In the early days of virtual system backup, backup agents were installed on each virtual machine, same as physical systems. This worked fine except for one major problem. Physical machines typically use 20-30% of the available CPU power while virtual system hosts run closer to 90% utilization. This leaves precious CPU power to run backup agents. When multiple VMs backups are running at the same time, all VMs running on the virtual host will be impacted and cause severe performance issues and possibly application failure.
Best practice for virtual system backup is to use image-based backup using snapshot technology. Image-based backup is popular for protecting large database applications. It is more efficient to backup only the changed volume blocks vs. copying the entire large database file. Virtual machines are also large files that change continuously, so just like a database application, virtual machines are best protected with image-based backup.
Image-based backup uses snapshot technology to capture the changed blocks for an entire data volume. All the VMs stored on a volume are protected with one quick snapshot. Backup agents are not required to be installed on each VM (Guest OS). The snapshot software that tracks the changed blocks and quiesces the data volume, is provided by the Host OS. Microsoft Volume Shadow Copy Service (VSS) is one popular snapshot technology and the other one is VMware vSphere vStorage APIs.
There are two major advantages of image-based backup vs. traditional file-based backup. First, image-based backup copy much less data during each backup job. Only the changed block are copied for image-based backup; while for file-based backup the entire file is copied, even if only a small portion of the file was changed. This difference has a huge impact depending on the size of the file. Keep in mind that VMs can be GBs in size.
Second is backup frequency. Because image-based backup has very little impact on the application being protected, it can be run more frequently in intervals of hours or minutes for the most mission critical applications. Previously, separate replication solution were required for business continuity, now near continuous data protection can be provided for virtual systems with image-based backup.
Because of these two major advantages, image-based backup is the preferred backup methodology for all virtualized systems. It is fast and non-obtrusive to applications, stores less backup data and can be run as often as required to protect critical applications.
If you are still using a traditional file-based backup solution with agents installed on each system, you should take a look at new modern backup solutions that protect both physical and virtual systems with fast, efficient image-based backup.