Broadly speaking, the invention pertains to improved techniques for accessing data storage systems. These techniques can be used to detect, correct and prevent undesirable access delays experienced in storage systems. Here's Apple's background on the invention: "The present invention relates generally to data storage systems for use with computing systems and, more particularly, to techniques for accessing data from the data storage systems.

"Most computing systems use a storage device to store data. A disk drive is an example of a storage device that is commonly used with computers, including desktop computers, notebook computers and servers. Disk drives are used for many purposes. Over the years storage devices (e.g., disk drives) have become more advanced so as to offer greater performance and storage capacity. Manufacturers of disk drives provide data sheets or specifications that detail performance and capacity of their disk drives. This is partially attributed to higher performance and storage capacity requirements for some applications."

"Data throughput rate for disk drives is essential for certain applications. For example, in the case of streaming data (e.g., audio or video data), time performance is of particular concern because if a data stream is temporarily delayed, the recipient of the data stream receives no data during the delay. The delay can cause slow spots, jitter or other undesired artifacts to occur in the presentation of the data. By way of example, a real time video playback stream typically requires a relatively high constant data rate. For a 10-bit High Definition (HD) video that outputs about 30 frames per second, this constant data rate amounts to about 165 MB of data per second. Data rates such as this, however, cannot always be maintained using conventional techniques. As a result, significant delays in data access are experienced and often manifest in undesirable effects (e.g., dropping one or more frames of a real time video play back)."

The inventors are Michael Eng, David Wong and Lamont Benaresh. The graphic below depicts a computing system in accordance with one embodiment of the invention.

Apple says the advantages of the invention are numerous. One advantage of the invention is that improved performance (e.g., data throughput) of disk drives or RAIDs can be achieved by reducing delay time of slow access operations. Another advantage of the invention is that overall performance can be enhanced by taking preventative measures that reduce the reoccurrence of slow access problems. Yet another advantage of the invention is that preventative measures can be performed cost effectively during maintenance times. Still another advantage is that the invention can be cost effectively implemented to use parity information which is typically provided for RAIDs.