Verifying the capacity of your source environment is important in assuring that backups can run without negatively impacting the performance of the running systems. The source refers to the storage that is used by your virtualized environment, whether it be shared SAN, NFS, or directly attached to your virtual hosts.
Overlooking the impact that will be caused by the taking and committing of snapshot data (discussed in Impact of Snapshot Operations section above), as well as the added impact of data transfers, can lead to significant degradation of the production environment during backups and cause backups to take longer than required.
When a virtual backup is taken, a snapshot must be created of the VM. It’s the snapshot data that is copied by Veeam Backup & Replication to the backup storage. However, while that backup is taking place, the VM is still running, thus changes to the disk must be written to a temporary location, the “snapshot file”. For lightly loaded VMs this is unlikely to be a major issue, but for VMs with moderate to heavy transactional load this snapshot can grow quickly.
Thus, you must verify that your source storage can handle both the growth of your snapshots, and the extra I/O load that is created by the snapshots, as well as the actual backup I/O. This impact is most notable during full backups, since this involves transferring significant amounts of data and requires snapshots to be held open for longer periods of time.
As Veeam Backup & Replication leverages the hypervisor to create snapshots, the impact of snapshots can be estimated prior to implementation by manually taking snapshots via the management console and observing their growth and impact.
Veeam uses built-in intelligence to check if there is enough free space on datastore before starting the backup:
•Veeam Backup & Replication will not attempt to take VM snapshot if free space on datastore hosting snapshot file is less than 2 GB.
•You will receive a warning message when the free space on production datastores reaches 10 GBs.
Also, a situation may occur when Veeam causes heavy IOPS on the datastore of the source VM (and a target VM, in a replication job), impacting performance of other VMs in the same datastore. This is not specifically because of Veeam and can be addressed by VMware SIOC feature (requires VMware Enterprise Plus licensing). For details, see this Veeam Knowledge Base article.