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Synchronous vs. Asynchronous Data Replication

Synchronous data replication, however, is sensitive to latency, and it is sometimes difficult to extend the distance between primary and DR sites beyond approximately 100 miles. To span regional, national, or international distances, asynchronous data replication is more suitable. Asynchronous data replication accumulates multiple transactions before processing an update to the secondary disk array, and so is less sensitive to latency.

This enables the distance between primary and DR sites to stretch over hundreds or thousands of miles, well beyond the circumference of potential disruption. The tradeoff for distance is the fact that, in the event of disaster, the backup site may not be fully synchronized with the production site and a few business transactions may be lost.

Synchronous and asynchronous disk-to-disk data copying offers the most efficient means to implement high availability replication. Typically, the disk-based synchronous and asynchronous solutions are closely integrated into the disk vendor’s architecture and so can leverage other advanced features offered by the vendor’s array.

There is also less data handling compared to other methods such as host- or file-based data copying or replication appliances. All major platform providers offer their own flavors of disk-based replication – e.g. Symmetrix Remote Data Facility (SRDF) for EMC Symmetrix; MirrorView or SAN Copy for EMC CLARiiON; TrueCopy for HDS; Data Replication Manager (DRM) for HP; Remote Volume Mirror (RVM) for LSI Logic, IBM, and StorageTek; and REDI SANlinks for XIOtech.

For heterogeneous storage environments, host- or appliance-based data replication provides data movement between the primary storage of one vendor and the secondary storage of another. The host- and appliance-based replication solutions vary widely in implementation, but typically involve multiple data copy processes and may convert block data to files for transport over IP networks. While not as straightforward as disk-to-disk data replication, these solutions do provide a means to accommodate mixed storage environments or replication between enterprise-class storage and lower cost JBODs.

For less mission-critical data, traditional tape backup at least provides a means to restore data if the primary array is lost. Restoring from tape, however, is time consuming and without periodic testing, it may not be possible to verify the integrity of tape cartridges. A bare metal restore to a new disk array is not an optimum choice for most administrators, but is a means to leverage existing tape archiving routines for data recovery.

Some customers combine disk-to-disk replication and tape backup methods, using the secondary disk array as a spooler for tape backup. That makes it possible to move aged data to tape without impacting production storage.

Page 3: Data Replication via Storage over IP

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