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Aggregate I/O rates and their affect on SAN performance

Combining data means consolidating the I/O rate. Does SAN have an aggregate I/O rate at which point the performance is drastically reduced? I realize that there are probably enough paths, but it does not indicate or have a way of indicating what the peak demand of I/O is.

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It is true that consolidating data so it is accessed through a single SAN port would also consolidate the I/O. But SANs certainly don't have to work that way. The storage can be accessed through many different ports in the SAN. The nummber of ports that can be used depends on the configuration and number of subsystem(s) deployed and the application mix. If you are talking about a single application, then yes, there probably is a limitation for a single port. But that's a rare case.

Otherwise, the I/O consolidation is a function of the network topology. Fibre Channel has two: loops and switched (fabric)

A Fibre Channel loop network has shared media. At some point the maximum capability of the loop will be reached. The theoretical max transer rate for FC is 100MB/s (one direction). With fast servers and subsystems it is possible to attain close to 90% utilization in benchmarks.

That said, mileage wll vary with driving habits. The more servers and storage there are on a loop, the more time is spent managing access and the efficiency drops off. I'll make a stab and say that loops with multiple servers and storage subsystems can probably support approximately 50MB/sec aggregated/sustained.

Switched (fabric) SANs are another (and more powerful) animal. By virtue of switching, each system and subsystem has its own link to a switch. Fibre Channel switches all tend to be very fast and very efficient. If you run out of bandwidth with the switch you have, it is possible to add another switch and divide the work. This has not been a major problem in practice, however as most switched SANs have planty of bandwidth to spare.

In my mind, it's not the I/O channel that is the gating factor in I/O performance, it's the disk drives behind the curtain that are limited by mechanical processes.

This was first published in March 2001

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