual child disks or any of the snapshot configuration files because doing so can compromise the snapshot tree and result in data loss. This restriction includes disk resizing and making modifications to the base parent disk using vmkfstools.

Each snapshot creates an additional delta .vmdk disk file. When you take a snapshot, the snapshot mechanism prevents the guest operating system from writing to the base .vmdk file and instead directs all writes to the delta disk file. The delta disk represents the difference between the current state of the virtual disk and the state that existed at the time that you took the previous snapshot. If more than one snapshot exists, delta disk scan represent the difference between each snapshot.

Warning Delta disk files can expand quickly and become as large as the entire virtual disk if the guest operating system writes to every block of the virtual disk.

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