Get ready for virtualization


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Split-path architectures
Split-path virtualization software runs on hosts or SSPs, and splits the data and control paths with each path running on a different appliance. For host-based configurations, a host agent communicates with the management appliance over an IP connection and serves as a volume manager on the host. With SSP configurations, no agent is required on the SSP, and the control appliance uses FC and the SSP's APIs to communicate with the SSP.

In host-based designs you must prepare the host, management appliance, FC directors and storage array to allow the host to access its storage. The agent on the host communicates with the management appliance over IP, and also inserts itself into the host's data path as a volume manager that accesses and manages the storage over the host's FC connections. A host reboot is usually required to complete the agent's configuration.

Completing access to the array LUNs requires the following steps: First, FC directors must be zoned to allow the FC HBAs on both the management appliance and host to access the array LUNs. Then the array LUNs must be masked to allow the management appliance and the host to access them. Next, the management appliance accesses the LUNs on the storage array and configures them so they can be accessed by the host. The management appliance then sends that volume configuration information over IP to the agent on the host. At that point, the host can access the LUNs

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assigned to it over the FC SAN.

Successful implementation of this approach is predicated on the following assumptions: First, it's assumed that the same individual or group will be responsible for all aspects of the process—including the configuration on the hosts, FC directors and storage arrays. Second, the virtualization application's agents have to work on all of the different types and versions of operating systems that are—or will be—in the environment. Finally, it assumes that installing an agent on each host is OK and that the agent is permitted by network policies to communicate with the management appliance. Failure to satisfy any of these conditions precludes the virtualization technique from being used on a specific host.

EMC's Invista and Fujitsu Computer Systems Corp.'s Eternus VS900 each offer software that runs on a switch vendor's SSP. An SSP has three unique characteristics:

  1. Instead of requiring agents on the host or SSP, the management appliance uses FC to communicate with the SSP via APIs and uploads the data path code to the SSP via the FC connection. Exceptions to the data path code loaded onto the SSP, such as the introduction of new worldwide names (WWNs) or LUNs, are routed to the management appliance.

  2. The data path code resides on ASICs on the SSP in the data path between the server and storage. This configuration allows switch vendors to develop and deploy SSP hardware that optimizes the breaking apart of FC packets and lets storage vendors focus on virtualization software.

  3. No cache resides on the SSP. Eliminating the cache in the SSP reduces the risk of an appliance outage causing data loss.

This was first published in December 2005

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