If you discover a disk bottleneck, what can you do? The first thing you need to determine is whether it's really more memory that you need. To restate a critical truth, if you are short on memory, you will see the lost performance reflected as a disk bottleneck. We'll take a look at that issue in the next chapter.
If memory isn't the problem, there are a number of possible avenues to pursue to improve disk throughput. First, think about your controller card. Find out from the manufacturer if it does 8-bit, 16-bit, or 32-bit transfers. The more bits in the transfer operation, the faster the controller moves data.
Your I/O bus architecture comes into play here. EISA and MCA buses transfer data at much higher rates than ISA buses. Some computers also have a "turbo switch" that affects bus speed. Make sure it is set to on if you have such a switch. ISA buses cannot see above 16 MB of RAM. To place data above 16 MB of RAM, the driver must arrange a copy of the data from the area below 16 MB. This can slow you down. Changing the bus within a computer is usually not an option. But it's about time you got that new computer anyway, and now if you've isolated your performance problems to the bus, you have a reason!
Also determine if your disk adapter uses direct memory access (DMA) or not. DMA can noticeably improve transfer speeds.
Some disk adapters feature built-in caches. The benefit of this depends a lot on the access patterns of your system. If you are going to purchase the RAM anyway, you might want to consider putting it in the computer's main memory instead of on the adapter card. Because Windows NT features adaptive disk cache size, when memory is needed for something besides disk caching, it is available. It is also available to disk drives on other adapters, as well as to the LAN file systems. But some computers are limited on the amount of main memory they can use, in which case adding memory to a disk adapter may be just the ticket.
If you've done the best you can with your adapter, you might want to think about your disk subsystem configuration. If you have all your activity focused on one disk and one adapter, consider getting a second disk and even a second adapter. Try to spread the load across them. This idea segues into our next topic.