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Choosing a VSS Provider (Microsoft Hyper-V Server 2012 R2 and Earlier)

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Before you configure backup jobs, it is recommended that you decide what VSS provider will be used to create and maintain volume shadow copies.

By default, Veeam Backup & Replication automatically selects a VSS provider on every volume. Every 4 hours it rescans all Microsoft Hyper‑V hosts added to the backup infrastructure and updates information about connected volumes. Veeam Backup & Replication also collects information about software and hardware VSS providers available on every volume.

If hardware providers are available, Veeam Backup & Replication selects a hardware provider to create volume shadow copies on a volume. If no hardware providers are installed, Veeam Backup & Replication selects the VSS system software provider. If necessary, however, you can assign a VSS provider on every volume manually.

If both software and hardware providers are available for a volume, it is recommended that you select a hardware provider. Although software providers are generally applicable to a wider range of storage platforms, they have a number of limitations:

Software providers do not support transportable volume shadow copies and cannot be used for off-host backup.
By default, jobs working with the same volume can take up to 4 snapshots of a volume simultaneously. If necessary, the number of snapshots can be increased.
Hardware providers operate at the storage system controller level. Software providers operate at the software level, between the file system and the volume manager, and can cause a significant performance overhead on the source host.
[For Microsoft Windows Server 2008 R2] Hardware providers can work with several snapshots simultaneously. That is, if you have several jobs that work with the same volume, you can run them in parallel. If you use a software provider, Veeam Backup & Replication serializes VM processing. You will not be able to start several jobs working with the same volume simultaneously. The volume on which VM disks reside remains locked by one job for the whole period of data processing. After the job completes, the volume becomes accessible for other jobs.
[For Microsoft Windows Server 2008 R2] Software providers are not suitable for backup on CSV's — a significant backup window is required to back up VMs that reside on the same volume but are registered on different hosts. When a cluster node initiates a snapshot on a CSV, the CSV is switched to the Backup in Progress, Redirected Access mode.

If a hardware provider is used to take a snapshot in such case, the CSV stays in the redirected mode while the snapshot is taken. After a volume shadow copy is created, the CSV resumes direct I/O.
If a software provider is used to take a snapshot, the CSV stays in the redirected mode until the backup process completes. If large virtual disks are processed, backup time can be significant.