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Introductie Windows Server 2016

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64 C H A P T E R 3 | Storage Synchronous replication Synchronous replication guarantees that the application writes data to at least two locations at the same time before completion of the write operation. This replication is most suitable for mission- critical data because it requires network and storage investments and carries a risk of degraded application performance. Synchronous replication is suitable for both high availability (HA) and disaster recovery (DR) solutions. As Figure 3-1 illustrates, when application writes occur on the source data copy (1), the originating storage does not acknowledge the I/O immediately. Instead, those data changes replicate to the remote destination copy (2) and log data is written (3). The remote site then returns an acknowledgment (4). Only then does the application receive the I/O acknowledgment (5). This ensures constant synchronization of the remote site with the source site, in effect extending storage I/O across the network. In the event of a source site failure, applications can failover to the remote site and resume their operations with assurance of zero data loss. Figure 3-1: Synchronous replication performed by Storage Replica Note In the diagram, T indicates data flushed to the volume at the source site, and T1 indicates data flushed to the volume at the remote site. In all cases, logs always write through. Asynchronous replication In contrast to synchronous replication, asynchronous replication means that when the application writes data, that data replicates to the remote site without immediate acknowledgment guarantees (see Figure 3-2). This mode facilitates faster response time to the application as well as a DR solution that works geographically. With its higher-than-zero recovery-point objective (RPO), asynchronous replication is less suitable for HA solutions such as failover clusters because they are designed for continuous operation with redundancy and no loss of data.

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