Owner`s manual

PlatinumNAS Owner’s Manual
51
by any RAID array as a backup. In the event a hard disk in a RAID array fails, this backup
is automatically mobilized by the RAID controller to step in place of the failed hard disk.
The data in the failed hard disk is “reconstructed” and written into the new hard disk. In the
case of a RAID 1, data is reconstructed by simply copying the contents of the surviving disk
into the spare. In the case of all other RAID types, reconstruction is performed using parity
information in the working hard disks of that RAID array. This backup hard disk is known as
a “hot” spare since the fail-over process is performed dynamically on a server within the same
session i.e., without the necessity for re-booting or powering down.
IDE Acronym for “Integrated Device Electronics”. A hard disk drive interface standard
developed by Western Digital and introduced. Also knows as Parallel ATA.
Logical Drive A logical drive is comprised of spaces from one or more physical disks and
presented to the operating system as if it were one disk.
MAC (Media Access Control) Address In computer networking a Media Access Control
address (MAC address) is a unique identifier attached to most forms of networking equipment.
All Ethernet devices have unique MAC addresses.
Online Capacity Expansion The ability to add space to an existing RAID array within a
session while preserving the RAID type and data within the array is known as online capacity
expansion. The availability of this feature enables the user to add space to a RAID array as and
when required without rebooting, thereby obviating the need for precise forecasts of capacity
requirements for the future.
Parity A mathematical function that serves as a method for error verification and correction.
In strict technical terms the parity of a group is set to 1 if the number of bits in the group that
are set to 1 is odd, and 0 otherwise. For instance, the parity of N bytes of data is obtained by
determining the number of ith bits in the N bytes that are set to 1. If that number is odd, then
the ith bit of the result is set to 1. This may sound complicated, but in reality the result can
be obtained by simply evaluating the XOR of the N bytes. Parity allows one error in a group
(of bytes) to be corrected.
Partition The space contributed to each array on a physical drive is referred to as a
partition.
PCI An acronym for “Peripheral Component Interconnect”. It is Intel’s local bus standard
that supports up to four plug-in PCI cards per bus. Since PCs can have two or more PCI
buses, the number of PCI cards they can support are a multiple of four. The current PCI
bus implementation (version 2.2) incorporates two 64-bit slots at 66 MHz. Consequently, the
highest throughput achievable using such a bus is 528 MB/sec.
PCI-X An enhanced version of PCI version 2.2. It supports one PCI slot per bus when running
at 133 MHz, two slots when running at 100 MHz and four slots when running at 66 MHz.
It is intended to provide throughputs in excess of 1 GB/sec using a 64-bit wide 133 MHz
implementation.
D-Glossary