SVC Global Mirror uses an asynchronous version of IBM's peer-to-peer remote copy technology, now called Metro Mirror, to move data over fiber networks. It requires two Fibre Channel links per storage system and is similar to EMC's Symmetrix Remote Data Facility, remote mirroring software for the Symmetrix DMX series of storage devices, with one important exception: SVC Global Mirror works with any SVC-virtualized storage on the back end.
With SVC 4.1, Metro and Global Mirror are sold as a single feature with no additional licensing fee for existing Metro Mirror users. Pricing for SVC Global Mirror/Metro Mirror starts at $10,500 per terabyte, plus one year's maintenance.
"With asynchronous, the secondary site will always be slightly behind the primary site," cautioned Chris Saul, marketing manager for SVC at IBM. "But it's much cheaper and the distances are virtually unlimited."
Version 4.1 of SVC also supports 4 Gbps fabrics, and 2 Gbps nodes can be upgraded to the higher speed. IBM has added support for Hewlett-Packard Co. Open VMS, Hitachi Data Systems Inc. TagmaStore, Network Appliance Inc. 3020 and 3050s and its own DS4700 and N series to the list of almost 80 systems that now work with SVC.
Greg Schulz, founder of the StorageIO Group, noted that simply adding 4 Gbps connectivity to a storage system does not guarantee a performance boost unless any and all bottlenecks in the data path (processors, memory, I/O bridge busses, PCI, etc.) are also removed. "Performance aside, 4 Gbit Fibre Channel will benefit SVC to support connectivity of more server and storage ports for consolidation purposes," he said.
IBM has more metrics on how the nodes, Fibre Channel interfaces and storage are performing under different application workloads, which Schultz says will enable better decisions to be made for performance tuning, configuration and load balancing. "Hopefully, IBM will leverage these metrics moving forward with some type of policy-based workload manager to load balance across the various SVC nodes," he added. It's possible to balance I/O across SVC nodes today, but it involves a short interruption to I/O, which is unacceptable for many applications.
Further improvements to the product include the ability to replace old nodes in a cluster with new boxes, while hosts continue to access data. "That's like performing heart surgery while carrying on a conversation with the patient," Saul said.
SVC 4.1 will be available in limited numbers on June 23. IBM is holding out on a general release of the product until Sept. 8 to ensure its support for global replication is ready. "SVC Global Mirror is complex, -- we need to make sure the support structure is skilled in it," Saul said.
IBM claims it has over 2,000 SVC customers who are virtualizing more than 15 petabytes of data with SVC. Meanwhile, EMC just acquired Kashya Inc. to add long-distance replication to its virtualization product, Invista. This product has been shipping since Dec. 2005 but with limited functionality and no customers to speak of.