PlatinumNAS Owner’s Manual
Appendix D: Glossary
Active Directory
an implementation of LDAP directory services by Microsoft for use in
Windows environments. Active Directory allows administrators to assign enterprise wide
policies, deploy programs to many computers, and apply critical updates to an entire
organization. An Active Directory stores information and settings relating to an organization
in a central, organized, accessible database. Active Directory networks can vary from a small
installation with a few hundred objects, to a large installation with millions of objects. Active
Directory was released first with Windows 2000.
ATA
Acronym for “
AT
Bus
A
ttachment” - a standard interface to IDE hard disks. Western
Digital’s IDE disk interface was standardized by ANSI to form the ATA specification using a
16-bit ISA bus.
Cache
cache is a fast-access memory bank that serves as an intermediate storage for data that
is read from or written to secondary storage. Typically, high-speed caches are implemented in
RAM, though they can also be implemented on disk when speed is not a critical requirement.
Caches generally improve the efficiency of read operations due to the principles of “spatial
and temporal locality of data”. They can also improve the efficiency of write operations.
See
also: Write Back Cache, Write Through Cache
Common Internet File System (CIFS)
a network protocol for sharing files, printers, serial
ports, and other communications between computers. CIFS is based on the widely-used SMB
protocol.
Degraded Mode/Status
All arrays, with the exception of
RAID
0, are designed to handle
disk failures. However, there is limit on the number of hard disks that can fail before the
array is rendered inoperative. For instance, this limit value is 1 for
RAID
1, 3, and 5. In the
case of
RAID
10 or 50, the upper bound is equal to the number of parity groups. When the
number of disk failures occurring in an array are less than or equal to this upper bound, the
array is denoted to be in a degraded state. The failure of the disks does not impair reading
from or writing to the array. However, it impairs the efficiency of throughput in all RAID
types (with the exception of RAID 1) since data requested by read operations may have to be
“reconstructed” using parity. In the case of RAID 1 the throughput of read operations is cut in
half if a drive fails. Operating in degraded mode is considered an acceptable alternative only
for short durations. Generally this duration should span no more time than that required to
inform the user of the failures and to replace the failed disks with suitable spares.
Device Driver
A piece of software that controls a hardware device. Typically drivers provide
an interface by which applications can use the device in a uniform and hardware-independent
manner.
Dirty Data
Dirty data is data that has been written to a cache but has not been “flushed,” or
written to its final destination, typically some secondary storage device.
D-Glossary
