Operating System and Controller Disk Caching

The process of caching describes the use of buffers to separate operations that differ significantly in speed, so the fast one is not held up by the slower one (or at least, not as much). In a system there are many levels of caching that are used to allow different-speed components to run unimpeded; in the disk subsystem there are usually two levels.

The disk drive's logic board contains an integral cache. This cache is used to separate the internal mechanical read/write operations from transfers over the bus, and to hold recently accessed data. A larger cache will result in improved performance by cutting down on the required number of physical seeks and transfers on the platters themselves. Smarter caching algorithms can have the same effect.

In addition to this hardware caching, most operating systems use software disk caching. Since the system memory is many orders of magnitude faster than the hard disk, a small area of system memory (usually a few megabytes) is set aside to buffer requests to the hard disk. When the disk is read, the data are stored in this cache in case they are needed again in the near future (which they often are). If they are needed again, they can be supplied from the cache memory instead of requiring another read of the hard disk.

As with the disk's internal buffer, increasing the size of the cache improves performance--to a point. If you increase it too much, your operating system will run out of usable memory for programs and data, and the system will be forced to rely on much slower virtual memory. In this case your use of memory as virtual disk is causing the system to also need to use your disk as virtual memory, defeating your original intent!

In addition to the two typical caches in the hard disk subsystem, some SCSI host adapters add a third level of cache on the controller itself; these are sometimes called caching controllers. This cache can be several megabytes in size and logically sits between any system disk cache and any buffer on the hard disk. Again, this improves performance by reducing the number of accesses required to the disk. In this case when the system tries to read data from the hard disk, the controller will intercept the request and if it is in the cache, satisfy it from there instead of going to the hard disk. This both improves system speed greatly and also cuts down on traffic on the SCSI bus.

Next: Redundant Arrays of Inexpensive Disks (RAID)