VMware 4817V62 Administration Guide - 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 vmdk file. The delta disk is an additional vmdk file to which the guest 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. 202 VMware, Inc.