Achieving data consistency

IBM Storage Fusion backs up application data consistently across all the Persistent Volumes (PVs). It ensures that the recovered data is error-free, reliable, and usable.

The IBM Storage Fusion supports application consistency and crash consistency approaches:

  • Application consistency
    In this approach, application I/O must be paused before a snapshot is taken by using hooks. Though it achieves the highest level of consistency, it comes at the cost of application downtime that can be disruptive. The pause can be a long time depending on the application.
  • Crash consistency
    If IBM Spectrum Scale is used for storage (and not Red Hat® OpenShift® Data Foundation), then you can consider crash consistency. In this approach, application I/O is not paused at all and hence results in zero disruption. It is achieved by what is called a consistency group. All the PVs of an application are placed in a consistency group. During backup, a snapshot is taken for all the PVs in the group at exactly the same instant in time to ensure that all the PVs are consistent with each other. But the disadvantage of this approach is that the application might have some data in memory that is not flushed to the PVs and not included in the snapshot.

    In IBM Storage Fusion with IBM Spectrum Scale, all PVs in a namespace are placed in a consistency group automatically and you do not have to do anything.

Choose the type of data consistency that is appropriate for your application. If the application can restore itself without the data in memory with just what is there in the PVs, then backups are nondisruptive. Otherwise, you must pause I/O and disrupt the application.