Promise Technology 66 Pro - Manual (Page 154)

SuperTrak66™ User's Manual Appendix G
G-5
parity A mechanism for making the sum of two binary digits (bits) equal to a
pre-specified digit. Such a system may use "even parity," in which
case the sum is made even, or "odd parity," in which the sum is made
odd. As an example if we had two pieces of data, ‘A = 4’ and ‘B =
6’, parity data would be defined as A+B = 10. In RAID 3 or 5
applications, such parity data can be used to reconstruct corrupt or
missing data from a failed drive. From the example above, if hard drive
‘A’ has failed, and the only data we have is that on hard drive ‘B’
and parity data contained on another drive, the RAID controller will
work backwards to rebuild the missing data on to a replacement drive.
The data on hard drive ‘A’ can be calculated since ‘A = parity - B’
(or ’A = 10 - 6’), yielding ‘A = 4’.
partial read cache hit
A partial read cache request has data which is partially in the cache
and part of which must be read directly from the disk.
partial write cache hit
A partial write hit occurs when part of the data transfer is already in
write back cache and part has to have new memory blocks allocated
to it. Partial write hit requests usually result in additional disk
operations because of the block allocation. By contrast, a full write hit
would not require an additional disk operation for block allocation.
partition A logical division created on drive media, as recognized by the O/S. A
drive with a single partition defines up to 100% of the drive as
belonging to it, depending on the file system used to set up the drive.
Multiple partitions may be used to break large drives into smaller
chunks which may be more manageable. Each partition may be
addressed by the O/S as a separate drive letter.
PIO Programmed Input/Output: A method of performing read/write
operations on an IDE drive in a programmed timing interval between
block requests. This type of I/O requires some sort of CPU utilization
in order to handle the I/O and store the data transferred to/from the
drive.
RAID Redundant Array of Independent Disks: A model of logically organizing
multiple hard drives accessible by an O/S as a single logical device.
Such an architecture of drives may improve I/O performance by
accessing multiple drives simultaneously, and/or add fault tolerance by
providing redundancy and parity configuration options.
RAM Random Access Memory: A type of memory which is used in all types
of computer systems and by peripheral devices, accessible for read or
write at any time. RAM loses its contents after a power-down.