About Alternate Pathing and MPIO
Alternate Pathing (AP) is an extension of mirrors handled by collaborating storage servers. It allows an application server interruption-free access to the virtual volume in the event of a component-volume failure or even the failure of a storage server. AP is able to redirect I/Os down a secondary path to the virtual volume when it detects a fault (indicated by a timeout condition while waiting for a response from the disk) anywhere in the primary path to its primary volume.
Multi Path I/O (MPIO) is a high availability software feature that provides path redundancy to multi-path storage devices. Support for MPIO by DataCore Software simulates multi-pathing by means of multiple mappings of the same volume.
Alternate Pathing software running on the application server automatically switches to the path of a “healthy” storage server. After a failure has been found and rectified (faulty cable replacement, for instance), the storage server will automatically re-synchronize the mirror and switch the application server back to the primary storage server.
With Alternate Pathing, an application server has two separate and redundant paths to the virtual volume. The Alternate Pathing software will only allow the application server to access the virtual volume over one path at a time and controls which path is active any given moment.
Note that a physical path is not designated as primary or secondary, nor is a particular storage server primary or secondary. Each path and storage server can be primary or secondary for specific virtual volumes for optimal distribution and protection.
