6.0.1

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 base .vmdk file.
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.
NOTE The parent snapshot is not always the snapshot that you took most
recently.
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. A parent snapshot can have more than one child. Many snapshots have no children.
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.
Snapshot Behavior
Taking a snapshot preserves the disk state at a specific time by creating a series of delta disks for each
attached virtual disk or virtual RDM and optionally preserves the memory and power state by creating a
memory file. Taking a snapshot creates a snapshot object in the Snapshot Manager that represents the
virtual machine state and settings.
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 disks can represent the difference between each snapshot. 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.
Chapter 10 Managing Virtual Machines
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