Veeam Backup & Replication leverages the vSphere functionality for snapshots to create backups of VMs. When Veeam Backup & Replication begins the backup of a VM, it communicates with vSphere to request a snapshot of the VM, and after the backup of the VM is complete, Veeam requests that the snapshot be removed. The creation and removal of snapshots in vSphere creates a significant impact on the environment that must be taken into account.
Here we will discuss the various factors that should be considered regarding this process, and recommend techniques to minimize the impact of snapshot operations.
As a concept, vSphere snapshots are a simple technology. A VM generally contains at least one virtual disk, which is represented by a VMDK file. When a snapshot is taken, VMware continues to read blocks from the file as normal, however, for any new blocks that are written to the disk, these writes are redirected to a new “thin” VMDK file called the delta file. Since the original VMDK file is only being used for reads, it provides a consistent view of the blocks that made up the VM at the time the “snapshot” was taken - this allows Veeam Backup & Replication to read this based disk as a consistent image for backup and replication. When the snapshot is removed, the blocks that were written to the delta file are read and written back into the original VMDK, and finally the delta file is discarded.
As with many things in technology, although the concept is simple, the actual implementation is a little more involved. The following is a quick look at the impact of various operations on the VM and underlying infrastructure.