Promise Technology 66 Pro - Manual (Page 118)

SuperTrak66™ User's Manual Chapter 6
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random writes (such as e-mail file servers, for example). For larger writes or
sequential writes, the performance is still fairly fast. Because only one drive in the
array stores redundant data, the cost per megabyte of a level 3 array is fairly low.
RAID 3 requires hardware support for most efficient operation such as the
processor employed by SuperTrak66 because of parity calculation.
Block and parity striping (RAID 5)
Block and parity striping (RAID 5) is useful for the performance gains offered from a
striping array, with the added security feature of single fault tolerance. RAID 5 uses
all drives to stripe data and also stripe parity data. Since parity is calculated on-
the-fly during write operations, RAID 3 (which uses a dedicated drive for parity)
experiences a performance bottleneck when the system constantly writes out
parity data to a single drive. Under RAID 5, parity is striped across all drives. Parity
calculation and data writes are much faster, keeping all the drives in the array busy.
This vastly improves random write performance.
In a nutshell, block and parity striping is best suited for those who wish to enjoy
the performance advantages of a striping array, but who require at least single drive
fault tolerance.
Spanning
In some instances, spanning may be a preferred model of combining drives
together rather than striping. One reason that an array might be configured as
spanning instead of striping would be to make full use of all the capacities of all
drives in the array -- even if drives are mismatched in size. With striping, the array
size is restricted to the number of drives times the storage capacity of the smallest
drive. This cuts off portions of any drives that are larger than the smallest.
The other reason that spanning might be considered over striping may be
performance. 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. 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.