> # Information About SDV

> As of Cisco SAN-OS Release 3.1(2) and later, you can use Cisco SAN device virtualization to create virtual devices that represent physical end-devices. Virtualization of SAN devices accelerates swapout or failover to a replacement disk array, and it also minimizes downtime when replacing host bus adapters (HBAs) or when rehosting an application on a different server.

> SAN device virtualization enables you to:

> - Reduce the amount of time it takes for data migration, and ultimately the overall amount of downtime.
> - Improve ease-of-use and reduce the possibility of user-introduced errors during the failover by performing the operation in a single step.
> - Easily scale to larger numbers of targets.

> SAN devices that are virtualized can be either initiators or targets. You can virtualize targets to create a virtual target and also virtualize initiators to create a virtual initiator. SAN device configurations do not distinguish between virtual initiators and virtual targets .

> Note While most of the examples in this chapter describe target virtualization, the initiator virtualization functions similarly.

> Typically, today’s deployments for handling device failures are designed for high availability (HA), with redundancy being a key part of this design. Consider the situation where a target is designed to be redundant. Two arrays are deployed–a primary and secondary in this situation. Enterprises often use some type of consistency technology (such as EMF SRDF) between the primary and secondary arrays to ensure that the secondary is a mirrored copy of the production LUN. However, if the primary array fails, it must be replaced by the secondary because all I/O must occur on the secondary array. Problems can occur because the time required to bring the secondary array up and have it working often takes longer than most can afford.

> If a storage array is replaced without using Cisco SDV, then it may require the following actions:

> - Taking down a server to modify zoning and account for the new array.
> - Changing the Cisco NX-OS configuration to accommodate Fibre Channel IDs (FC IDs) and pWWNs of the new array.
> - Changing a server configuration to accommodate the new FC IDs and pWWNs.

> More specifically, without SDV you might experience the following conditions:

> - It can take a considerable amount of time to configure a secondary device for a typical production environment.
> - In the zoning configuration, all the initiators must be rezoned with the secondary device, and certain initiators must also be reconfigured. For example, the WWN and FC ID of the secondary device are different, so driver files must be changed and the server must be rebooted.
> - Clustering (multiple initiators) compounds the problem, and the failover procedure must be repeated for each server of the cluster. Think of a server cluster as a set of HBAs–any storage array FC ID changes must be performed for each HBA.

> SDV enables you to achieve the following performance targets:

> - Reduce the amount of time it takes for data migration, and ultimately the overall amount of downtime.
> - Easily scale to larger numbers of devices.

> When disk array X was deployed, the user created virtual devices for all the Fibre Channel interfaces using SDV. After data replication from disk array X was completed, the user briefly pauses activity on the application server and relinked disk array Y to the virtual devices used by the server, completing the swapout of disk array X. No zoning changes or host operating system configuration changes were required during the time-critical period when the swap was performed; this significantly minimized application downtime.

> Note The array administrator will likely have to perform actions on array Y for it to become a primary device and accept server logins before linking the virtual device to the array Y pWWN.

> This section includes the following topics:

> - Key Concepts
> - Automatic Failover and Fallback
> - Resolving Fabric Merge Conflicts

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