Drive arrays and fault-tolerance methods 107
With an array 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
in the figures in
this section). Then, the read/write heads of all the constituent physical drives are active simultaneously,
reducing the total time required for data transfer.
Because the read/write heads are active simultaneously, the same amount of data is written to each drive
during any given time interval. Each unit of data is called a
block
(denoted by B
n
in the figure), and adjacent
blocks form a set of data
stripes
(S
n
) across all the physical drives that comprise the logical drive.
For data in the logical drive to be readable, the data block sequence must be the same in every stripe. This
sequencing process is performed by the array controller, which sends the data blocks to the drive write heads
in the correct order.
A natural consequence of the striping process is that each physical drive in a given logical drive will contain
the same amount of data. If one physical drive has a larger capacity than other physical drives in the same
logical drive, the extra capacity is wasted because it cannot be used by the logical drive.
