Nutanix supports two main types of snapshots: crash-consistent and application-consistent. A crash-consistent snapshot is taken at the storage layer without any awareness of the guest OS state; it is equivalent to pulling the power plug on a server. An application-consistent snapshot is more complex because it requires the Nutanix cluster to " quiesce " the applications (such as SQL Server or Exchange) inside the guest OS, flushing pending writes to disk to ensure data integrity.
To perform this quiescence on Windows VMs, the Nutanix cluster must communicate with the Windows Volume Shadow Copy Service (VSS). This communication is handled exclusively by Nutanix Guest Tools (NGT). If NGT is not installed on the VM, or if the NGT agent is not running/communicating with the cluster, the system cannot trigger the VSS hardware provider. In this situation, any request for an " application- consistent " snapshot from a third-party backup solution will fail because the cluster lacks the necessary hooks into the guest OS. However, a " crash-consistent " snapshot will still succeed because it does not require any guest-level interaction. Therefore, when app-consistent backups fail but crash-consistent ones succeed, the most likely root cause is a missing or broken NGT installation on the affected virtual machines.