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Continuous Replication

Replication

Replication involves sharing information to ensure consistency between redundant resources, such as software or hardware components, to improve reliability, fault-tolerance, or accessibility.

Data replication is good idea for database replication but can also be used for change management where configuration at main site is replicated to the standby systems.

Purpose

The overall purpose of this document is to provide insight of different replication strategies for disk and datafiles replications.

Terminology

One speaks of:

• Data replication: if the same data is stored on multiple storage devices.
• Active replication: is performed by processing the same request at every replica.
• Passive replication: involves processing each single request on a single replica and then transferring its resultant state to the other replicas.
• Load balancing differs from task replication, since it distributes a load of different (not the same) computations across machines, and allows a single computation to be dropped in case of failure. Load balancing, however, sometimes uses data replication (especially multi-master replication) internally, to distribute its data among machines.
• Backup differs from replication in that it saves a copy of data unchanged for a long period of time.

Disk storage replication

Real time storage replication is implemented by distributing updates of block device to several physical hard disks. This way, any file system supported by the operating system can be replicated and this is achieved by disk mirroring.

A replication is extendable across the network, so the disks can be located in physically distant locations.

The cross-site replication can be achieved by following three characteristics:

1. Synchronous replication: guarantees zero data loss as an atomic operation (i.e. two-phase commits).
2. Asynchronous replication: write is happens with a small lag, but helps in performance.
3. Semi synchronous replication: guarantees that a transaction is correctly copied, but it does not guarantee that the commit on the slave takes place.

Examples of replication technologies:

• Hypervisor-based replication (vSphere Replication, Hyper-V Replica)
• Distributed data storage (EMC VPLEX, and alike)
• Host/fabric/array-based replication
• SAN replication
• Asynchronous NAS (often used for long-distance replication for performance reasons)
• Synchronous NAS
• Storage Mirroring

tip

The performance of the software and databases stored on network-based storage devices depends on the performance of the network connection between the server and the storage device.

For this reason, WorkFusion recommends that you connect the server to the storage device using a private dedicated network connection, which should be 1 Gigabit Ethernet or better.