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In 2006, Symantec significantly upgraded its Veritas Storage Foundation 5.0 Dynamic Multi-pathing (DMP) path management feature. Prior to Version 5.0, DMP provided only a round-robin path management algorithm; administrators can now choose from seven different algorithms to do path management. Its new default is to look at the minimum queue length on each FC path, identify the least-busy path and then send IO traffic down that path.
DMP 5.0 also improves its error detection and volume discovery by circumventing the OS when managing specific FC paths and communicating directly with the FC HBAs using their APIs. Working with the host bus adapters (HBAs), DMP can identify specific SCSI timeouts or commands issued by storage systems. These are normally received by the FC HBA, but not passed on to the volume managers or OS; however, DMP can spot specific FC path problems or storage system trespass errors, and use alternative paths to access LUNs on back-end storage systems.
DMP can also detect how different storage virtualization products present LUNs to the host. LUNs may be presented by storage controllers in active-active (A/A) or active-passive (A/P) states, which affects how Veritas Storage Foundation's Volume Manager treats them. A/A LUNs are simpler to manage because if the LUN is unavailable on one path, Volume Manager can simply try accessing the LUN on an alternative path.
| A/P LUNs are assigned to and managed by a specific storage system controller; if that controller becomes unavailable, however, it's not as simple as switching to another path as trespass errors on the storage system can occur. By monitoring FC commands issued by storage system-based or network-based virtualization and received by the FC HBA, DMP can ascertain which alternate path to use to access a LUN without causing a storage system trespass error.
The growth of server virtualization also drove Symantec to port the Veritas Storage Foundation's Veritas Volume Manager (VxVM) directly to the hypervisor level starting with the resource manager on Sun Microsystems Inc. Solaris Logical Domains (LDoms) (see "Improving virtual server backups," next page). By default, the native Sun Solaris LDom resource manager virtualizes the storage volumes and FC HBAs presented to it, which it then re-presents as virtual volumes and HBAs to its virtual hosts.
This was first published in January 2008