A snapshot can also be described as a »frozen« version of the virtual machine, and a consistent point-in-time copy of the VM can only be created under this condition. If the file system were not »frozen« during the copy process, the .vmdk (virtual VMware hard drive) and .vmx (configuration settings file of a VMware guest machine) files would most likely change during the copy process. The resulting copy would thus not be consistent at any point in time.
So a snapshot on its own does not make a backup, but it is necessary to create reliable VM backups. In addition to the manufacturer's native solutions, there are many different backup products for VMware infrastructures on the market. Even though some of these products differ greatly in design, virtually all solutions rely on snapshot technology.
The agony of choice.
If backup software has access to the virtual machine's files provided via a snapshot, then the creation of the actual backups varies from manufacturer to manufacturer. Various storage locations, redundant backup options, different methods of compression and data duplication, incremental approaches and countless restoration options are only a few of the conceivable selection criteria.
The restore scenarios required are a no less important factor in selecting the correct backup solution. For instance, some solutions are better suited if the machine is to be restored at its original location, while others can start up a virtual server directly from a backup.
Removing snapshots is another critical part of the backup process. If the backup process was successful, the snapshot will no longer be needed, and may even have a detrimental effect on the VM's performance and take up unnecessary storage space if left unmanaged. In normal cases, the backup solution will send a delete command to the ESX or vSphere environment to have the snapshot removed.
If communication problems occur in the process, it can lead to abandoned snapshots and the aforementioned disadvantages. A good backup solution will check for this scenario and remove the remnant during the next run.
Depending on the backup concept and the solution used, the software also archives backups, shifting the older backup images to second-tier storage and de-duplicates the data to be archived in the process.
Backup, snapshot and then there is the ...
... replication of virtual machines. Replication is another alternative to backing up virtual machines (specifically for virtual servers) which provide first tier applications and must thus have high availability. Replication offers by far the best RTO (recovery time objective), since VM replicas do not have to be compressed and are kept in their original format in near proximity to the original machine. In emergencies, replicas can immediately be put into operation without extra configuration work.
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