The lowdown on replication appliances


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Users, however, will generally want to avoid the pass-through approach for ongoing data replication. Load balancing and path failover software like EMC Corp.'s PowerPath, Hitachi Data Systems' Hitachi Dynamic Link Manager (HDLM) or IBM Corp.'s Subsystem Device Drive (SDD) may not work with the LUNs presented by these appliances or, if they do work, may not result in a vendor-certified configuration. Second, appliances using the pass-through method limit the number of LUNs that can be presented on each port. While McData's UltraNet Replication Appliance offers 256 LUNs per port, administrators must account for this limitation in larger deployments, especially if the ports will be shared among multiple servers. Finally, all I/O between the server and storage array must go through the replication device. So even though only the write I/O is intercepted and copied by the appliance, putting this device in front of the storage array in effect virtualizes the LUNs presented by that array--the only benefit realized is the new replication capability.

Sun partially circumvents this situation by deploying most of its MirrorStore replication appliances as data movers and using them for one-time replication requirements. Used that way, MirrorStore leaves the primary I/O path between the server and storage array intact. Instead, MirrorStore is given read-only access to the LUNs assigned to the server on the storage array. With this block-level access, MirrorStore migrates LUNs from the existing storage array to the new one in the background, while tracking changes to LUNs on the old array and moving those blocks as the changes occur.

Once all of the data on the LUNs is on the new array, the application is stopped so that the final writes can be synchronized by MirrorStore to the LUNs on the new storage array. The SAN is then reconfigured to allow the server to access the LUNs on the new array, and the server discovers the data on these new LUNs dynamically or is rebooted to allow LUN discovery to occur. The application is then restarted using data that resides on the LUNs on the new storage array.

This was first published in November 2005

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