By default, vRanger does not provide quiescing during backups. When you enable the Enable Guest Quiescing option, quiescing in vRanger is provided by leveraging the VMware® Tools installed in the VM.
• Crash consistent: A crash-consistent backup is analogous to pulling the plug on a server and then backing up the data. The state of the data that is being backed up with respect to the users of the data is indeterminate. Restoring a crash-consistent image is equivalent to rebooting a server after a hard shut-down.
• File-system consistent: File-system consistency is achieved through standard quiescing by using the VMware Sync Driver, which ensures that no file-system writes are pending when the snapshot is taken. For normal VMs, file-system consistency is adequate, although it can cause corruption in database applications.
• Application consistent: Consistency of VSS-compatible applications is achieved by freezing application I/Os prior to creating the VM snapshots. This option ensures that all application writes requests in the machines memory are committed to disk before the snapshot is taken.
The level of consistency provided by the Enable Guest Quiescing option depends on the version of VMware® ESXi™ — and the corresponding VMware Tools — and the guest operating system. The following table provides more detail on what is needed to achieve various levels of consistency:
Table 1. Achieving consistency
As shown in the preceding table, application consistency is not always available with the basic quiescing options. In these situations, you may use the vRanger VSS Tools — vzShadow.exe — for application-level consistency.
vRanger can provide two levels of consistency for Hyper-V® VM backups: file-system consistency and application consistency.
• File-system consistent: File-system consistency is achieved without any additional tools or configurations. File-system consistency ensures that no file system writes were lost during the backup process.
• Application consistent: vRanger engages the Microsoft Volume Shadow Copy Services (VSS) to put supported applications into a consistent state during a backup. This option ensures that all application writes requests in the machines memory are committed to disk before the snapshot is taken, which means that the application and data can be reliably recovered from the backup archive.
• The guest must be operating a Windows® operating system.
The vRanger agent installed on the Hyper-V host uses the VSS native to the Windows operating system to support the truncation of any supported application transaction logs. Log truncation automatically frees space in the logical log for the transaction log to reuse.