VMware 4817V62 - vSphere - PC Administration Manual page 202

vSphere Basic System Administration
Because you cannot revert to a snapshot with dynamic disks, quiesced snapshots are not used when backing
up dynamic disks.
Backup solutions, like VMware Data Recovery, use the snapshot mechanism to freeze the state of a virtual
machine. However, the Data Recovery backup method has additional capabilities that mitigate the limitations
of snapshots.
Multiple snapshots refers to the ability to create more than one snapshot of the same virtual machine.
Multiple snapshots are not simply a way of saving your virtual machines. With multiple snapshots, you can
save many positions to accommodate many kinds of work processes.
When taking a snapshot, the state of the virtual disk at the time the snapshot is taken will be preserved. When
this occurs, the guest cannot write to the
is allowed to write. The delta disk represents the difference between the current state of the virtual disk and
the state that existed at the time the previous snapshot was taken. If more than one snapshot exists, delta disks
can represent the difference (or delta) between each snapshot. For example, a snapshot can be taken, and then
the guest could write to every single block of the virtual disk, causing the delta disk to grow as large as the
entire virtual disk.
When a snapshot is deleted, the changes between snapshots and previous disk states are merged, and all the
data from the delta disk that contains the information about the deleted snapshot is written to the parent disk
and merges with the base disk only when you choose to do so. This can involve a large amount of disk input
and output. This may reduce the virtual machine performance until consolidation is complete.
The amount of time it takes to commit or delete snapshots depends on how much data the guest operating
system has written to the virtual disks since the last snapshot was taken. The required time is directly
proportional to the amount of data (committed or deleted) and the amount of RAM allocated to the virtual
machine.
For additional information about snapshot behavior, see the Knowledge Base article at
http://kb.vmware.com/kb/1015180.
Relationship Between Snapshots
The relationship between snapshots is like that of a parent to a child. In the linear process, each snapshot has
one parent and one child, except for the last snapshot, which has no children.
The snapshots taken form a tree. Each time you revert and take another, a branch (child) is formed.
In the process tree, each snapshot has one parent, but one snapshot may have more than one child. Many
snapshots have no children.
You can revert to a parent or a child.
Snapshots and Other Activity in the Virtual Machine
When you take a snapshot, be aware of other activity going on in the virtual machine and the likely effect of
reverting to that snapshot.
In general, it is best to take a snapshot when no applications in the virtual machine are communicating with
other computers. The potential for problems is greatest if the virtual machine is communicating with another
computer, especially in a production environment.
For example, if you take a snapshot while the virtual machine is downloading a file from a server on the
network, the virtual machine continues downloading the file, communicating its progress to the server. If you
revert to the snapshot, communications between the virtual machine and the server are confused and the file
transfer fails.
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file. The delta disk is an additional
vmdk
file to which the guest
vmdk
VMware, Inc.