Promise Technology FastTRAK66 User Manual page 51

Understanding Disk Array Concepts
A Spanning disk array (also aptly named as JBOD for "Just a Bunch of Drives") is
equal to the sum of all drives when the drives used are of different capacities.
Spanning stores data on to a drive until it is full, then proceeds to store files onto
the next drive in the array. There are no additional performance or fault tolerance
array features in this array. When any disk member fails, the failure affects the entire
array.
Spanning may be considered for performance in certain instances. With striping,
the performance is affected directly by the stripe block size. Block size should be
tailored to the typical I/O on the drive — whether it is generally more random or
sequential. However, what if there is no predictability of the type of I/O access?
What if both random and sequential I/Os occur unpredictably? The performance of
a striped array will fluctuate. In the end, this may result in no overall performance
gain. With spanning, the performance factor simply reflects a single drive's perfor-
mance level, offers a more predictable transfer rate, and allows the use of mis-
matched drives.
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