Resolving a Disk Bottleneck

The obvious that the solution to a disk bottleneck is to add another disk. That is an appropriate solution if you are out of space. However, if your disk is too slow, the addition of a new disk might not be the cheapest or most effective solution. Disk systems have many parts, and any one could be the limiting factor in your disk configuration.

• Rule out a memory shortage. When memory is scarce, the Windows NT Virtual Memory Manager writes more pages to disk, resulting in increased disk activity. Because memory is the most common bottleneck on workstations, this is the first place to look.

• Consider upgrading your disk controller card. If you have a card that does transfers in units of 8 or 16 bits, your transfer rate will be improved by switching to a 32-bit controller. Also, choose a bus-mastering controller instead of one that does programmed I/O.

• Consider the architecture of the system I/O bus. The I/O bus and disk controller determine the maximum transfer rate of your disk configuration. After you select an architecture, you can then choose from the range of components, including disk adapters, device buses, and disk controllers that use that architecture. The performance of I/O buses varies widely from 16-bit buses that transfer less than 30 MB per second to 32-bit buses that transfer as much as 100 MB per second. However, the slowest component limits the maximum transfer rate. Even if you have the fastest I/O bus, a slow disk controller can become a bottleneck.

• Upgrade the device bus, the cable-and-pin connector that plugs into the disk. Some device buses transfer 8-bits at a time, but more advanced ones transfer twice as much, 16-bits at a time. There are no 32-bit designs. The transfer size is limited by the number of pins, each of which corresponds to a wire in the cable. For each additional pin, the cable must be shorter to avoid turbulence between the wires. Using current technology, 32-bit designs would not allow enough cable to be practical.

Standard device buses support parallel transmission, that is, communication in one direction at a time. Serial buses allow simultaneous transmission across several paths in different directions. The performance of serial buses is, in theory, up to four times better than parallel buses.

• Upgrade your disk adapter to one that uses bus-mastering direct memory access (DMA) instead of programmed I/O. Programmed I/O uses the processor to set up disk transfers. In bus-mastering DMA, the disk controller managers the I/O bus and uses the DMA controller to manage DMA operation. This frees the processor for other uses.

• Finally, if you have decided that you must have another physical disk, consider placing one of more or your disks on a separate I/O bus. The benefit of multiple disks is reduced significantly when they need to share a crowded bus. One lesson of scaling—whether it is multiple computers, processors, or disks—is that if they have to share a scarce resource, their performance will be limited by the resource, not by their own performance potential.