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Highly Available Storage: Multipathing and the Microsoft MPIO Driver Architecture White Paper Published: October 2003 Abstract High reliance on the Internet and e-commerce means that organizations are increasingly requiring that their data be available 24x7, year round. Multipathing, the ability of a system to use more than one read/write path to a storage device, is a high availability solution that provides fault tolerance against single-point-of failure in hardware components. Multipathing can also provide load balancing of I/O traffic, thereby improving system and application performance. This white paper is of interest to the storage administrator who is ultimately responsible for ensuring that an organization’s data remains highly available, and to storage vendors who target multipathing solutions using the Microsoft MPIO (multipath input/output) solution in Windows 2000?Server, Windows Server?2003, and Windows Storage Server?2003. The paper begins with a discussion of the situations and environments in which multipathing is a smart business solution, then describes the critical factors that must be part of an effective multipathing solution, and concludes with a brief overview of the driver-level details of the Microsoft MPIO solution. The Microsoft MPIO driver development kit is not available as part of the Windows Driver Development Kit; it can be licensed by hardware and software vendors following execution of a Program Agreement. Organizations that seek Windows-based multipathing solutions should check with their vendors to ensure that their storage devices are Microsoft MPIO-compatible. ContentsIntroduction 1 High Availability Solutions 2 Application Availability through Server Clustering 2 Storage Redundancy through RAID 2 Storage Availability through Multipathing 2 How Data Becomes Unavailable 3 Failure Modes 3 Points of Failure 3 Multipathing Solutions 6 Third Party Solutions 6 Operating System-Based Solutions 6 A Joint Solution: MPIO and De