1
Appendix B RAID arrays and fault
tolerance
RAID arrays
Physical drives
The capacity and performance of a single physical drive is adequate for home users. However,
business users demand higher storage capacities, higher data transfer rates, and greater protection
against data loss when drives fail.
As shown in Figure 1, connecting extra physical drives (denoted by P
n
) to a system increases the
total storage capacity, but has no effect on the efficiency of read/write (R/W) operations. Data can
still be transferred to only one physical drive at a time.
Figure 1 Data can still be transferred to only one physical drive at a time
Logical drives
As shown in Figure 2, with a storage controller installed in the system, the capacity of several
physical drives can be combined into one or more virtual units called logical drives (also called
logical volumes and denoted by L
n
). The read/write heads of all the constituent physical drives are
active simultaneously, reducing the total time required for data transfer.
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