ARCHIVED: In Mac OS 9 and earlier, what is Disk Caching, and how do I use it?

The time it takes a hard disk to fetch and retrieve requested data can be described by a variety of measures, such as average seek time and data interleave. However, speed is always limited by the physical facts. A disk drive is a fast-spinning platter with a flying recording head that looks for specific bytes among millions of bytes. RAM, on the other hand, has no moving parts, and fetch and retrieval functions are performed so much faster that access speeds are measured in nanoseconds instead of milliseconds.

Mac OS uses the speed of solid state RAM to alleviate common data bottlenecks. The Memory control panel can allocate an area in RAM for storing the contents and results of a previous hard disk search. The cache provides an alternate area for the system to check before going to the disk. During subsequent searches, if the system finds requested information in the cache, a disk search is not required, saving potential hard disk seek time and speeding up the retrieval process.

Operations that require repetitive pieces of information to be loaded and unloaded are ideal for cache storage. Because Mac OS builds and runs its applications by loading and unloading resources in small, discrete chunks of code, it is an ideal system for caching. However, operations that require accessing large chunks of information spread out unevenly over the hard disk defeat the purpose of a cache. Large word processing or database systems may actually slow down performance because the system must check both the user-definable Disk Cache and the disk buffer.

As a rule of thumb, you want to set the Disk Cache at the ratio of 32KB for every megabyte of physical memory in your Mac OS computer. Thus, if you have a 4MB system, a cache setting of 128KB would be normal. For an 8MB system, a setting of 256KB will be normal.

Note: Settings above 512KB may degrade performance on many older PPC computers and all 68k Macs. For example, if you have 32MB of physical memory in a Quadra 660AV, the highest setting you would want to use is 512KB, even though with the 32KB per megabyte rule you would think it should be 1024KB. On newer PPC computers, especially any running Mac OS 8 or higher, you can get better performance out of higher cache settings. The default setting is usually best. To set the default setting, from the Memory control panel, click Default.