The advantages of asynchronous replication

Key selection criteria

Some key areas to consider while comparing asynchronous replication software.

Delta change or write-order fidelity. Storage systems employ two methods to capture changes to block data: delta changes and write-order fidelity. Deltas are changes since the last replication interval and they capture only the changes at the time for which the replication is scheduled. Write-order fidelity captures every change and sends all of them during each replication interval.

Different model support. If you need to replicate from many-to-one, one-to-many or many-to-many configurations, confirm that the asynchronous replication works the same way on multiple models from the vendor.

Firmware upgrades. Firmware updates may disrupt asynchronous replication until all firmware is applied on all systems.

Read-write snapshots. Snapshots taken of volumes previously replicated or scheduled for replication by some storage systems are sometimes read-only. This is fine for recoveries and backup, but if you're planning on performing testing and development with these snapshots you need to determine if they support read-write capabilities.

Replication bandwidth management. You can manage the network bandwidth used by the asynchronous replication software

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by setting policies in the network, the storage system or both. Most storage systems leave most of the management to the network, but some storage systems let you prioritize replication traffic and fine-tune quality of service queues.

Recovery point objective (RPO) is the point in time to which data must be restored to satisfy application owners. For applications that require more specific point in times of recovery down to seconds, minutes or specific writes, you should select a high-end storage system that supports write-order fidelity. Users who can recover to an approximate point in time (give or take 15 minutes), will find snapshots taken at specific intervals adequate for their needs.

Recovery time objective (RTO) defines how soon businesses must recover before unacceptable risks to the business may occur. For applications that must be up in seconds or minutes, you should consider only high-end storage systems that support both synchronous and asynchronous replication software. For applications that have 60 minutes or more to recover, asynchronous replication on midrange storage systems should suffice.

This was first published in September 2007

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