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MaxNAS Owner’s Manual
72
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.
IEEE 802.3ad Link Aggregation a method for using multiple Ethernet network cables/
ports in parallel to increase the link speed beyond the limits of any one single cable or port,
and to increase the redundancy for higher availability. The following modes of operation are
available:
• Failover: When one port fails, the other one will take over.
• Load Balance: Ethernet traffic will flow along both Ethernet ports.
• 802.3ad: Linkage two Ethernet ports in parallel to increase throughput.
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.
iSCSI (“Internet SCSI”) a protocol allowing clients (called initiators) to send SCSI commands
(CDBs) to SCSI storage devices (targets) on remote servers. It is a popular Storage Area
Network (SAN) protocol.
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.
NFS (Network File System) a network file system protocol originally developed by Sun
Microsystems in 1983, allowing a user on a client computer to access files over a network
as easily as if the network devices were attached to its local disks. NFS, like many other
protocols, builds on the Open Network Computing Remote Procedure Call (ONC RPC) system.
The Network File System protocol is specified in RFC 1094, RFC 1813, and RFC 3530
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
E-Glossary