Virtualization software company VMware develops and markets virtual machine or VM management software that lets you create and maintain backup copies of your VM. One method VMware uses lets you create a picture or snapshot of a working VM and track the changes to it using a special VMDK or Virtual Machine Disk redo log.
Virtual Machine Snapshots
Just like a digital camera captures a moment in time as an image, you can take a snapshot of a functioning virtual machine that captures its RAM contents, the data stored on its virtual hard disk, the applications installed and its configuration state. If for some reason your virtual machine develops serious problems, you can use the snapshot to revert the VM to the state it was in when the snapshot was taken.
After you make a snapshot, the changes you make to the working virtual machine are not stored in the VMDK file, but in a separate, redo log file. Redo log files have the file extension ".vmdk.REDO" attached to the VM name. The size of the working virtual machine stays the same, while the redo log file size grows to accommodate future changes. Redo logs are also known as child disks or disk links.
Restoring Original State
You can configure VMware to delete changes written to a redo log once a VM session is ended. This allows you to safely and thoroughly vet a new application, operating system update or system configuration change before you commit it to your production VM. You can also restore the VM state by deleting the redo log on the physical machine where your VM is running.
Merging Redo Logs
If you are satisfied with the current state of your VM, you can use tools like the VMware-Snapshotmanager to automatically merge the redo log with the snapshot, delete the current redo log and then create a new redo log file based on the newly merged VM. This process is also called consolidating or committing a snapshot. Another way to merge the redo log is to take a snapshot of an existing VM snapshot. This results in the creation of a new redo log based on the consolidation of the previous snapshot and redo log.
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