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To protect against outages which can interrupt data availability, it is crucial to have a redundant copy of data. To protect against a storage system outage, you can use replication to create a copy of data on a remote system. Replication is a software feature which synchronizes data to a remote system within the same site or a different location. Replicating data helps to provide data redundancy, and safeguards against storage system failures at the main production site. Having a remote Disaster Recovery (DR) site protects against system and site-wide outages and provides a remote location to resume production and minimize downtime due to a disaster. For Dell Unity, the platform offers many replication solutions to easily meet disaster recovery needs in customer environments.
Synchronous replication is a data protection solution which ensures each block of data written to a storage resource is first saved locally and to a remote image before the write is acknowledged to the host. This ensures that in the event of a disaster, there is zero data loss. In synchronous replication solutions, there are also trade-offs. As each write needs to be saved locally and remotely, added response time occurs during each transaction. This response time increases as distance increases between remote images. Synchronous replication has a distance limitation based on latency between systems. This limitation is generally 60 miles or 100 km between sites. It is recommended to ensure the latency of the link between the local and remote system is less than 10 milliseconds.
Asynchronous replication is primarily used to replicate data over long distances, but also can be used to replicate storage resources between Pools within the same system. Asynchronous replication does not impact host I/O latency as host writes are acknowledged once they are saved to the local storage resource. Because write operations are not immediately replicated to a destination resource, all writes are tracked on the source. This data will be replicated during the next synchronization. Asynchronous replication introduces the concept of a Recovery Point Objective (RPO). RPO is the acceptable amount of data, measured in units of time, which may be lost due to a failure. This delta of time also affects the amount of data which needs to be replicated during the next synchronization, and the amount of potential data loss if a disaster scenario were to occur.
Dell Unity’s synchronous and asynchronous replication features can easily be configured using Unisphere, Unisphere CLI, or REST API. Dell Unity also supports manual replication, which does not automatically update a destination image with changes on the source. Manual replication will be discussed with asynchronous replication. RecoverPoint also supports block replication for Dell Unity and will be discussed later in this paper. RecoverPoint uses physical or virtual appliances to replicate data between systems and is configured using the Unisphere for RecoverPoint user interface.
The Dell Unity File synchronous replication feature and MetroSync Manager are covered in the Dell Unity: MetroSync white paper, which can be found on Dell Technologies Info Hub.