**6.1 Introduction to replication**

It is necessary for cloud service providers to protect mission-critical data and minimize the risk of service disruption. If a local outage or disaster occurs, faster data and VM restore, and restart is essential to ensure business continuity. One of the ways to ensure BC is replication, which is the process of creating an exact copy (replica) of the data. These replica copies are used for restore and restart services if data loss occurs. Based on the availability requirements for the service being offered *Network Function Virtualization over Cloud-Cloud Computing as Business Continuity Solution DOI: http://dx.doi.org/10.5772/intechopen.97369*

to the consumer, the data can be replicated to one or more locations. Service provider should provide the option to consumers for choosing the location to which the data is to be replicated in order to comply with regulatory requirements. Replication can be classified into two major categories: local replication and remote replication. Local replication refers to replicating data within the same location. Local replicas help to restore the data in the event of data loss or enables to restart the application immediately to ensure BC. Snapshot and mirroring are the widely deployed local replication techniques. Remote replication refers to replicating data across multiple locations (locations can be geographically dispersed). Remote replication helps organizations to mitigate the risks associated with regional outages resulting from natural or human-made disasters. During disasters, the services can be moved to a remote location to ensure continuous business operation. In a remote replication, data can be synchronously or asynchronously replicated.

Replicas are immediately accessible by the application, but a backup copy must be restored by backup software to make it accessible to applications. Backup is always a point-in-time copy, but a replica can be a point-in-time copy or continuous. Backup is typically used for operational or disaster recovery but replicas can be used for recovery and restart. Replicas typically provide faster RTO compared to recovery from backup.

#### **6.2 Local replication: snapshot**

A snapshot is a virtual copy of a set of files, or volume as they appeared at a specific PIT. A snapshot can be created by using compute operating environment (hypervisor), or storage system operating environment. Typically the storage system operating environment takes snapshot at volume level, that may contain multiple VMs data and configuration files. This option does not provide an option to restore a VM in the volume. The most common snapshot technique implemented in a cloud environment is virtual machine snapshot. A virtual machine snapshot preserves the state and data of a virtual machine at a specific point-in-time. The VM state includes VM files, such as BIOS, VM configurations, and its power state (powered-on, powered-off, or suspended). This VM snapshot is useful for quick restore of a VM. For example, a cloud administrator can snapshot a VM, then make changes such as applying patches, and software upgrades. If anything goes wrong, administrator can simply restore the VM to its previous state using the previously created VM snapshot.

The hypervisor provides an option to create and manage multiple snapshots. When a VM snapshot is created, a child virtual disk (delta disk file) is created from the base image or parent virtual disk. The snapshot mechanism prevents the guest operating system from writing to the base image or parent virtual disk and instead directs all writes to the delta disk file. Successive snapshots generate a new child virtual disk from the previous child virtual disk in the chain. Snapshots hold only changed blocks. This VM snapshot can be used for creating image-based backup (discussed earlier) to offload the backup load from a hypervisor.

#### **6.3 Local replication: mirroring**

Mirroring as shown in **Figure 11** can be implemented within a storage system between volumes and between the storage systems. The example shown on the slide illustrates mirroring between volumes within a storage system. The replica is attached to the source and established as a mirror of the source. The data on the source is copied to the replica. New updates to the source are also updated on the replica. After all the data is copied and both the source and the replica contain

**Figure 11.** *Local Replication: Mirroring.*

identical data, the replica can be considered as a mirror of the source. While the replica is attached to the source, it remains unavailable to any other compute system. However, the compute system continues to access the source. After the synchronization is complete, the replica can be detached from the source and made available for other business operations such as backup and testing. If the source volume is not available due to some reason, the replica enables to restart the service instance on it or restores the data to the source volume to make it available for operations.
