Understanding Snapshots

A good rule of thumb is to allow for disk space of at least 25% of the virtual machine's (VM's) total disk size. But this amount can vary depending upon the type of server, how long you keep the snapshots, and if you plan on using multiple snapshots. If you plan on including the memory state with your snapshots, you'll also need to allow for extra disk space equal to amount of RAM assigned to the VM.

A VM with only one snapshot requires no extra disk space when deleting, or committing, it. (The term committing is used because the changes saved in the snapshot's delta files are now committed to the original virtual machine disk file, or VMDK.) But if you have multiple snapshots, you will need extra disk space available when deleting all snapshots. This is because of the way they are merged back into the original disk file.

An alternate method of deleting multiple snapshots that requires less additional disk space is to delete the snapshots one by one, starting with the VMs farthest down the snapshot tree. This way, the snapshots grow individually when they are merged into the previous snapshot, and subsequently deleted. If a little more tedious, this method requires far less extra disk space.

Important: Don't run a Windows disk defragmentation while the VM has a snapshot running. Defragment operations change many disk blocks and can cause very rapid growth of snapshot files.

If you've created a snapshot of a VM, and run the VM, the snapshot is active. If a snapshot is active, the performance of the VM will be degraded because ESX writes to delta files differently and less efficiently than it does to standard VMDK files. Because there is a lock on the metadata, nothing else can be written to the delta file when a write is made to the disk. Also, as the delta file grows by each 16 MB increment, it will cause another metadata lock. This can affect your VMs and ESX hosts. How big an impact on performance this will have varies based on how busy your VM and ESX hosts are.

Never expand a disk file with a snapshot running, If you do expand a virtual disk using vmkfstools while a snapshot is active, the VM will no longer start and you will receive an error: "Cannot open the disk ".vmdk" or one of the snapshot disks it depends on. Reason: The parent virtual disk has been modified since the child was created."

delta.vmdk file - This is the differential file created when you take a snapshot of a VM (also known as REDO log). When you snapshot a VM it stops writing to the base vmdk and starts writing changes to the snapshot delta file. The snapshot delta will initially be small and then start growing as changes are made to the base vmdk file, The delta file is a bitmap of the changes to the base vmdk thus is can never grow larger than the base vmdk. A delta file will be created for each snapshot that you create for a VM. These files are automatically deleted when the snapshot is deleted or reverted in snapshot manager

A snapshot can never grow larger the the original disk.