Virtual Backup, Physical Backup: A Tale of Two Backups

Fundamentally, there are two ways to protect virtual machines. You can perform the backup at the hypervisor-level (also called the host-level backup) or you can do backup within the virtual machine (also called guest-level backup.)

Backup vendors typically want to sell you whatever technique that they support. Thus there are companies that can only operate at the host-level or only operate within the virtual machine. Both will tell you that whatever they do is better. I recommend being pretty careful with these claims – there are a lot of ways to misrepresent numbers.

What’s an example of this? I can remember talking to someone from a company that did only backup within the virtual machine. They claimed they were superior to host-level backups because they used less resource – specifically less CPU time. I asked how much they used. They told me 5%. And that host backup used 70% of the CPU.

The issue with this was that they were measuring CPU utilization of a backup that occurred every few minutes within the virtual machine – all day long – for multiple virtual machines – versus the CPU utilization of a single backup a day at the host level for all the virtual machines. It’s not what they were doing was evil – there are 24x7x52 environments in which a continuous CPU load is better than a once a day spiked CPU load – but positioning a backup within a virtual machine as “solving” this is just plain wrong.

So are there no differences between virtual and physical backup? There are – and these have to do with the limitations of hypervisor-level backup. These are

  • Backup of VMware unlicensed (free) ESX/ESXi virtual machines.
  • VMware RDM (Raw Device Mapping) in physical compatibility mode.
  • Backup and recovery of NAS or iSCSI devices attached directly to the virtual machine.
  • Backup and recovery of an iSCSI.
  • Greater ability to include and exclude file system data at the file and/or folder level.
  • Distinct and customizable backup strategies between the VM and the operating system.
  • Dissimilar bare metal between the VM and other physical or virtual environments.

Any questions?


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