Understanding Crash Consistency and Application Consistency
In the context of snapshots and backups and data protection in general, there are two types or states of data consistency:
• Crash consistency - 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. To better understand this type of consistency, imagine 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.
• Application consistency - A backup or snapshot is application consistent if, in addition to being write-order consistent, running applications complete all their operations and flush their buffers to disk (application quiescing). Application-consistent backups are recommended for database operating systems and applications such as SQL, Oracle, and Exchange.
SteelFusion ensures continuous crash consistency at the branch and at the data center by using journaling and by preserving the order of write operations across all the exposed LUNs. For application-consistent backups, you can directly configure and assign hourly, daily, or weekly snapshot policies. Edges interact directly with both VMware ESXi and Microsoft Windows servers, through VMware Tools and VSS, to quiesce the applications and generate application-consistent snapshots of both VMFS and NTFS data drives.