Expanding Options for Clustering
Finally, some applications do require physically shared disks. However, because shared SCSI remains an unusual configuration, putting together complete systems using this approach requires care—often complete hardware/software system certification by a vendor. (When was the last time you checked your disk firmware level?) Even when this is all arduously put together, as we have seen, there are severe limitations on both the number of nodes that can share data over SCSI and possible physical cable layouts. Fiber channel relaxes some of these limitations but comes with drawbacks of its own, including steep price points, problems with multivendor interoperability and a set of management issues unfamiliar to many users of Linux thin servers.
The next year will see a huge change in the face of shared storage. Industry leaders are pushing two new major storage interconnects that will make shared storage available at a price point appropriate for thin servers that will also support sharing by large node counts seen in Internet data centers. Cisco and hot startups like 3ware are backing SCSI-over-IP, which will allow SCSI block protocols to run over a switched Ethernet fabric. Intel and the server vendors are lining up behind a new I/O standard called Infiniband. It will provide a switched I/O fabric that could eventually be implemented in the chip sets included on every commodity server motherboard. Indeed, these developments are complementary—Gigabit Ethernet cards will be able to sit on the Infiniband fabric and run the SCSI-over-IP protocols. As a consequence, we can look forward to having dozens or even hundreds of systems share storage over modern interconnects in the near future. Clustering solutions that provide the basis for using these interconnects to create truly manageable and scalable distributed solutions out of sets of Linux boxes will set the tone for the data center architectures of the future.
Ken Dove is chief architect of PolyServe, Inc. Previously, he was a distinguished engineer at IBM and before that principal software architect at Sequent Computer Systems for 12 years.
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