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One of the best technologies for enhancing the reliability and resilience of servers is multipath I/O, and it is coming to Microsoft's Windows 2000 and Windows .NET Server operating systems soon. Multipath I/O does exactly what its name suggests: It provides multiple paths between a server's operating system and I/O ports and the I/O devices attached to that server, and is particularly useful for improving the performance and availability of storage arrays and storage area networks linked to servers.

Multipath I/O does two thing. First, by having two redundant paths between a server and all of its I/O devices (or at least the ones that are deemed critical), in the event that one link is broken (say a SCSI card fries or a cable is accidentally disconnected), there is a second live link between that server and the device that allows the server to continue to send and receive information from that device, as if nothing bad had happened. Perhaps more significantly, since component failure is rare, is the fact that multipath I/O allows load balancing across those multiple links when both are connected, which increases the throughput across I/O links.

Multipath I/O, like many server technologies, first made its debut in the IBM mainframe line, specifically for the 3990 disk controllers for ES/9000 and S/390 mainframes in the late 1980s. All of IBM's core mainframe operating systems--z/OS (formerly OS/390 and MVS), z/VM, and VSE--support multipath I/O. IBM has introduced this technology years ago in the OS/400-based iSeries product line a few years ago, and is set to debut an improved version of it for its AIX 5L V5.2 Unix operating system in a matter of weeks. Hewlett-Packard's HP-UX and Tru64 Unix servers have supported multipath I/O for years, and Sun Microsystems' Solaris supports multipathing I/O through adjunct products like the file managers from Veritas and Oracle's Real Application Clusters. SCO Group added multipath I/O to UnixWare with version 7 several years ago, and Sistina offers a global file system for the open source Linux operating system that provides some of the functions of multipath I/O, and the open source community is working to add multipath I/O into the kernel of the next generation of Linux (which means 2.5 for development and 2.6 for production). Microsoft is playing catch up here, and the fact that it is working on multipath I/O only goes to show that it finally understands some of the things that Windows server environments need these mainframe-class technologies to compete in the data center, where winning deals is not just based on price and raw processor scalability.

Microsoft announced its Multipath I/O initiative late last week, and said that it is getting broad support for this technology from the major storage vendors in the Windows market, including HP, EMC, Hitachi, Veritas, Emulex, QLogic, among many others. Microsoft says that it will backcast support for multipath I/O into Windows 2000 Server (and presumably Advanced Server and Datacenter Server) as well as putting it into the future "Whistler" edition of Windows, now called Windows .NET Server 2003. Exactly when this multipath I/O support will be available is unclear, and in any event, even when Microsoft does put it into Windows 2000 and Windows .NET Server 2003, it will only be useful once the third party storage vendors that supply products for the Windows platform make use of the code Microsoft has created.