The advantages of asynchronous replication

Replication caveats
As EMC's SAN Copy illustrates, there are caveats to using any storage system's asynchronous replication technology. There's considerable variation among products related to the degree of flexibility users have in their choice of storage systems, what replication configurations are supported and how well the storage systems manage the asynchronous replication process.

Companies with Hewlett-Packard (HP) Co.'s StorageWorks Continuous Access Enterprise Virtual Array (EVA) software can replicate data between any of HP's EVA models, in either direction from the high-end EVA8000 to the lower end EVA3000, as well as any model in between. The catch with the EVA Continuous Access software is that it doesn't replicate to other HP storage systems such as HP's StorageWorks Modular Smart Array (MSA) or XP Series. The MSA offers no support for asynchronous replication software, while the asynchronous replication software for the XP Series isn't incompatible with the EVAs. Cost-conscious users will also need to pay a premium for disk drives on the EVA because it supports only Fibre Attached Technology Adapted (FATA) disk drives as opposed to the more economical SATA drives.

The lack of interoperability among different vendors', and even a single vendor's, models is a clear downside when using asynchronous replication, although some vendors give administrators some flexibility in reusing existing capacity on competitor's storage systems. For

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example, admins may present LUNs on other vendors' storage systems to Compellent, HDS and Network Appliance (NetApp) Inc. controllers so they can use the capacity on these storage systems as part of their management scheme.

As users deploy storage system-based replication more extensively, they'll find it will become more difficult to migrate off those storage systems. Another potential problem with using asynchronous replication is that in write I/O-intensive environments there may be a performance hit on the application running on the storage system. The severity of the problem depends on the nature of the write I/Os. If the write I/O represents new data to the storage system, the impact is negligible. But when an application makes frequent changes to existing blocks, the storage system must first copy the existing data to a new block location so it can replicate the data at the next replication interval and then write the new data to the old block location. To address this delay, storage systems like those from NetApp eliminate the extra copy step by writing the new data to a new block and then updating the set of pointers the asynchronous replication software uses.

This was first published in September 2007

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