Quantum DLT 2000 Handbook page 202

Overhead. Command overhead refers to the processing time
required by the controller, host adapter, or drive prior to the
execution of a command. Lower command overhead yields higher
drive performance.
Overwrite. To write data on top of existing data thus erasing the
original data.
P
POH. Acronym for Power On Hours. The unit of measurement for
Mean Time Between Failures (MTBF), expressed as the number of
hours that the drive is powered on. See MTBF.
PRML. See partial response, maximum likelihood.
Parallel Channel Architecture. Allows DLTtape drives to
read/write multiple channels simultaneously, providing a data
transfer rate of 6MB/s for the DLT 8000 drive. With this
architecture, data blocks are not required to be located on any
particular track or in consecutive order. This channel-independent
block structure provides a powerful write-error handling system
that allows bad blocks to be rewritten on the next available
channel.
Partial Response, Maximum Likelihood (PRML). A technology
that allows a disk or tape drive's read channel to pack more data in
the same amount of space on magnetic media. Super DLTtape
technology incorporates a new, high-efficiency PRML channel.
Peak Transfer Rate. The maximum speed with which information
moves inside a tape drive or between drive and host. Usually
measured in megabytes per second.
Performance. A measure of the speed of the drive during normal
operation. Factors affecting performance are seek times, transfer
rate, and command overhead.
G.14 GLOSSARY