Promise Technology FastTrak66 Anwenderhandbuch Seite 54

Manual EN 05-08-1999 11:30 Pagina 50
Understanding Disk Array Concepts
Spanning (JBOD)
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 performan-
ce 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 stri-
ping, 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 performance level, offers a more predictable transfer
rate, and allows the use of mis-matched drives.
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