The symptoms of a memory shortage are very similar to those of a disk bottleneck. When physical memory is scarce, the system starts writing the contents of memory to disk and reading in smaller blocks more frequently (this process is called paging. The less memory you have, the more the disk is used, resulting in a greater load on the disk system. Therefore, it's important to monitor memory counters along with disk counters when you suspect a performance problem with your disk system.
Monitor paging activity along with disk reading and writing, using the following counters:
Avg. Disk Queue Length
Figure 8.6 shows how a memory shortage can cause disk counters to indicate a problem.
Figure 8.6 Paging Activity Compared with Disk Activity
Notice that this figure shows a long disk queue, accompanied by a high rate of paging. Compare the number of page reads against the number of disk reads to see how many times the system accessed the disk to retrieve pages that were not found in memory, or to write pages to free up memory for new data coming in from the disk. When these values are high, the system does not have sufficient memory. Without inclusion of the memory counters to reveal this behavior, you might have assumed that the disk was inadequate. Upgrading the disk in this situation would not have cured the problem.
For more information about measuring memory and identifying memory shortages, see "Evaluating Memory and Cache Usage" in this book.