RAID Technology Overview - September 2007

Performance and Data Redundancy
Increasing Logical Drive Performance
Without an array controller, connecting extra physical disks to a system increases the total storage
capacity. However, it has no effect on the efficiency of read/write operations, because data can
only be transferred to one physical disk at a time (see Figure 1-1).
Figure 1-1 Physical Disks Added to System
With an array controller, connecting extra physical disks to a system increases both the total
storage capacity and the read/write efficiency. The capacity of several physical disks is combined
into one or more virtual units called logical drives (also called logical volumes).
The read/write heads of all of the physical disks in a logical drive are active simultaneously,
improving I/O performance and reducing the total time required for data transfer (see Figure 1-2).
Figure 1-2 Physical Disks Configured into a Logical Drive (L1)
Because the read/write heads for each physical disk are active simultaneously, the same amount
of data is written to each disk during any given time interval. Each unit of data is called a block.
The blocks form a set of data stripes that are spread evenly over all the physical disks in a logical
drive (see Figure 1-3).
Performance and Data Redundancy 13