In the context of snapshots and backups and data protection in general, two types or states of data consistency are distinguished: crash and application.

A backup or snapshot is crash consistent if all of the interrelated data components are as they were (write-order consistent) at the instant of the crash. This type of consistency is similar to the status of the data on your PC’s hard drive after a power outage or similar event. A crash-consistent backup is usually sufficient for nondatabase operating systems and applications like file servers, DHCP servers, print servers, and so on.

A backup or snapshot is application consistent if, in addition to being write-order consistent, running applications have completed all their operations and flushed their buffers to disk (application quiescing). Application-consistent backups are recommended for database operating systems and applications such as SQL, Oracle, and Exchange.

The product family ensures continuous crash consistency at the branch and at the data center by using journaling and by preserving the order of WRITEs across all the exposed LUNs. For application-consistent backups, administrators can directly configure and assign hourly, daily, or weekly snapshot policies on the Core. Edge interacts directly with both VMware ESXi and Microsoft Windows servers, through VMware Tools and Volume Snapshot Service (VSS) to quiesce the applications and generate application-consistent snapshots of both Virtual Machine File System (VMFS) and New Technology File System (NTFS) data drives.

Through the vSphere client, you can view inside the LUN to see the VMs previously loaded in the data center storage array. You can add a server that contains vSphere VMs as a datastore to the ESXi server in the branch. This server can be either a regular hardware platform hosting ESXi or the hypervisor node in the Edge appliance.