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More Information On “Interleaving”

Our Word For The DayInterleaving” prompted reader Jim B. To send in the following technical information:

“Some hard disk drives support “interleaving.” On disks, this is a method where what appears to be logically consecutive blocks of data are actually written on the disk with one or more intervening physical blocks between them.

For example, a disk holding 6 records – 1, 2, 3, 4, 5, 6 may actually be written as 1, 4, 2, 5, 3, 6. This was done for performance reasons — some disk drive electronics were so slow, that the drive could not keep up with the speed of the rotating media. This “solution” gave the drive the time to transfer record 1 while “moving” over record 4, so that by the time it got to “2” it was again ready to read the data. (actually, some of this time was needed to verify the correctness of the data by running the error detection algorithms).

The only other (slow) alternative was to go “all the way around” and get record 2 on the next rotation of the disk.

This “interleaving” could be specified by a user when the disk was formatted, and was usually specified by x:y, where 1:1 meant NO interleaving. The above example would have been 2:1. Some formatting programs ran performance tests and “suggested” the best interleave factor based on the results.

Since musicians probably could care less about all of this, I’ll save you the torment of thinking about seek time, rotational delays, alternate tracks and blocks, and so on. Suffice it to say, disks are STILL most often the single greatest performance bottleneck in a computing system, and the components most prone to failure (after software of course).”

Thanks Jim!

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